Abstract
Agricultural organisms, such as insect herbivores, provide unique opportunities for studies of adaptive evolutionary processes, including effects of insecticides on movement and oviposition behavior. In this study, Brassica leaves were treated with one of two non-systemic insecticides and exposed to two individual strains (referred to as single or double resistance) of diamondback moth (Plutella xylostella) (DBM) exhibiting physiological resistance. Behavioral responses by these two strains were compared as part of characterizing the relative effect of levels of physiological resistance on the likelihood of insects showing signs of behavioral avoidance. For each DBM strain, we used choice bioassays to quantify two possible types of behavioral avoidance: 1) females ovipositing predominantly on leaf surfaces without insecticides, and 2) larvae avoiding insecticide-treated leaf surfaces. In three-choice bioassays (leaves with no pesticide, 50% coverage with pesticide, or 100% coverage with pesticide), females from the single resistance DBM strain laid significantly more eggs on water treated leaves compared to leaves with 100% insecticide coverage (both gamma-cyhalothrin and spinetoram). Females from the double resistance DBM strain also laid significantly more eggs on water treated leaves compared to leaves with 100% gamma-cyhalothrin, while moths did not adjust their oviposition behavior in response to spinetoram. Larvae from the single resistance DBM strain showed a significant increase in mobility in response to both insecticides and avoided insecticide-treated portions of leaves when given a choice. On the other hand, DBM larvae from the double resistance strain showed a significant decrease in mobility in response to insecticides, and they did not avoid insecticide-treated portions of leaves when given a choice. Our results suggest that pest populations with physiological resistance may show behavioral avoidance, as resistant females avoided oviposition on leaves without gamma-cyhalothrin. Thus, physiological resistance and behavioral avoidance do not appear to be controlled by the same selection pressures, and the mechanisms responsible for behavioral avoidance may vary among life stages. Our analysis also suggested that a population with lesser physiological resistance to insecticides may be under a stronger selection pressure and therefore be more likely to develop avoidance behaviors than a population with higher levels of physiological resistance.
Highlights
Agricultural cropping systems provide unique opportunities for studies of behavioral adaptations and evolutionary processes, because: 1) crop management practices impose strong selection pressures, 2) a wide range of environmental variables can be experimentally controlled, 3) replicated field conditions at considerable spatial scales are fairly easy to establish, and 4) interactions among competing species and among species on different trophic levels can be manipulated and studied in detail
From resistance bioassays with the three DBM strains exposed to spinetoram, we found that (Fig 1b): 1) resistance levels in the Waite reference and single resistance strains were very similar and about 20 times lower than in the double resistance strain, and 2) the application rate of 360 mg/L was considered high enough for all DBM third instar larvae to be susceptible to spinetoram treatments
There were no significant effects of DBM strain (DF = 1,39, F = 1.85, P = 0.18) or active ingredient (DF = 1,39, F = 0.09, P = 0.77) on total oviposition, when DBM adults were presented with a choice of three Chinese cabbage leaves subjected to different treatments [water only (0%), distal portion of leaf (50%), and complete insecticide coverage (100%)] (Fig 2a)
Summary
Agricultural cropping systems provide unique opportunities for studies of behavioral adaptations and evolutionary processes, because: 1) crop management practices impose strong selection pressures, 2) a wide range of environmental variables can be experimentally controlled, 3) replicated field conditions at considerable spatial scales are fairly easy to establish, and 4) interactions among competing species and among species on different trophic levels can be manipulated and studied in detail. Regarding agricultural herbivores, another important aspect of adaptations and evolutionary processes is that organisms are represented by wide ranges of diets and feeding guilds [1, 2]. The overall outcome of such physiological and behavioral adaptations to selection pressures is a perceived “resistance” of the target species to a given pesticide
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