JPE572.fm Page 88 Monday, March 5, 2001 11:48 AM
Journal of Applied
Ecology 2001
38, 88 – 99
Living where the food is: web location by linyphiid spiders
in relation to prey availability in winter wheat
Blackwell Science, Ltd
J.D. HARWOOD*†, K.D. SUNDERLAND† and W.O.C. SYMONDSON*
*School of Biosciences, Cardiff University, PO Box 915, Cardiff CF10 3TL, UK; and †Horticulture Research
International, Wellesbourne, Warwick CV35 9EF, UK
Summary
1. Spiders form a major component of the generalist predator fauna, potentially able to
restrict pest population growth, but their populations may be food-limited under
current farming regimes. This study aimed to quantify food availability to spiders in
winter wheat and to determine whether spider web locations are positively associated
with available food resources.
2. Mini-sticky traps (availability rate per 24 h, including prey falling from the crop) and
mini-quadrats (instantaneous density on the ground by day) were used, in combination,
to monitor the availability of potential prey to web-building species of money spider
(Linyphiidae) in fields of winter wheat in Warwickshire, UK, 1997–98.
3. These methods were applied to web sites of individual spiders and to non-web sites
located randomly up to 30 cm away from each web. A total of 18 546 invertebrates were
captured using these methods.
4. Overall, significantly more potential prey were available in web sites than in non-web
sites (both on sticky traps and in quadrats).
5. Prey availability in May and July was about a third of that in June (both on sticky
traps and in quadrats) and may have been below that known to be necessary for spiders
to realize their maximum population growth rate.
6. The peak rate of capture of linyphiid spiders on mini-sticky traps was 0·6 trap–1 day–1
at web sites, and approximately half this value at non-web sites. Numbers of spiders
captured by mini-sticky traps and mini-quadrats increased exponentially as the season
progressed. The high capture frequency in relation to population density, and the
differential between web and non-web sites, points to a dynamic and aggregated distribution of spiders in winter wheat, which is consistent with what is known about
mate-searching and web site abandonment rates by the Linyphiidae.
7. The combination of techniques described here is recommended for monitoring prey
availability in prey-enhancement programmes and may prove useful in quantitative
studies of both intra- and interspecific interactions between spiders.
Key-words: aggregative responses, Collembola, generalist predators, mini-quadrats,
mini-sticky traps.
Journal of Applied Ecology (2001) 38, 88–99
Introduction
In Britain, the area of cereal crops treated with pesticide has increased by 24% since 1994, with a concomitant 16% increase in the amount of active ingredients
applied ( Thomas, Garthwaite & Banham 1996). Although
chemical pesticides are convenient to use, and often
efficient and cost-effective in the short term, it is
now appreciated that they are not a long-term option
© 2001 British
Ecological Society
Correspondence: Dr W.O.C. Symondson (fax 02920 874305;
e-mail [email protected]).
because of the associated cumulative problems (such as
pest resistance and environmental pollution) that render
pesticide-based agriculture unsustainable (Pimentel
1995). Biological control is a strong alternative option,
and for the majority of low-value outdoor European
crops this must be achieved through conservation of
endemic biocontrol agents (i.e. ‘conservation biological
control’; Ehler 1998) rather than by classical biological
control or rear-and-release methods. Small soft-bodied
pests (such as aphids) that are accessible on the vegetation and ground surface, are attacked by a range of
natural enemies (pathogens, parasitoids, specialist and
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Web location by
linyphiid spiders
© 2001 British
Ecological Society,
Journal of Applied
Ecology, 38,
88–99
generalist natural enemies) that interact in complex
ways (Sunderland et al. 1997). Generalist predators
(e.g. spiders, carabid and staphylinid beetles) have the
valuable attribute of being able to subsist on alternative
non-pest prey. They can therefore either simply be
present in the field before the pest arrives, performing a
lying-in-wait strategy (Murdoch, Chesson & Chesson
1985; Chang & Kareiva 1999), or build up their populations on alternative foods early in the season and
then impact on the pest population with a favourable
predator : pest ratio during the early phase of pest
population growth (Settle et al. 1996). Manipulative
field experiments have demonstrated that generalist
predators can often (Edwards, Sunderland & George
1979; Chiverton 1986; Duffield et al. 1996), but not
always (Holland & Thomas 1997), contribute to commercially valuable reductions of aphid populations
on wheat. Web-building species of money spider
(Linyphiidae), such as Lepthyphantes tenuis (Blackwall)
and Erigone atra (Blackwall), consume cereal aphids
(Sunderland et al. 1987) and trap considerable numbers in their horizontal sheet webs (Sunderland, Fraser
& Dixon 1986a; Alderweireldt 1994a), which can cover
up to half of the surface area of a wheat field (Sunderland, Fraser & Dixon 1986b). Death of pests trapped in
webs may contribute to pest control even if the spider
does not consume them (Sunderland 1999).
Reviews of the literature have demonstrated that
spider density is likely to be increased by within-crop
habitat diversification (Samu & Sunderland 1999; Sunderland & Samu 2000). The mechanisms underlying
this effect are not fully understood but it is probable
that prey diversification is an important factor. Laboratory investigations suggest that the value of different
prey types for supporting spider population growth
varies greatly between major taxonomic groups (orders
and families) of prey (Toft 1995; Sunderland et al.
