Isolated females and limited males: evolution of insect resistance in structured landscapes

Abstract

To delay evolution of insect resistance to insecticidal Bacillus thuringiensis Berliner (Bt) transgenic crops, USA and Canadian maize growers commonly set aside a portion of each field as a refuge, where susceptible pests can develop without exposure to the toxin(s) expressed in the insecticidal crop. Abundant mate-seeking refuge adults are expected to move into insecticidal crop areas and mate with rare, resistant insects. Production of heterozygous offspring, rather than homozygous offspring reduces the rate of resistance evolution. Expectations about the refuge strategy are predicated on assumptions about when and where pest insects move and mate. Accumulating evidence indicates that the behavior of western corn rootworm beetles, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) (WCR), in refuge and corn rootworm (CRW)-active Bt-transgenic maize does not always conform to assumptions about random mating and individual movement. Contrary to expectations, up to 23.6% of unmated refuge females left refuge and were non-teneral by the time they mated in CRW-active maize. Consequences of protandry, limited male mating capacity, premating male and female movement patterns, skewed sex ratios, and delayed adult emergence from CRW-active maize all may contribute to unexpected or undesirable patterns of WCR reproductive behavior. Modeling suggests that temporal and spatial components of WCR mating that limit interactions in block refuges can be reduced by deploying blended refuges.

Measuring rootworm refuge function: Diabrotica virgifera virgifera emergence and mating in seed blend and strip refuges for Bacillus thuringiensis (Bt) maize

“…Dispersal is a key determinant of mate selection in WCR. Typically, males will not travel unnecessarily to find mates, and females will mate near emergence sites . In our 20% strip refuges, few beetles from refuge plants moved four to ten rows from refuge borders; thus, Bt and refuge beetles mixed almost exclusively at refuge boundaries.…”

“…There were more days between the time at which adults first emerged and the time at which the peak mating occurred in strip refuges compared with seed blends. It is possible that the longer period between emergence and peak mating disproportionately limited available mates both early and late in the emergence curve because: (i) males complete sexual development post‐emergence, (ii) males lose mating ability as they age, and (iii) females typically mate within hours of emergence; the latter is supported by the very high percentages of mated females in our field experiments.…”

Effects of adult emergence timing on susceptibility and fitness of Cry3Bb1‐resistant western corn rootworms

“…) and dispersal (immigration/emigration) taking place within and among fields (Spencer et al. , ; Hughson and Spencer ) which could alter population level fitness over time. The field Cuming‐1 had been planted to a single trait Cry3Bb1 hybrid from 2007–2011 (Wangila et al.…”

Male Reproductive Competition and Components of Female Fitness in Relation to Body Size in Northern Corn Rootworm (Coleoptera: Chrysomelidae)

“…Although not always accurate (Gwynne 1989), it is widely assumed that reproductive activity increases the risk of predation (Magnhagen 1991). Because of the relatively long duration of D. barberi copulation and the probability that most D. barberi females, like most D. v. virgifera females (Quiring and Timmins 1990, Hammack 1995, Spencer et al 2013, mate while teneral before their exoskeleton is fully sclerotized, the vulnerability of beetles to predation could be increased during copulation. Although corn rootworms in general are considered to have few natural enemies (Levine and Oloumi-Sadeghi 1991), and there is relatively little information speciÞcally regarding the natural enemies of D. barberi adults or the relative susceptibility to predation of solitary beetles compared with copulating pairs, Bollinger and Caslick (1985) reported signiÞcant beetle predation by red-winged blackbirds in maize Þelds before the maize reached milk-stage, when half the gut contents of the birds consisted of D. barberi beetles.…”