Semiochemicals — Chemical Control of Behavior
Much of an insect’s behavior is mediated by chemicals in its environment. By turning these chemicals to our own advantage, it is often possible to attract pests to traps or baits, or repel them from our homes, our crops, or our domestic animals. Behavioral messages are delivered by a wide array of chemical compounds. As a group, these compounds are known as semiochemicals. In some cases, they may facilitate communication between the members of a single species (e.g., pheromones) or between members of different species (e.g., allelochemicals). Functionally, semiochemicals may have a wide range of activity. They may serve as attractants or repellents, they may stimulate or inhibit feeding, they may provoke flight or inhibit it, or they may simply elicit behavior patterns at inappropriate times.
Attractant pheromones and allelochemicals can be used as lures or baits in a wide variety of insect traps, or they can be mixed together with toxicants to produce an “elixir of death”. Protein hydrolysates, for example, serve as feeding attractants for fruit flies (Rhagoletis spp.). These chemicals can be applied to sticky traps to improve catch, or combined with an insecticide and sprayed on fruit crops to suppress active infestations. Phenethyl propanoate, eugenol, and geraniol can be mixed in a 3.5:3.5:3 ratio and used as an attractant for Japanese beetles (Popillia japonica). These are the active ingredients in the “floral attractant” found in popular bag traps for Japanese beetles. In some cases, chemists have produced synthetic compounds that are even more attractive than naturally occurring chemicals. Trimedlure, a synthetic substitute for alpha-copaene, is produced commercially as an attractant for the Mediterranean fruit fly (Ceratitis capitata). Improved food lures and baits are among the most promising new developments for controlling cockroaches (Blattoidea) in homes and businesses. These are the active ingredients in a new generation of “roach motels” where the insects “check in but don’t check out.”
Sex pheromones are among the most powerful of chemical attractants. Ever since they were first discovered by A. A. Budenandt in 1959 (from silkworm moths, Bombyx mori), these chemicals have aroused great interest because of their potential as pest control agents. During the past 30 years, chemists have identified the sex pheromones for over 300 insect species. Many of these compounds are now sold commercially. In some cases, pheromones are packaged (or encapsulated) in slow-release dispensers (rubber septa, hollow fibers, or rope wicks) that are used as lures in traps of various designs. At low densities, these pheromone traps are a valuable monitoring tool, providing information on the density and distribution of pest populations. At high densities, they can be used for mass trapping sexually active adults (usually males) in efforts to reduce population density and lower a pest’s reproductive potential.
Slow-release formulations of sex pheromones can also be used for mating disruption. By increasing the concentration of pheromone in an insect’s environment, it may be possible to make everything smell like a prospective mate. Males wear themselves out courting inanimate objects or become habituated to the odor and stop responding to it. This approach, variously known as air permeation or the innundation technique, has shown promise for controlling a number of fruit and vegetable pests, including the codling moth (Cydia pomonella), the cabbage looper (Trichoplusia ni), the oriental fruit moth (Grapholita molesta), and the peachtree borer (Synanthedon exitiosa).
Chemical repellents and feeding deterrents are also useful tools for managing insect behavior. As their name suggests, these compounds cause insects (or other arthropods) to disperse or to discontinue normal feeding behavior. Repellents such as dimethyl phthalate, benzyl benzoate, and N,N-diethyl-m-toluamide (DEET) have been developed to protect humans from biting flies, ticks, and chiggers (immature trombiculid mites). Other compounds, like di-n-butyl succinate or butoxypolypropylene glycol, are used as fly repellents for cattle. Moth balls and flakes (paradicholorobenzene or alpha-naphthalene) are placed in drawers and closets to prevent infestation by a variety of insects that feed on stored products and natural fibers. Woolen cloth can be manufactured with colorless dyes (e.g., Eulans and Mitin FF) that bond permanently to the fabric and make it unpalatable to clothes moths and carpet beetles. Wood preservatives, such as pentachlorophenol, act as feeding deterrents to termites and other wood-dwelling insects.
The neem tree, Azadirachta indica (Meliaceae) is a promising new source of feeding repellents that may be developed for use on selected non-crop plants. The leaves, twigs, and seeds of this tree, which is grown commercially in India, contain at least 25 biologically active compounds that act as insect repellents, feeding deterrents, or growth regulators. Azadirachtin, the most abundant of these active ingredients, is now commercially available in the United States. Sold under the trade name Margosan-O, this new feeding deterrent has been approved for use on non-food greenhouse crops, ornamentals, and turf.