How adjacent nonhost plants affect the ability of an insect herbivore to find a host

Studies using nonhost plant odors to deter insect pests and protect host plants have reported mixed results. Moreover, how adjacent nonhost plants affect the ability of insect herbivores to locate hosts is unclear. We examined the effects of location of two nonhost rosemary plants (Rosmarinus officinalis) relative to a host peanut plant (Arachis hypogaea) under fixed wind direction on the host-finding ability (number of eggs laid) of Spodoptera exigua. We then correlated the effects with the degree of odor mixing between the host and nonhost plants. Subsequently, we investigated whether row orientation affects the ability of insects to find hosts. The degree of odor mixing between host and nonhost plants was greater when the rosemary plants were upwind and downwind than when they were crosswind from the peanut plants. Closer plant spacing resulted in more odor mixing. The number of eggs laid on peanut plants was reduced when the degree of odor mixing was high. When rosemary plants were replaced with odorless plastic plants, the odorless plants upwind and downwind from the peanut plant were also associated with fewer eggs laid. In the field, significantly fewer eggs were laid on peanut plants when the row orientation was perpendicular to the prevailing wind direction. Nonhost rosemary plant location relative to the host plant significantly affected the host-finding ability of S. exigua by influencing the degree of odor mixing between host and nonhost plants and by physical barrier effects of nonhost plants. Our results may provide insights for future pest management strategies.