1996; Beck & Toft 2000; Bilde, Axelsen & Toft 2000)
and even between congeneric species [e.g. between the
collembolans Isotoma anglicana Lubbock and Folsomia
candida Willem (Toft & Nielsen 1997; K.D. Sunderland,
J.D. Harwood & W.O.C. Symondson, unpublished
data) and Isotoma tigrina Nicolet (T. Bilde & S. Toft,
unpublished data)]. To guide the future development
of improved practical techniques aimed at enhancing
spider populations and increase their impact on pests,
quantitative information (from the field or under
simulated field conditions) will be needed on (i) the
absolute and relative availability of different prey types,
(ii) prey preferences, and (iii) diet-related spider population growth rates. In this paper we address the first of
these aims and develop the methodology needed to
quantify prey availability in the field. For this purpose
it is necessary to quantify how many prey are available
close to the webs of individual spiders, which are to be
found on the ground or up to 10 cm above the ground
(Sunderland, Fraser & Dixon 1986a). Sampling methods
such as vacuum insect nets (Potts & Vickerman 1974;
Moreby et al. 1994), pitfall traps (Nentwig 1982) and
large sticky traps (Kajak 1965; Sunderland, Fraser &
Dixon 1986b) fail to provide density estimates, or seriously
underestimate density (Sunderland & Topping 1995;
Sunderland et al. 1995), and lack the spatial precision
needed for this study.
To be able ultimately to optimize the spatial distribution of within-field diversified prey resource it is
necessary to know how efficiently and dynamically
spiders are capable of relocating their webs in relation
to temporal and spatial fluctuations of prey availability.
Some web-building spiders are known to be efficient in
this respect (McNett & Rypstra 1997) but others are
relatively insensitive to prey availability (Schaefer 1978).
There are, however, very few such studies of spiders in
agricultural habitats (Sunderland & Samu 2000).
In this study we (i) quantified spatial and temporal
variation in the availability of potential prey to webbuilding linyphiid spiders in fields of winter wheat,
using a combination of ‘instantaneous’ and cumulative
sampling techniques, and (ii) determined whether web
location is correlated spatially with the abundance of
potential prey. As far as we know, this is the first time
that these topics have been investigated in an agricultural setting.
Methods
   
The study sites were winter wheat (cv. ‘Hereward’)
fields of approximately 7 ha planted on a predominantly sandy loam soil at Horticulture Research
International (HRI), Wellesbourne, Warwickshire,
UK (52°12.18′ N, 1°36.00′ W). The fields were farmed
according to standard farming practice and no
insecticide applications were required during the
period of investigation. The study sites were surrounded by fields of spring barley, winter barley,
winter wheat and by hay meadows.
   
Sampling was carried out from late April until harvest
(in late July/early August) in 1997 and 1998. The abundance of invertebrates (including potential prey) was
determined by two different ground-based sampling
methods: mini-sticky traps and mini-quadrats. These
methods were designed to be small and precise enough
to monitor potential prey in the immediate area of a
web without the interference of ‘sampling noise’ from
invertebrates that would never be encountered by the
spiders. They also form a complementary pair of methods;
the mini-sticky trap is a passive sampling technique that
relies on activity of the prey, but can sample throughout
24 h and catch prey that fall or descend from higher
strata, whereas collection from a mini-quadrat is an
active sampling method, confined to the ground during
a very limited time period (thus providing an ‘instantaneous’ density estimate). The use of mini-quadrats
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90
J.D. Harwood,
K.D. Sunderland &
W.O.C. Symondson
© 2001 British
Ecological Society,
Journal of Applied
Ecology, 38,
88–99
enables the collection of potential prey that are temporarily inactive and hiding under weeds, small stones
and at the bases of cereal stems.
The plastic mini-sticky traps were 7·5 cm2 (1·5 cm
× 5 cm, 2 mm thick), which is of comparable area to
webs constructed by the common erigonid spiders
E. atra and Erigone dentipalpis (Wider) (Sunderland,
Fraser & Dixon 1986a; Alderweireldt 1994a). The traps
were coloured with black acrylic paint (to minimize
visual attraction by merging in colour with the ground
surface) and were covered on the upper side with a
thin acetate sheet coated with Oecotak A5, a non-toxic
polybutene-based adhesive (Oecos, Kimpton, UK).
The black traps were unlikely to absorb much additional heat from sunlight because they were placed on
the ground surface, and were therefore shaded by the
winter wheat. After traps were removed from the field,
the detachable acetate sheet was placed in a bath of
white spirit to dissolve the Oecotak, enabling recovery
of trapped invertebrates into alcohol for storage and
later identification. The mini-quadrats were circular
sampling areas (diameter 10 cm, area approximately
78·5 cm2) defined by a template. These were placed
on the ground, surrounding the web or non-web site.
All invertebrates were collected from within the miniquadrat by pooter, searching under loose soil and
vegetation.
