Predatory insects avoid incidental ingestion by mammalian herbivores

The direct trophic links between mammalian herbivores and plant-dwelling insects have received little attention. Insects are ubiquitous on plants consumed by mammalian herbivores and are thus likely to face the danger of being incidentally ingested by a grazing mammal. Some herbivorous hemipterans are able to avoid this peril by dropping to the ground upon detecting the heat and humidity on the mammal's breath. If this incidental ingestion is common, other insects from distinct groups should develop similar mechanisms to avoid it. We assessed the ability of three species (adults and larvae) of coccinellid beetles, important aphid predators, to avoid incidental ingestion. In feeding experiments, both larvae and adults were able to avoid incidental ingestion by goats effectively by dropping to the ground, demonstrating the possible importance of this behavior in grazed habitats. Remarkably, all adult beetles escaped by dropping to the ground and none used their functional wings to fly away. In controlled laboratory experiments, we found that human breath caused 60-80% of the beetles to drop. The combination of heat and humidity was the most important component of mammalian herbivore breath in inducing adult beetles and larvae to drop. The fact that the mechanism of dropping in response to mammalian breath developed in distinct insect orders and disparate life stages accentuates the importance of the direct influence of mammalian herbivores on plant-dwelling insects. This hitherto unresearched direct interaction should be given its due place when discussing trophic interactions.