DISCUSSION
Sucking pests
Whiteflies
Jambhunkar et al. (1998) found that intercropping cotton with legumes
reduced incidence of B.tabaci.
Balasubramanyam et al. (1998) also reported low
incidence of sucking pests such as whiteflies, leaf hoppers, aphids and thrips in cotton
intercropped with cluster beans or green gram. The results of two years study revealed
that whitefly populations were always high in cotton monocrop compared to cotton
intercropping systems.
Particularly cotton intercropped with soybean consistently
showed lesser incidence of whiteflies which corroborates with the findings of Sheshadri
and Natarajan (1998).
Among the intercrop combination cotton with black gram attracted more
whiteflies compared to red gram or green gram intercropping. Hirano et al. (1995)
pointed out the population dynamics of whiteflies B.tabaci was influenced by temporal
variations in quantity and quality of host plants in the area. Intercropping systems in
cotton which broughts such variations resulted in differential levels of infestation.
Balasubramanian et al. (1998) also found the among six intercrops green gram was more
effective in suppressing early season sucking pests than black gram. Venkatesan et al.
(1987) also reported whitefly incidence was higher in cotton intercropped with black
gram than in green gram. The population behaviour of whitefly showed higher numbers
of peeks in monocrop than in intercropped with cotton. It was interesting to note that in
cotton with soybean intercrop the population of whiteflies remained stable. Activity of
whiteflies was high during high temperature and humidity and declined during winter as
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reported by Horowitz (1986). Though unlike aphids for instance, whitefly infestation can
lost throughout the whole cropping season.
Thrips
Thrips appeared early in the season and continued till the break of winter
in the first of week of November. Thrips were significantly less in cotton + soybean
compared to cotton + green gram, red gram or black gram. Balasubramanian et al.
(1998) also reported lesser incidence of thrips in cotton + green gram. In soybean
intercropping there was an abrupt crash of thrips population compared to continued
infestation in other intercrop situations. Thrips (Megalothrips sp.) attack is more on
green gram and attracted higher incidence of natural enemies of thrips (predatory thrips,
lady birds etc) which migrated to main crop of cotton and resulted reduction of thrips in
cotton. Sreekanth et al. (2002) also reported that thrips cause at least 40% yield loss in
green gram. It was observed that thrips were unusually high in cotton intercropped with
red gram. Intercropping red gram with cotton was found to be unfavourable (AICIP –
2007) though many workers reported that with short duration pegionpea as intercrop,
thrips damage was much reduced in cotton.
Den Belder et al. (2000) reported increased damage by western flower
thrips in chrysanthemum intercropped with subterranean clover.
Increasing thrips
population in cotton + red gram with intercropping may be congenial micro climate
offered by long duration pigeonpea utilized in the experiment. High humidity positively
correlated with increased incidence of thrips in cotton Khan et al. (2000). Though
natural enemy complex was found to be higher in cotton + red gram, it was not exercising
any influence on the thrips population in cotton. The predators found on red gram were
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found to be more aphidophagous insects where as in soybean intercropping thrips
population remained very low all through the cropping season as observed by
Venugopala Rao et al. (1995).
Jassids
Jassids are early season sucking pests. The incidence was observed late
August and terminated in first week of November in 2004 and two weeks later in 2005.
Jassids were found at least 16 – 50% higher in sole crop of cotton than various
intercropping situations. Cotton + soybean intercropping consistently showed significant
lower infestation levels compared to other intercrop combinations. There was not much
variations in population rise in different intercropping situation as observed for thrips and
whiteflies. The findings were in conjunction the results obtained by Venkatesan et al .