Sheet webs were located at random within the study
fields, and linyphiid spiders present within these webs
were captured and preserved for subsequent identification. Vacant webs were not categorized as web sites
because spiders are known to leave their webs in active
pursuit of prey (Alderweireldt 1994a; Schütt 1995) or
abandon their web sites (Samu et al. 1996) in search of
more profitable hunting grounds (Vollrath 1985; Gillespie
& Caraco 1987). Therefore, only webs in which linyphiid
spiders were present were included in the analysis. The
webs were removed because spiders are known to be
attracted by the presence of silk (Leborgne & Pasquet
1987; Hodge & Storfer-Isser 1997). This also minimized
any potential interference with the traps. One mini-sticky
trap was centred horizontally on the ground at the
position from which a web was removed, and another
placed at a random non-web site nearby (up to 30 cm
away). Sampling was always carried out in pairs, to
enable direct comparisons between each set of traps,
which were left in situ for 24 h. Great care was taken
not to disturb surrounding vegetation and dislodge
arthropods onto the traps during placement and
collection.
When sampling by mini-quadrat, the same pairwise sampling procedure was used as described above
for mini-sticky traps. The ground within the miniquadrat was searched thoroughly, and all arthropods
present were transferred immediately into alcohol.
No attempt was made to collect arthropods from the
crop vegetation above the mini-quadrat. Samples
were taken during the daytime between 08.00 hours
and 16.00 hours.
Comparisons between web-centred and non-webcentred mini-sticky traps were made from mid-June to
mid-July in 1997 (n = 120 paired samples), and a more
extensive monitoring programme performed in 1998 at
regular intervals from late April until harvest (n = 250).
Mini-quadrats were used to sample the abundance of
potential prey during July 1997 (n = 52) and from late
April until harvest in 1998 (n = 271). No comparisons
were made for quadrat catches between the two years
due to the different sampling dates.
 
Meteorological data (maximum and minimum air temperatures; soil temperature; rainfall; hours of sunshine;
wind speed; rate of evaporation; relative humidity)
were obtained from the weather station located at HRI
Wellesbourne, which was within 1200 m of all field sites.
  
To stabilize variances, all sample data were transformed
(log10 (x + 1)) prior to analyses. For the purposes of
making direct comparisons between mini-sticky traps
and mini-quadrats, data were converted into standard
unit areas (per cm2). Analysis of variance () was
used to analyse potential prey populations captured by
mini-sticky traps and mini-quadrats. Catches of potential
prey were pooled over time for comparisons of web vs.
non-web sites, and mini-sticky traps verses mini-quadrats.
A non-parametric Mann–Whitney U-test was used
where the assumptions of  could not be met, and
paired sample t-tests were used to compare numbers
of individual prey taxa captured by web and non-web
traps. Where less common prey taxa were analysed,
data were grouped into means per sampling session
and the analysis was performed on mean numbers
captured per session.
Potential prey items in the samples were separated
from other invertebrates on the basis of size (the
linyphiid species found in winter wheat are rarely able
to capture and kill prey > 5 mm long) and in relation
to information from the published literature for prey
of linyphiids in European arable crops (Sunderland,
Fraser & Dixon 1986a; De Keer & Maelfait 1987a,
1988; Nyffeler & Benz 1988; Alderweireldt 1994a;
Jmhasly & Nentwig 1995; Toft 1995) and the results of
laboratory prey acceptability trials (K.D. Sunderland,
unpublished data).
Results
Data for web sites of all Linyphiidae were pooled and
analysed collectively. Table 1 shows the numbers of
each species/genus/subfamily of spider captured at web
sites prior to sampling by mini-sticky traps and quadrats
(i.e. the web owners). The total numbers of linyphiids
recorded exceeded the number of web sites because,
occasionally, more than one spider was present in a web.
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linyphiid spiders
Table 1. Number of each species/genus/subfamily caught in web sites prior to sampling by mini-sticky traps (MS) and miniquadrats (MQ) during 1997 and 1998
Species/genus/subfamily
1997 MS
1997 MQ
1998 MS
1998 MQ
Erigone atra (Blackwall)
Erigone dentipalpis (Wider)
Milleriana inerrans (O.P.-Cambridge)
Oedothorax spp.
Lepthyphantes tenuis (Blackwall)
Bathyphantes gracilis (Blackwall)
Meioneta rurestris (C.L. Koch)
Porrhomma errans (Blackwall)
Micrargus subaequalis ( Westring)
Pachygnatha degeeri Sundevall
Erigoninae juveniles
Linyphiinae juveniles
Total captured
25
18
3
2
25
–
39
–
1
1
10
9
133
5
4
1
2
25
1
6
–
–
–
5
7
56
54
32
–
6
103
37
8
1
–
1
12
9
263
76
34
–
19
113
23
11
–
–
–
10
9
295
Fig. 1. Mean number of potential prey, non-prey and spiders captured (per cm 2) by mini-sticky traps and mini-quadrats within
web sites during (a) 1997 and (b) 1998. Bars are ± SE. Numbers above columns are the ratios of numbers of prey captured by ministicky trap/mini-quadrat.
