Plasticity in Chemical Host Plant Recognition in Herbivorous Insects and Its Implication for Pest Control

Simple Summary Insects feeding on plants constitute a serious problem for agriculture because they produce large populations, especially in monocultures. Insect pests are mainly controlled with chemical pesticides, but with major problems caused by the development of resistance and toxic effects on non-target organisms, alternative management of insect pests has become an important goal. Understanding how insects find and choose their host plants is one major research line in order to develop environmentally acceptable methods to protect cropped plants. We review here information on how herbivorous insects use chemical cues, such as volatiles and contact cues, in a sequential way to find and finally choose their host plants. Then we put forward evidence that insect behavior, guided by chemical cues, strongly depends on the chemical environment and in addition varies as a function of the insect's physiological state, i.e., if they are sexually mature, have mated, have fed, and had previous experience, thus displaying a high degree of plasticity. In order to successfully manipulate pest insect behavior for sustainable crop protection, we need to implicate and further increase our knowledge firstly on interactions between chemical cues and their environment, and secondly on the different types of plasticity. Chemical communication is very important in herbivorous insects, with many species being important agricultural pests. They often use olfactory cues to find their host plants at a distance and evaluate their suitability upon contact with non-volatile cues. Responses to such cues are modulated through interactions between various stimuli of biotic and abiotic origin. In addition, the response to the same stimulus can vary as a function of, for example, previous experience, age, mating state, sex, and morph. Here we summarize recent advances in the understanding of plant localization and recognition in herbivorous insects with a focus on the interplay between long- and short-range signals in a complex environment. We then describe recent findings illustrating different types of plasticity in insect plant choice behavior and the underlying neuronal mechanisms at different levels of the chemosensory pathway. In the context of strong efforts to replace synthetic insecticides with alternative pest control methods, understanding combined effects between long- and close-range chemical cues in herbivore-plant interactions and their complex environment in host choice are crucial to develop effective plant protection methods. Furthermore, plasticity of behavioral and neuronal responses to chemical cues needs to be taken into account to develop effective sustainable pest insect control through behavioral manipulation.