(1987) where in significantly less population of the leaf hopper was observed in cotton +
green gram and cotton + black gram. Balasubramanian et al. (1998) also observed
similar effect in cotton + cluster bean. Swaminathan et al. (2002) also postulated crop
diversity approach to manage cotton leaf hopper. Aswathanarayana Reddy et al. (2006)
also obtained similar results with intercropping of irrigated chillies with onion, marry
gold, maize and beans. Hormachan et al. (2009) in Thailand used okra, sunflower, castor
as trap crops to decrease leaf hopper populations in cotton. They used non-host plants as
intercrops and host plants as trap crops for leaf hopper. In our study all the intercrops
were non-host plants (Sharma and Singh 2002) because of non-host plants of the leaf
hopper intercropped with cotton the reduction over monocrop was greater. In yet another
experimentation of intercropping affects on sucking pests population. Tingey and Lamont
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(1998) showed that Empoasca fabae and Aphis fabae were significantly less in field
beans intercropped with winter wheat compared to sole crop of bean.
Aphis gossypii
In 2004 a initial colonization of aphid population was noticed in August.
Reduction in percent infested plants of cotton in different intercropping situations was 19
– 66% over monocrop. During September – October the reductions were much lesser
over previous observations which shows adaptability of aphid population to intercropped
habitat except in cotton + green gram where in reduction increased to 20%. In 2005 no
such non preference to green gram intercropping was observed. The initial colonization
appeared in August and terminated in October and again reappeared in December first
week. It was also observed by Rathod and Bapodra (2004) that A.gossypii populations
initialize infestation of young cotton plants in India during July and activity peaks reach
at last week of November and continue till December. Cotton + soybean consistently
showed lower percentage of plants infested by the aphid. However, cotton + black gram
the infestation continued at a level equal to monocrop situation. Viera et al. (1983) also
reported that when cotton was intercropped with maize or cowpea (Vigna unguiaculata)
the infestation of A. gossypii was 14 times less than in monoculture. Chamune et al.
(2007) also observed the same effect in cotton intercropped with pigeonpea and crotalaria
area orchroluca, further they observed the population of natural enemies were abundant.
Jackson (2005) in his experiments in organic farming in cotton in Kutch,
Gujarat (India) observed that cotton aphids appear early in August on young cotton plants
and reduce to insignificant levels and when aboitic / biotic conditions favour, they
buildup to damaging levels by last week of November or early December, which was also
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observed in our experiment. Mohapatra (2008) also made similar observation in Orissa
(India) with major sucking pests of cotton namely leaf hoppers, aphids and whiteflies
with three peeks in August – September, October and November last week respectively.
Flint and Roberts (1998) in their review of cotton cultivation of moderate and small scale
growers in California (USA) concluded that intercropping cotton with alfalfa and oats as
companion plants reduced pest infestations. Girma et al. (2004) in Kenya (Africa)
reported such reduction aphid, R.maidis infestation in maize associated with certain
insectary plants as hedgerows. Balasubramanian et al. (1998) in a similar study of
intercropping effects in cotton on sucking pest incidence showed that cotton + cowpea
and cotton + green gram had significantly reduced incidence of aphids, thrips, leaf
hoppers and whiteflies than in monocrop. Rathod and Bapodra (2004) in Rajasthan
(India) monitored aphid infestation on cotton which showed initiation of infestation from
July onwards in remain highly active from October to December which corroborates our
findings. Parajulee and Slosser (1999) in their study on utilization of relay strip crops
viz., canola, Vicia villosa and wheat observed effects of strip cropping on aphid incidence
was significant in cotton. Weekly buildup of aphid, Lipaphis erysimi in mustard was
significantly reduce by intercrops like garlic or onion (Sarker et al. 2007).
It is also important that adequate host population has to be present for aphid
predator activity particularly the lady beetles, Phoofolo et al. (2010) observed when relay
intercrops in sorghum, like winter wheat, alfalfa and cotton were unable to attract and
retain high population of lady beetles while in main crop sorghum with higher infestation
of aphids resulted in more retenction of lady beetles. In our study the higher levels of
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coccinellid activity in monocrop of cotton compared to intercrop cotton at certain times
may be attributed to continuation of aphid infestation while it was terminated in
intercropped situations.
Lepidopterous pests
H . armigera
H.armigera populations initiated in October in both the years and
continued till January last week.