© 2001 British
Ecological Society,
Journal of Applied
Ecology, 38,
88–99
     
  
In 1997, web-centred mini-sticky traps (cumulative
sampling over 24 h) caught more potential prey items,
per cm2, than were found in mini-quadrats (instantaneous sampling during the day) (F1,340 = 5·67, P < 0·001).
However, comparisons between individual prey taxa
were not possible due to the small number of sampling
dates (n = 4 for both mini-sticky traps and mini-quadrats).
JPE572.fm Page 92 Monday, March 5, 2001 11:48 AM
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J.D. Harwood,
K.D. Sunderland &
W.O.C. Symondson
Fig. 2. Mean number ( per trap per day) of (a) total potential prey, (b) all Collembola, (c) Isotoma anglicana (Collembola) and (d)
Isotomurus palustris (Collembola) captured by web-centred (filled circles) and non-web-centred (empty circles) mini-sticky traps
from May to July 1998. Bars are ± SE.
Similarly in 1998, significantly more potential prey
items were captured, per cm2, by web-centred mini-sticky
traps than by web-centred mini-quadrats (F1,1038 = 1199·43,
P < 0·001). There were also large differences between
the two methods in the numbers of certain invertebrate
taxa captured during both 1997 and 1998 (Fig. 1).
Significantly more Collembola (F1,1038 = 532·05, P <
0·001), Diptera (Mann–Whitney, U = 0·0, n = 35,
P < 0·001), Hymenoptera (U = 64·0, n = 35, P < 0·01)
and Araneae (U = 77·5, n = 35, P < 0·05) were caught
by web-centred mini-sticky traps than were sampled by
web-centred mini-quadrats. There were large differences in the ratio of individual prey taxa captured, per
cm2, between the two sampling methodologies (Fig. 1).
The small dipterans that were classified as potential
prey were especially diverse, with representatives
from the families Cecidomyiidae, Lonchopteridae,
Phoridae, Sciaridae, Mycetophilidae, Drosophilidae
and Dolichopodidae.
       
© 2001 British
Ecological Society,
Journal of Applied
Ecology, 38,
88–99
In 1997, significantly more potential prey items were
captured in sites where Linyphiidae had constructed
their webs than in non-web sites, as monitored by
both mini-sticky traps (F1,238 = 6·80, P < 0·01) and
mini-quadrats (F1,102 = 12·22, P < 0·01). Similar results
were found in 1998, on different fields, again when
monitored by both mini-sticky traps (F1,480 = 96·69,
P < 0·001) and mini-quadrats (F1,520 = 75·51, P < 0·001).
The temporal abundance of potential prey in 1998 was
found to be highly variable (F9,480 = 28·45, P < 0·001)
(Fig. 2a).
Collembola, which constituted > 60% of all potential prey items captured by web-centred mini-sticky
traps during 1998, were significantly more abundant in
web sites (mean per cm2 = 0·60 ± 0·04) than in non-web
sites (mean per cm2 = 0·26 ± 0·02) (F1,480 = 109·02,
P < 0·001). Similar distribution patterns were also
evident when individual species of Collembola were
analysed. The two most abundant isotomid collembolans,
I. anglicana (sensu. Fjellberg 1980) and Isotomurus
palustris Müller, were present in greater densities on
web-centred, than on non-web, mini-sticky traps (I.
anglicana: F1,480 = 39·69, P < 0·001; I. palustris: F1,18 =
6·41, P < 0·05) and in mini-quadrats (I. anglicana:
F1,520 = 29·59, P < 0·001; I. palustris: F1,520 = 22·54, P <
0·001). The temporal capture frequencies of these
species, and of Collembola as a whole, on mini-sticky
traps varied significantly throughout the monitoring
period (Fig. 2b–d) (all Collembola: F9,480 = 45·56, P <
0·001; I. anglicana: F9,480 = 55·60, P < 0·001; I. palustris:
F9,480 = 17·22, P < 0·001). Similar significant differences
between numbers captured over time were recorded by
mini-quadrat sampling (all Collembola: F10,520 = 59·67,
P < 0·001; I. anglicana : F 10,520 = 99·30, P < 0·001;
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Web location by
linyphiid spiders
Fig. 3. Mean number of entomobryid collembolans (Lepidocyrtus cyaneus and Entomobrya multifasciata) captured per cm2 by
web-centred mini-sticky traps and mini-quadrats during 1998. Points are log 10 (means) for individual sampling occasions.
Mini-sticky trap regression: log10 y = 0·00648x – 0·117, r 2 = 0·80, t14 = 7·10, P < 0·001. Mini-quadrat regression: log10 y = 0·00862x
– 0·164, r2 = 0·42, t19 = 3·64, P < 0·01.
Table 2. Differences between web and non-web sites in numbers of potential prey (Diptera, Hymenoptera, Coleoptera,
Aphididae) and spiders (Araneae) captured by (a) mini-sticky traps and (b) mini-quadrats, in winter wheat in 1998. The area of
a mini-sticky trap (7·5 cm2 ) is approximately a tenth that of a mini-quadrat (78·5 cm2 ). Mean number of Araneae captured per
web-centred mini-quadrat are presented as (x + 1) to account for the web owner
Variable
(a) Mini-sticky traps
Collembola
Diptera
Hymenoptera
Coleoptera
Aphididae
Araneae
(b) Mini-quadrats
Collembola
Diptera
Hymenoptera
Coleoptera
Aphididae
Araneae
© 2001 British
Ecological Society,
Journal of Applied
Ecology, 38,
88–99
Mean per web
site ± SE
Mean per non-web
site ± SE
Ratio web/
non-web
d.f.
