MAXIMIZATION OF HOST ENCOUNTERS BY PARASITOIDS FORAGING IN THE FIELD - FEMALES CAN USE A SIMPLE RULE

Searching animals maximizing resource encounters should use reliable f oraging cues. Biosteres juglandis, a braconid parasitoid of Rhagoletis juglandis larvae feeding in fruits of the Arizona walnut, searches fo r hosts both on walnuts that are on the tree and those that have falle n to the ground. Field and laboratory assays were conducted to determi ne the cues used by B. juglandis to choose ground walnuts on which to alight. After walnuts are infested with fly larvae, they change color from yellow to black. In field studies, wasps preferentially landed on walnuts on the ground that had at least some yellow on them. At one s outheastern Arizona site, yellow fruits were more likely to contain fl y larvae than those that were all black. Yellow fruits also contained younger larvae in which wasps had greater oviposition and/or developme ntal success. At another site, black fruits were more likely than yell ow to contain larvae, but wasps were searching yellow walnuts only. A laboratory experiment suggested that visual contrast may be one reason why wasps prefer to land on yellow walnuts in the field.