Assessing the Role of Associational Susceptibility in Declines of a Native Shrub
Sarah O’Neill, Rick Redak, Matt Daugherty
Department of Entomology, University of California, Riverside
• The annual forb Brassica tournefortii (Sahara mustard) is highly invasive in portions
of California’s deserts
• Dominant stands of mustard are associated with declines in native annual plants and
select arthropod and vertebrate taxa
• Anecdotal observations suggest mustard is also associated with declines of the
perennial native shrub Atriplex canescens (Four-winged saltbush)
• In 2010, the most recent wet year in the region, extensive early season recruitment
of Br. tournefortii led to high population densities of the invasive stinkbug Bagrada
hilaris, which prefers brassicaceous plants
• As the mustard senesced, B. hilaris moved en mass onto nearby Atriplex
• The following season, an unusually large number of Atriplex were dead in areas
where mustard had been prevalent
• Is mustard increasing herbivory on Atriplex?
Study organisms: Bagrada hilaris, Atriplex canescens, Brassica tournefortii.
Objectives:
We characterized the relative importance of associational susceptibility of a native
shrub, as mediated by an invasive plant and herbivore, via a suite of greenhouse and
field experiments:
Obj 1: Assess Bagrada’s relative preference for Atriplex versus Brassica
Figure 6. Bagrada
performance on Atriplex
versus Brassica.
0
• Bagrada failed to complete their lifecyle on just Atriplex
• Pairs of Bagrada produced on average 100 new adults on Brassica and none
on Atriplex
Obj 2: Quantify Bagrada’s relative performance on Atriplex versus Brassica
Obj 3: Estimate the effect of Brassica on spillover herbivory by Bagrada hilaris
Objective 3: Does mustard encourage herbivore spillover?
2x2x2m field cages with a factorial combination of 0, 10, or 100 Brassica
plants, 0 or 50 initial Bagrada adults, and 1 Atriplex plant
Objective 1: Herbivore plant preference
1a. Herbivore choice. Greenhouse behavioral bioassays with individual adults given
the choice between mustard, at one of 3 stages (basal rosette, recently bolted, or
senescent), and Atriplex
Figure 4. Total # of
Bagrada found on lone
Atriplex, Atriplex next
to mustard, or mustard
next to Atriplex..
Cages were censused regularly for more than 2 days - first choice and proportion of
censuses on each plant were recorded
Censused weekly for # of Bagrada and damage to Atriplex
Figure 7. Individual Atriplex canescens planted in cages with one of three densities of
Brassica tournefortii: a) 0, (b) 10, or c) 100 Brassica.
• Bagrada much more commonly found on Brassica
• Bagrada only found on Atriplex when Brassica was nearby
Figure 1. Trials were
conducted in bugdorms with
both plants and one adult Figure 2. Results from choice trials, measured via a proxy for
Bagrada hilaris.
relative time Bagrada spent on Brassica or Atriplex.
• Bagrada preferred 4 to 6-fold Brassica over Atriplex, unless the mustard was strongly
senescent
1b. Herbivore aggregation. Field experiment with Atriplex deployed alone or adjacent
to Brassica plants
Objective 2: Herbivore performance
Figure 8. Bagrada a)
spillover onto Atriplex
and b) damage to
Atriplex
No-choice greenhouse bioassays with pairs of mated adult Bagrada confined on
Brassica or Atriplex
Plants censused regularly to estimate:
1. Population growth (# of new adults produced)
2. Development time to adulthood
3. Adult longevity
Plants were inspected daily and the number of Bagrada were recorded and removed
• Brassica increases
the frequency of
Bagrada on Atriplex
• Increased damage
on Atriplex in the
presence of Brassica
Conclusions
Figure 3. Bagrada hilaris
aggregation trial: (a) lone
Atriplex, and (b) Atriplex
with Brassica neighbors.
Bagrada strongly prefers Brassica over Atriplex, which is a poor quality host
Figure 5. Bagrada performance (population growth, development time, adult longevity) when
confined on Atriplex or Brassica.
Brassica presence encourages herbivore spillover and damage on Atriplex
-sufficient for mortality?