P
7·52
1·02
1·32
2·02
1·49
4·26
14
14
14
14
14
14
< 0·001
0·326
0·209
0·063
0·157
0·001
4·48 ± 0·31
0·29 ± 0·04
0·14 ± 0·03
0·12 ± 0·02
0·20 ± 0·04
0·30 ± 0·04
1·93 ± 0·18
0·25 ± 0·03
0·08 ± 0·02
0·06 ± 0·01
0·11 ± 0·02
0·08 ± 0·02
2·32
1·16
1·75
2·00
1·81
3·75
10·85
3·24
3·94
5·25
1·40
32·47
19
19
19
19
19
19
< 0·001
0·004
0·001
< 0·001
0·117
< 0·001
27·66 ± 1·81
0·20 ± 0·03
0·08 ± 0·02
0·75 ± 0·07
0·21 ± 0·04
1·78 ± 0·08
13·66 ± 1·12
0·08 ± 0·02
0·04 ± 0·01
0·34 ± 0·04
0·16 ± 0·03
0·19 ± 0·04
2·02
2·50
2·00
2·21
1·31
9·37
t
I. palustris: F10,520 = 77·06, P < 0·001). However, despite
this temporal variability, more Collembola (total and
isotomid) were captured on all dates at web-centred
sites, regardless of sampling method.
The entomobryid collembolans Lepidocyrtus cyaneus
Tullberg and Entomobrya multifasciata (Tullberg)
constituted 25·4% of total potential prey items on
mini-sticky traps and 57·6% in mini-quadrats in 1997,
although this difference was not significant. They were
relatively less dominant in 1998 (5·6% for mini-sticky
traps and 14·3% for mini-quadrats), with no significant
difference between the two sampling methods, although
there was a significant difference in numbers captured
by mini-sticky traps between the two years (U = 6198·5,
n = 240, P < 0·05). No comparison was made for quadrat
catches between the two years due to the different sampling dates. However, during 1998 the density of Entomobryidae increased logarithmically as the season
progressed (Fig. 3). Analysis of covariance indicated
that there was no significant difference between the
slopes for mini-sticky traps and quadrats (F1,31 = 0·40,
P > 0·05) or the y-axis intercepts (F1,31 = 0·52, P > 0·05).
Linyphiidae were significantly more abundant in
web than non-web sites both on mini-sticky traps and
in mini-quadrats, and Diptera, Hymenoptera and
Coleoptera were more abundant in web than non-web
sites within mini-quadrats (Table 2). The sex ratio of
spiders captured also varied considerably. Male linyphiids were captured on significantly more occasions than
females on mini-sticky traps in web sites (U = 56·5,
JPE572.fm Page 94 Monday, March 5, 2001 11:48 AM
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J.D. Harwood,
K.D. Sunderland &
W.O.C. Symondson
Fig. 4. Mean number of Linyphiidae sampled by web and non-web-centred mini-quadrats during 1998. Data for web quadrats
are presented as log10(x + 1) to account for the web owner. Web regression: log10y = 0·00621x – 0·080, r 2 = 0·72, t19 = 6·78,
P < 0·001. Non-web regression: log10y = 0·00330x – 0·360, r2 = 0·64, t19 = 5·63, P < 0·001.
Fig. 5. Mean number of Linyphiidae captured by web and non-web-centred mini-sticky traps during 1998. Web regression:
log10y = 0·00388x – 0·395, r2 = 0·37, t14 = 2·73, P < 0·05. Non-web regression: log10y = 0·00195x – 0·250, r2 = 0·45, t14 = 3·25,
P < 0·01.
© 2001 British
Ecological Society,
Journal of Applied
Ecology, 38,
88–99
n = 30, P < 0·01); 3·3 times more male than female
spiders were also collected in non-web traps, although
these differences were not statistically significant.
Linyphiid density, monitored by mini-quadrat
sampling, increased logarithmically with time (Fig. 4).
Analysis of covariance indicated a significant difference
between the slopes of the web and non-web regressions
(F1,36 = 7·17, P < 0·05) and the y-axis intercepts (F1,36 =
76·51, P < 0·001), showing that linyphiid density was
increasing more rapidly at web sites. The number of
linyphiids captured by mini-sticky traps (both web and
non-web) showed a similar pattern (Fig. 5) but analysis
of covariance indicated no significant difference between
the two slopes (F1,26 = 1·57, P > 0·05) or y-axis intercepts
(F1,26 = 0·32, P > 0·05), but the two slopes are significantly different from zero (F1,26 = 14·28, P < 0·01). The
log10 number of spiders captured by these methods
within web sites also increased when regressed against
maximum air temperature (mini-sticky traps: r2 = 0·27,
t14 = 2·18, P < 0·05; mini-quadrats: r2 = 0·57, t19 = 4·91,
P < 0·001) and soil temperature (mini-quadrats:
r2 = 0·53, t19 = 4·47, P < 0·001).
