Influence of Prey Availability and Induced Host-plant Resistance on Omnivory by Western Flower Thrips

Theory predicts that the balance of nutritional needs, food availability, and the quality of particular food items are important factors in the feeding decisions of omnivorous animals. In this study we investigate factors that affect the feeding decisions of an omnivorous thrips (Frankliniella occidentalis) that eats both animal prey and plant foliage. In the presence of prey (mite eggs), adult and larval thrips consumed nearly half the amount of plant tissue as did thrips without prey. We manipulated host-plant quality by exposing plants to spider mite feeding. In the absence of prey, systemically induced plant responses following herbivory reduced the amount of feeding by thrips compared to that on uninduced control plants. In the presence of prey, induced responses caused a shift in the feeding preferences of thrips: thrips consumed half the amount of plant material and twice the number of prey on induced plants as did thrips on uninduced control plants. Our findings are relevant to understanding the complex factors that shape food-web interactions in nature. Variation in host-plant quality and availability of prey can exert a strong influence on the feeding preferences of omnivores. In addition, induced resistance may be a particularly useful biological-control strategy because it reduces herbivorous pest populations directly, and indirectly by causing a shift toward predation in naturally occurring omnivores.