The Dangers of Growing Old: Adult Moths Face Higher Predation Pressures than Caterpillars in Hyles lineata

TL;DR

Adult white-lined sphinx moths face higher predation than caterpillars, with most attacks happening during the day, likely due to their larger and more visible size.

Contribution

This study provides empirical evidence that adult moths are more vulnerable to predation than caterpillars in a natural setting.

Findings

Adult moth replicas were attacked twice as often as caterpillar replicas.

Predation was six times more likely to occur during the day than at night.

Birds were responsible for the majority of attacks on both adult and caterpillar replicas.

Abstract

Predation on insects can depend upon numerous factors, including morphology, size, age, environment, time, and many others. Most insects undergo complete metamorphosis, in which they begin as eggs, then become larvae, then pupae, and then reproductive adults. Complete metamorphosis has been hypothesized to be advantageous for numerous reasons, including reduced predation between the different stages. Here, we tested the attack rates on a common moth, the white-lined sphinx, in both caterpillar and adult moth stages, using clay replicas. We paired caterpillar and moth replicas in a natural desert environment and monitored attack rates at dawn and dusk for 72 h. By checking attack rates at dawn and dusk, we were able to quantify if the replicas had been attacked during the day or night. We found that most attacks occurred during the day and that the chance of adult moths being attacked was more than twice that of caterpillars. Our study supports the hypothesis that non-adult stages may have reduced predation risk; however, our methodologies are only appropriate for assessing visually guided predators. Thus, future work quantifying predation involving all modalities (e.g., acoustic and chemical) is necessary to fully understand differential predation across life stages in insects.

Holometabolous insects display drastically different morphologies across life stages (i.e., larvae vs. adults). Morphological differences across life stages, such as different sizes and coloration, likely result in differential survival, as predators may find individuals of one life stage more conspicuous and/or more energetically profitable than another. Furthermore, prey conspicuousness may vary temporally because both the sensory environment and predator sensory abilities differ between day and night. Here, we investigated how the interaction between life stage (caterpillar vs. moth) and time of day (day vs. night) influences predation of the white-lined sphinx (Lepidoptera: Hyles lineata). We predicted that caterpillars would be less susceptible to predation than adult moths, as adults are larger and have a more conspicuous shape. After quantifying predation for 72 h during dawn and dusk using 199 plasticine replicas each of adults and caterpillars, predation on adult replicas was twice that of predation on caterpillar replicas. Furthermore, replicas were six times more likely to be predated on during the day than during the night. Lastly, attacks were made mainly by birds, which carried out 86% of the attacks on adult models and 85% of those on caterpillar models. These data support the hypothesis that predation rates differ across life stages in holometabolous insects. This research lays a foundation for further investigation into how specific differences in morphology across life stages affect predation and survival in holometabolous insects.

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