A negative relationship was found to exist between
the number of Linyphiidae and Collembola captured
by web-centred mini-sticky traps (log10 Linyphiidae =
–0·721 log10 Collembola + 0·636, r2 = 0·40, t14 = 2·97,
P < 0·05). No such relationship was found using data
from mini-quadrats.
-    

From the comparison of 4 consecutive weeks (mid-June
to mid-July) of mini-sticky trap data collected during
JPE572.fm Page 95 Monday, March 5, 2001 11:48 AM
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Web location by
linyphiid spiders
Table 3. Mean daily meteorological values (± SE) for climatic conditions in the months May, June and July during 1997 and 1998
Mean per day
Weather variable
1997
1998
F
P
Maximum air temperature (°C)
Minimum air temperature (°C)
Soil temperature at 10 cm (°C)
Rainfall (mm)
Sunshine (h)
Wind speed (m.p.h.)
Rate of evaporation
Relative humidity
18·23 ± 0·40
7·76 ± 0·42
14·85 ± 0·34
1·95 ± 0·34
5·85 ± 0·40
8·50 ± 0·45
2·60 ± 0·16
75·93 ± 1·09
17·52 ± 0·40
8·34 ± 0·35
14·22 ± 0·37
2·17 ± 0·60
4·83 ± 0·36
8·35 ± 0·39
2·32 ± 0·16
78·41 ± 0·99
1·47
1·07
1·55
0·10
3·65
0·07
1·63
2·83
0·227
0·302
0·214
0·747
0·057
0·795
0·203
0·094
1997 (mean number of total potential prey captured
per web site = 3·12 ± 0·22, non-web site = 2·38 ± 0·20)
with a data set for the same calendar period during
1998 (mean number per web site = 5·69 ± 0·50, non-web
site = 3·95 ± 0·27), the mean number of arthropods
captured during the second year was significantly greater
(F1,476 = 20·55, P < 0·001).
-   
Despite the large variation in the numbers of arthropods captured between years, there was no evidence to
suggest that meteorological factors were responsible
for such variability. Comparisons of data from 1997
and 1998 indicated that no category of meteorological
data varied significantly in the 4 weeks prior to sampling, or during the months in which the study was
undertaken (Table 3).
Discussion
© 2001 British
Ecological Society,
Journal of Applied
Ecology, 38,
88–99
Quadrats (Bilsing 1920; Edgar 1969) and sticky traps
(Kajak 1965; Sunderland, Fraser & Dixon 1986b;
Riechert 1991; Bradley 1993; Gillespie & Tabashnik
1994; Marshall 1997) have been used previously to
monitor the potential prey of spiders, but very few of
these studies were in agricultural habitats. This appears
to be the first study where both methods were applied
simultaneously to individual web sites to obtain a
balanced and spatially precise assessment of the
availability of potential prey. Overall, it revealed that
Collembola were the most abundant group of potential
prey. Biomass of prey items was not recorded in this
study, but it is likely that Diptera, Hymenoptera,
Coleoptera and Aphididae would increase in relative
importance if biomass were to be taken into account.
Densities of potential prey (per cm2) were usually greater
on mini-sticky traps than in mini-quadrats. This is, in
part, because the latter offer an instantaneous measure
of prey availability, whereas the former represent an
availability rate (per 24 h in this case). It is, however,
also likely that the cumulative differences are related
to diel cycles in availability of prey to spiders in webs
on and near the ground (Vickerman & Sunderland
1975; Leathwick & Winterbourn 1984; Sunderland
1996). Nocturnal prey availability could be an
important aspect of the trophic biology of agricultural
linyphiids because many species are particularly active
at night (Thornhill 1983; De Keer & Maelfait 1987b;
Alderweireldt 1994b). The ratio of aerial species,
such as Diptera and Hymenoptera, sampled by ministicky traps in relation to mini-quadrats was particularly high, indicating the likelihood of underestimating
the presence of such species if quadrat sampling is used
alone. The ratio of Aphididae captured by mini-sticky
traps in relation to quadrats was also relatively high,
probably due to the presence of alate aphids descending into the crop, and the high frequency with which
aphids are known to fall from the crop (Sunderland,
Fraser & Dixon 1986b; Sopp, Sunderland & Coombes
1987). It is clear, therefore, that use of daytime miniquadrats alone would have seriously underestimated
the abundance and diversity of prey available to spiders.
There was considerable temporal variation in prey
availability. Less than four prey items were available per
mini-sticky trap per day (c. 0·5 cm–2) in web sites during
May and July. This is approximately equivalent to four
items per day entering the 8-cm2 web of an adult female
Erigoninae spider (e.g. E. atra). If the 74-cm2 webs of
adult female Linyphiinae spiders (e.g. L. tenuis) were
on the ground they would receive a mean of 37 items
per day, but as they are located up to 10 cm above
ground they will receive only falling prey, and therefore
fail to benefit from the traffic of ground-active prey.
