Abstract
Dropping behavior is an effective antipredator defense utilized by many insects including aphids, which drop from plants to lower plant parts or underlying substrates to avoid attack from predatory invertebrates. While research commonly focusses on triggers of dropping, less attention is given to what happens to prey individuals following escape drops. In this study, the duration of tonic immobility, recovery rates, and cases of “instant recovery” (re-clinging to lower plant parts) exhibited by potato aphids (Macrosiphum euphorbiae) that dropped from potted seedlings in response to introduced ladybird (Adalia bipunctata) adults, lacewing (Chrysoperla carnea) larvae, and a standardized tactile stimulus were investigated in relation to a range of environmental factors. Air temperature had a negative correlation with the duration of post-dropping tonic immobility; as temperature increased, time spent motionless decreased. Aphids also showed a pattern of increased recovery rate at higher temperatures. Aphids may be selected to move off the substrate quicker to avoid risks of overheating/desiccation at higher temperatures; and/or higher body temperature facilitates locomotion. Stimulus type also influenced recovery rate back to the original seedling, with aphids generally recovering after the standardized stimulus quicker than after dropping triggered by a real predator. Considering cases of instant recovery onto lower-reaches of the host seedling, seedling height influenced the likelihood of re-clinging, with aphids that managed to instantly recover dropping from, on average, taller seedlings than aphids that dropped to the substrate. Plant architecture could mitigate the costs of dropping for aphids, but further studies quantifying understory foliage cover are needed.
Highlights
Dropping is an important antipredator behavior, wherein a prey individual releases itself from a substrate, passively or actively, such that gravity, wind or water currents power its subsequent escape from a perceived threat
For all analyses concerning Tonic immobility (TI), cases of instant recovery and cases where aphids fell off plant pots entirely were excluded
Comparing the mean durations of TI, no significant difference was found between aphids that dropped in response to ladybirds and those that dropped in response to lacewings (Welch twosample t-test: t = 1.8, df = 54.9, P = 0.078)
Summary
Dropping is an important antipredator behavior, wherein a prey individual releases itself from a substrate (or ceases powered flight), passively or actively, such that gravity, wind or water currents power its subsequent escape from a perceived threat (reviewed in Humphreys and Ruxton 2019). When an insect drops from a plant onto the underlying substrate to avoid predation, it is exposed to many costs and risks associated with being off the plant, such as: lost feeding time, risk of desiccation, and potential exposure to new predators (Losey and Denno 1998a, b; Nelson 2007; Agabiti et al 2016; Humphreys and Ruxton 2019). Previous studies have observed insects behaving in ways that prolong the time spent off plants after dropping (see later). This exploratory study aims to investigate under-explored links between dropping and post-dropping behaviors
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