Chemical conspicuousness of an herbivore to its natural enemy: Effect of feeding site selection

A physical refuge from the parasitoid Aphytis melinus is provided to the Ca lifornia red scale Aonidiella aurantii by the interior bark substrates of c itrus trees, even though scales have lower fitness on bark in the absence o f A. melinus. How bark-reared scales escape parasitism was unclear because A. melinus searches the interior of trees as effectively as the exterior. H ost identification in A. melinus is mediated by a kairomone, O-caffeoyltyro sine, in scale covers. O-caffeoyltyrosine concentration varies with scale a ge and rearing conditions. We hypothesized that the reduced acceptance of b ark-reared scale may be due, in part, to reduced quantities of O-caffeoylty rosine in their covers. We reared scales on bark, leaves, and fruit of lemo n and orange trees in the field at monthly intervals and then collected the scales and measured them. We bioassayed covers for their acceptability to A. melinus in the laboratory and then determined their O-caffeoyltyrosine c ontent. Even after adjusting for the differences in scale body size, O-caff eoyltyrosine content in bark-reared scale covers was 45-85% less than that in covers of leaf- or fruit-reared scales, depending upon cultivar and rear ing date. Covers of bark-reared scales were selected for probing only 40-45 % of the time when compared to leaf-reared scales. Covers with the highest levels of O-caffeoyltyrosine were most likely to be selected for probing. W e conclude that part of the mechanism by which California red scales on bar k avoids discovery is through reduced O-caffeoyltyrosine content in their c overs. This reduction is probably a consequence of the reduced nutritional quality of bark as a substrate for scale survival and growth.