This does not necessarily mean that these linyphiids
would receive less than this number of prey items,
because their location may allow them to intercept and
capture more flying species than they would at ground
level. Solid horizontal sticky traps at 10 cm above
ground caught half as many potential prey per unit
area as traps placed on the ground (Sunderland, Fraser
& Dixon 1986a), but solid traps might not be appropriate in an aerial location. Immature Linyphiidae,
which constitute the vast majority of spiders during the
growing season (Sunderland & Topping 1993; Topping
& Sunderland 1998), have much smaller webs (16 cm2
for Linyphiinae and 3 cm2 for Erigoninae; Sunderland,
Fraser & Dixon 1986a) and so will receive commensurately fewer prey. In laboratory studies, immature
E. atra that received approximately one collembolan per
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J.D. Harwood,
K.D. Sunderland &
W.O.C. Symondson
© 2001 British
Ecological Society,
Journal of Applied
Ecology, 38,
88–99
day at 20 °C had markedly slower development rates
than those that received an ad libitum supply (De Keer
& Maelfait 1988). Immature E. atra in the current
study are estimated to have received approximately 1·5
prey items per day in May and July, and so their development is likely to have been food-limited. De Keer &
Maelfait (1988) also showed that adult female E. atra
receiving less that three adult Drosophila melanogaster
Meigen (Diptera) per day at 20 °C produced fewer eggs
than those provisioned at higher rates. In May and July,
adult female E. atra in our study would have received
about four prey items per day, which were mostly small
Collembola. The biomass of prey available was probably
therefore less than the equivalent of three Drosophila
and reproduction is likely to have been suboptimal. The
situation was different in June, when prey availability
increased to 12 prey items per trap per day (c. 1·6 cm–2).
The seasonal pattern of prey availability recorded in
this study matches the seasonal pattern of hunger
assayed by Bilde & Toft (1998) for the linyphiid spider
Oedothorax apicatus (Blackwall) in a Danish winter
wheat field. They recorded hunger equivalent to 7 days
of starvation at 20 °C in May and July, but a value of 3–
4 days starvation during June. These results suggest
that spiders are unlikely to exhibit prey preferences in
May and July, when they may need to capture whatever
they encounter in order to survive. In June, however,
when prey availability is higher, it is possible that they
will reject cereal aphids (which were found to be lesspreferred prey in laboratory trials; Toft 1995; Beck &
Toft 2000) in favour of alternative prey.
The number of potential prey caught per trap during
a 4-week period was significantly greater in 1998 than
in 1997 and this difference did not appear to be directly
due to weather. However, different fields were investigated in the two years and so interfield variability may
explain the observed differences. A full study of interfield variation of potential prey availability (using the
methods developed here) would be worthwhile, as it
might point to farming practices that are favourable
and others that are deleterious in relation to fostering
the alternative prey of generalist predators. This could
include a comparison of winter and spring cereals,
as plant taxa that are important food resources for
arthropod herbivores (= potential prey for generalist
predators) occur at greater densities in spring cereals
(Hald 1999).
The linyphiid spiders at our study sites were not
locating their webs randomly with respect to the spatial
distribution of their food. This was true for all fields
and sampling methods in 1997 and 1998. More potential prey were available in the web sites of spiders than
in random non-web sites located up to 30 cm away
from the web. We do not have information at this stage
to determine whether Linyphiidae are responding
directly to food availability or whether their aggregation in prey-rich patches is a secondary outcome of a
response to other microhabitat variables such as high
humidity and structural complexity of the vegetation.
Further studies would be useful, matching web and
non-web sites within crop rows and within spaces between
rows, for example. It is possible that linyphiids were
simply responding passively to a lack of prey, relocating at random until they reached areas of high prey
availability, where further movements were arrested. In
fact, if Linyphiidae were selecting sites on the basis of
quality, the results suggest that their efficiency in selecting high-quality web sites improved with progression
of season. Our results justify a full study of the determinants of microhabitat selection by linyphiid spiders
in winter wheat. There are examples in the literature of
microhabitat selection being primarily driven by food, or
microclimate, or physical structure of the microhabitat,
or avoidance of conspecifics and enemies; the prime
determinant varies with habitat and species of spider
(Samu & Sunderland 1999; Sunderland & Samu 2000).
The fine-grain variation in the distribution of linyphiids
and their prey described in this study indicates that
mini-sampling techniques can be valuable for investigating spatial dynamics and predator–prey interactions
in agro-ecosystems, especially for small ‘sit-and-wait’
strategist predators. There are a growing number of
studies of the spatial dynamics of predators and prey in
agro-ecosystems that employ nested sampling grids at
a range of degrees of resolution, suitable for analysis by
new techniques such as Spatial Analysis by Distribution
Indices (Perry 1995, 1998). For large, highly mobile,
carabid beetles, grid scales ranging from 16 m down
to 0·25 m have been demonstrated to be appropriate
(Bohan et al. 2000). Our study suggests that for smaller,
less-mobile, predators important information will be
lost unless finer scales are also included. Therefore, the
scale should be chosen according to the species being
investigated.