Significant population reduction with respect H.
armigera throughout the two years study, while increased levels of infestation at par with
monocrop of cotton were noticed in case of cotton + red gram consistently. The rate of
increase of infestation levels in intercropped cotton was noticed during January due to
harvesting of the intercrops except in case of red gram. Though there was consistency in
population reduction from initiation to termination in cotton + soybean, the other
intercrop situations also were more or less at par with soybean. During pod formation
stage of green gram infestation of H. armigera also increased which might have
contributed higher levels of infestation in cotton + green gram. Green gram pods were
particularly more attractive to H.armigera than soybean or black gram.
Observations of reduction in H.armigera with maize and cowpea as
intercrops in cotton was reported by Amoako et al. (1993). Hedge (2003) observed that
intercropping sorghum with cotton due to predatory activity of lacewings was kept at
below ET levels. Pesticide post (2009) also reported that in central and southern India
intercropping in cotton with black gram, green gram and cowpea diverted population of
both sucking pests and American cotton bollworm H. armigera from cotton and
enhanced parasitism and predation by natural enemies. The increased infestations in
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cotton + red gram intercropping were also observed by Ravi et al. (2005) in cotton
ecosystem associated with pigeonpea (red gram) and chickpea which harboured
significantly greater egg and larval population of H. armigera than cotton. The H
.armigera population reduction with soybean or groundnut as intercrops in cotton was
reported by Venugopala Rao et al. (1995). Wu and Guo (2005) opined that large number
of small scale, family owned farms in China which practiced mixed plantings of Bt
cotton with corn, soybean and groundnut contributed to slower buildup of resistance to Bt
toxin in H. armigera though the Bt cotton was introduced over several years.
Though cotton + red gram traditional intercropping or mixed cropping
practice, it was reported by All India Cotton Implement Project (AICIP) experiments that
red gram contributed to higher incidence of H. armigera in cotton and also repels natural
parasitization by Trichogramma and thus unsuitable choice as intercrop in cotton in
modern cotton cultivation.
Surulivelu (2009) reported that intercropping cotton with
black gram and chillies will reduce the intensity of bollworm infestation in cotton.
Yelshetty et.al (2009) in a validation of integrated pest management modules for
management of pod borers H.armigera in pigeonpea based intercropping found that
intercropping with soybean reduced incidence of H. armigera compared with
intercropping with cotton, groundnut and green gram.
Spodoptera litura
The larval population of Spodoptera litura ranged from 1 – 2.5 larva per
plant in 2004-05 and 1 – 2.8 larva per plant in 2005-06. The infestations appeared from
October onwards in both the years. Cotton + soybean and cotton + red gram contained
lesser infestation than cotton intercropped with black gram or green gram.
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The
populations remained more or less constant without much fluctuations. Red gram is a
non-host plant for Spodoptera litura and that might be the reason for lower incidence of
S.litura in cotton + red gram where as similar effects in cotton + soybean may be
attributed to abundance of natural enemies of S.litura in cotton + soybean. Particularly
higher incidence of S.litura in cotton + black gram is due to the preferential oviposition
on black gram, however all the intercrops situations of cotton in both the years generally
showed lesser incidence of S.litura than monocrop.
Regarding lepidopterous pests, researchers Asman et al. (2001) observed
diamond back moth, Plutella xylostella can be reduced by intercropping in cabbage.
Similarlly, Alteri et al. (1978) recorded 14% reduction of armyworm Spodoptera
frugiperda in maize by poly cultures, which also collaborates our observations with
H.armigera and S.litura infestation reductions in cotton intercropping over sole crop of
cotton.
Insect infestation in intercrops
The insect pest infestation in intercrops when grown separately was on par
with the monocrop of cotton on the natural enemies populations among the intercrops
were not significantly differing. However the population levels were as good as those
observed when the intercrops were grown in companion with cotton.
Natural enemies
Cheilomenes sexmaculata
Coccinellid predators C. sexmaculata populations were monitored at all
their life stages namely egg, larva, pupa and adult. Activity of predator coincided with
the appearance of aphids which started colonization on young cotton plants in August.
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Fortnightly observations were made from August – January. Significantly highest egg
populations ranging from 14 - 50% was recorded in cotton + red gram and cotton +
soybean. Cotton + green gram attracted greater oviposition by the predator than cotton +
black gram. However towards the end of the season in January due to the senescent
period of intercrops more oviposition was noticed in sole crop of cotton. The migratory
nature of lady beetles was shown by Lin et al. (2003) in China when cotton intercropped
with alfalfa which attracted greater number of lady beetles, lacewings and spiders and
when alfalfa was cut to force migration of natural enemies on to cotton. Egg laying by
Coccinella septumpunctata and Menochilus sexmaculata was reported to be 4 – 5 weeks
before infestation of Lipaphis erisimi in mustard (Rana, 2006). Aphid infestation during
2004 in cotton + green gram was noticed only for a fortnight (August 4th week to
September 1st week), in other intercrop situations aphid infestation continued till last
week of September. Egg populations of C.sexmaculata also reached high levels during
September and continued till December second week.
Adult activity, oviposition and aphid infestation showed positive trends
with each other and exhibited favourable response to increasing aphid infestation. Wells
et. al (2001) also reported similarly that densities of Coccinellids in cotton closely tracked
the aphid population. It was also established by Evan and Dixon (1986) in a laboratory
study that cues for oviposition of lady beetles were aphid odours and aphid honey dew.
Larval and pupal population of C.sexmaculata was also found to be higher in cotton +
soybean and cotton + red gram systems, however all the intercropped situations were
better than monocrop as regards to presence of higher number of larvae and pupae of C.
sexmaculata.
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Chrysopa Sp.
The egg larval and adult population of Chrysopa species were
significantly high in cotton intercropped with soybean or red gram. However at certain
times larval populations were equally high in cotton monocrop which may be due to
migration of larvae. Cotton plants it self is highly attractive to Chrysopa Sp. due to the
host plant released synomones like caryophyllene and aphid honey dew containing indole
acitic acid. Hedge et.al (2003) found similar response of Chrysoperla carnea due to
intercropping grain sorghum in cotton. Hanumantharaya et al. (2008) also found that
intercrop of lucrene in 1:1 ratio increased the effectiveness of C. carnea.
The
observations of Lin et al. (2003) also corroborate our findings that intercropping cotton
with legumes increase the abundance of the lacewing. Kranthi and Russel (2009) in their
review of changing trends in cotton pest management mention about the lady beetles and
lacewings as important predators in cotton intercropped with different pulse crops.
Spiders
In cotton eco system 21 species of spiders grouped under 16 genera
belonging to 8 families were reported and the predatory potential of the spider Paradosa
viridianum was maximum on sucking pests in cotton (Rajeswaran et al. 2005). Spiders
are generalist predators and essential in community balance of the cotton insect eco
system. In all cotton intercropping systems spiders were more abundant than in cotton
raised as monocrop. They were significantly high in cotton intercropped with soybean
followed by red gram, compared to other intercrops. Balasubramanian et al. (1998)
observed higher incidence of spiders in cluster bean intercropped in cotton. In maize
cultivation Brust et. al. (1986) reported higher population of spiders and carabids in
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intercropped situations than monocultures. Mensah (1999) quantified that 2.1, 2.5 and
1.2 times abundance of predatory beetles, bugs, lacewings and spiders per meter row in
intercropped cotton compared to monocultures. The species composition of spiders was
rich in contrast with monocrop.
Geocorids
Cotton intercropped with either soybean or red gram contained highest
number of geocorid bug activity.
Cotton in other intercrops was also better than
monocrop in retaining the presence of geocorid bugs in both the years. In western Kenya
Van den Berg and Cock (1995) reported that anthocorids and geocorids are important
mortality factors of H.armigera in cotton. It was also shown by Johnson et al. (2006)
that the mere presence of the predator, Geocoris lubra on a plant can have a strong
influence on the movement and behaviour of H. armigera. Predators do more than just
reduce numbers of herbivours, they influence, feeding, displacement and subsequently
the distribution of the pest in the plant canopy.