Numbers of both spiders and Entomobryidae
(Collembola), captured by mini-sticky traps and
mini-quadrats, increased logarithmically as the season
progressed. At the same time, trap capture frequencies
of isotomid Collembola (Fig. 2c,d) showed a sharp
decline in July, when spider activity density was at a
maximum. While it is possible that predation by spiders
(and other natural enemies) was affecting population
densities of isotomid Collembola (see below) this does
not explain why capture frequencies of the Entomobryidae continued to increase. One possible explanation is
that the Entomobryidae have powerful protection
mechanisms against predators (Bauer & Pfeiffer 1991).
Figure 3 clearly indicates that entomobryid activity
must have been very low, with relatively few encountering
webs, because the cumulative capture rates (mini-sticky
traps) over time were not significantly different from
the instantaneous densities (mini-quadrats). This too
could contribute to lower levels of predation. Alternatively, Entomobryidae could be a non-preferred
prey item, as the nutritive value of different species
of Collembola is known to vary ( Toft & Nielsen 1997).
However, a number of other factors could be responsible
for the differential trends in capture rates for the
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© 2001 British
Ecological Society,
Journal of Applied
Ecology, 38,
88–99
different groups of Collembola, including responses
to changing climatic conditions, or some species of
Collembola locating to different microhabitats such
as the large cracks which appear in the soil late in the
season. The negative relationship between numbers of
spiders and Collembola captured by mini-sticky traps
indicates that, as Collembola density decreases, spider
activity density increases. Increases in predator activity,
in response to reduced prey availability, have been found
amongst other generalist predators such as carabid beetles
(Chiverton 1984). However, increasing spider density
may also be a factor. Although it is possible that the
associations between Collembola and linyphiid numbers
were in part a result of predation, quantitative immunological studies would be needed to confirm such a link in
the field (Symondson et al. 1996; Bohan et al. 2000).
A surprisingly large number of linyphiid spiders was
captured on the mini-sticky traps. These were not the
original web owners returning to the web site because
we removed and killed them as a routine part of the
sampling protocol. Peak linyphiid densities (including
immatures) in winter wheat are typically about 100 m–2
(Dinter & Poehling 1992; Sunderland & Topping 1993;
Volkmar et al. 1994), which is equal to one spider per
100 cm2, or 0·08 spiders per trap. The actual capture
rate per trap peaked at 0·6 linyphiids day–1 (web sites
in July), which implies that either these spiders are
extremely active throughout the field, or their activity
is lower and concentrated in areas that they find attractive. Support for the latter hypothesis comes from the
fact that peak capture rate on non-web mini-sticky traps
was only 0·3 trap–1 day–1. Thus, the sampling programme
has detected the aggregated and dynamic nature of
spider distribution in winter wheat. The dynamic aspect
is likely to be due to males searching for females, and
females searching for new web sites. Male linyphiids are
known to be, in general, more active than females, e.g.
sex ratios in pitfall traps are strongly biased towards
males (Sunderland 1987; Topping & Sunderland 1992),
and they can be attracted to sex pheromones secreted
onto web silk by the female (Watson 1986). Adult female
linyphiids, such as L. tenuis, compete strongly for
desirable web sites, a larger female being able to evict a
smaller one. Web site abandonment from this and other
causes results in the mean duration of an individual
L. tenuis in a web site being less than 2 days (Samu et al.
1996). These mechanisms probably contributed to the
high capture rate on mini-sticky traps and are also
likely to have caused a fairly high encounter rate between
individual spiders. Although, in general, spiders are
highly cannibalistic (Edgar 1969; Wise 1993), it is known
that web-weavers (including Linyphiidae) are much
less so than hunters such as Lycosidae, Salticidae and
Thomisidae (Nyffeler 1999), and so the encounters
implied by the results from this study probably did not
result in high rates of mortality. We suggest, however,
that mini-sticky traps could, more generally, be a useful
tool for quantitative arachnological investigations of
intra- and interspecific interactions.
This study has shown that money spiders locate their
webs in prey-rich areas of winter wheat fields. In spite
of this behaviour, which enables them to maximize
their access to food, it seems likely that prey availability
in May and July is not sufficiently great for them to
realize fully their potential population growth rate.
Studies are needed to find practical modifications to
current farming practice that will achieve enhanced
prey availability, especially at times of year when
prey densities are low. The mini-sampling techniques
developed here, especially when used in combination,
will enable prey availability to be monitored more
efficiently in such prey-enhancement studies.
There is no certainty, however, that increased prey
and spider density will always translate into improved
pest control (Sunderland & Samu 2000). Indeed, the
increased availability of high-quality alternative food
could, to some extent, divert spiders from feeding on
pests. Clearly, there is a need for manipulative field
studies to investigate rigorously the interactions between
spiders, pests and alternative prey before sound recommendations can be made to optimize the spider
predation component of pest control.
Acknowledgements
We are very grateful to John Fenlon (HRI) for statistical
advice, Sally Mann (HRI) for meteorological data, and
Jørgen Axelsen (National Environmental Research
Institute, Silkeborg, Denmark) for Collembola identification. J.D. Harwood was funded by a joint Cardiff
University and HRI studentship, and K.D. Sunderland
by the UK Ministry of Agriculture Fisheries and Food.
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