Soil dwelling insects
Carabids
Ground dwelling carabids (Coleoptera : carabidae) are important in
regulating the soil inhabiting life stages of foliar insects of cotton. Diverse habitats and
diversity of cropping system are proposed as promoters of ground beetle diversity in
intensely managed agricultural crops like cotton, maize and wheat (Liu et al. 2010).
Carabids were found to be highest in cotton + red gram intercropping systems followed
by other cropping modules in both the years of study significantly contained higher
populations than those found in monocrop.
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Millipedes
Millipedes are the macro arthopods important in primary recycling of
plant litter in the form of dead leaves, twigs, flowers and flower buds etc. millipedes
were found to be active during the months of September – November in cotton cropping
systems. In both the years of study they were found to be high in cotton + red gram and
cotton + soybean. The other two cropping systems were at par with monocrop. Brevault
et al. (2009) in their project report similarly mentioned leguminous soil cover harboured
more individuals of particularly earth worms (Lambricidae), millipedes (Julidae),
centipedes (Scolopendridae) and spiders (Arachinidae) and detrivorous millipedes are
mostly numerous in soil cover mulches that provide habitat.
Collembola
Collembolans are the meso arthropod fauna essential in as secondary
trophic level of the detrivourous soil recycling community. They are also designated as
indicators of soil health and eco system sustainability and are known as springtails. In
this study of cotton intercropping system cotton + red gram significantly had the highest
density of collembolans and other cropping modules were at many instances at par with
the monocrop. Crop residues and minimum tillage operations encourage these delicate
members of the soil biota. Poveda et al. (2008) in their review of research papers on crop
diversification in monocultures reported that out of 62 detailed reviews of original studies
of last 10 years showed that diversification practices (a) enhance natural enemies in 52%
(b) reduce pest pressure in 53% (c) increased yield only 32% of the cases.
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Parasitization by Trichogramma sp
Trichogramma species are ubiquitous egg parasites of many lepidopteran
insects besides parasitizing eggs of spiders and members of other insect orders.
T.chilonis in cotton eco system is used as an inundative release biocontrol agent. Cotton
+ soybean intercropping system exhibited highest degree of parasitization by T.chilonis
in the two years observation. In a study of intercropping systems in cotton Hegde et. al
(2003) recorded similar observations in cotton intercropped with sorghum. As was the
experiment of Scholz and Parker (2004) with the same intercropping system. Andow and
Risch (1987) also establish the fact that poly cultures of maize / bean / squash and maize /
clover improved parasitism rates of T.minutum by 1.9 times over monocultures of cotton.
In our study that cotton + pigeonpea system recorded the lowest parasitization rates by
T.chilonis which was reported by many workers that the cotton + pigeonpea eco system
surely inhibited the efficiency of the parasitoid.
Larval parasitization of H. armigera
Larval parasitization of H. armigera was significantly improved with different
intercropping systems over monocrop. Consistently high levels of parasitization was
observed with cotton + soybean intercropping.
Parasitization by Encarsia sp.
Encarsia sp. the aphilinid parasitoids are of world wide distribution
parasitizing many species of whitefly pupae and are mass multiplied for inundative
releases. The obseravations on encarsia parasitization of whitefly pupae revealed that
cotton + soybean with highest rates of 4-5% parasitization was better than other cropping
modules. Deguine (2007) in their review of sustainable pest management for cotton
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production reported the use of wheat as intercropped with cotton in China which
increased the abundance of natural enemies of sucking pests including encarsia sp. these
reports collaborate our observations that significant increase in parasitization in cotton
was the result of intercropping.
Parasitization by Aphilinus sp
Aphid parasitoids like Aphilinus and Aphidius in conjunction with the
community of aphidophagous predators limit aphid populations below ETLs.
The
intercropping system practices in this study namely cotton + soybean and cotton + red
gram improved the parasitization of aphids in cotton by Aphilinus over the monocrop.
Particularly cotton + soybean exhibited 3-4 times improvement in parasitization over
monocrop. Intercropping systems in cotton were reported to the encouraging for the
activity of aphid parasitoids (Deguine et al. 2008), which are inconjunction with our
findings.
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