Flight Capacity and Dispersal of Walnut Husk Flies in Laboratory and Field Studies

Although the walnut husk fly, Rhagoletis completa, is sometimes considered to be a weak flier, very little is known about the actual flight capacity of this fly. With funding from CDFA and CSU-ARI (California State University Agricultural Research Initiative), we initiated studies, in the laboratory and field, of flight capacity and dispersal ability of husk flies. Adult flies in the laboratory were fed a diet containing either sucrose alone or a mixture of sucrose plus yeast hydrolysate. In the lab, using a computerized flight mill, we found that flies fed a diet of sucrose alone flew significantly farther than flies fed sucrose plus yeast, and we found that males generally flew farther than females. In an unsprayed English walnut orchard using markrecapture techniques, we found that males flew significantly farther than females, but that diet did not significantly affect recapture distances. However, these recapture distances did not accurately reflect the maximum dispersal capabilities of flies because once a fly was captured on a sticky trap, it could fly no further. Thus, we conclude that while laboratory flight mill studies might provide overestimates of actual dispersal distances of flies, mark-recapture studies likely underestimate dispersal abilities of flies. In the field, insect physiological state (including feeding status, reproductive status, age and gender) as well as environmental conditions (i.e., wind, humidity, host development, food and water resources) have been shown to influence dispersal distance and direction of other insects and likely strongly influence movements by walnut husk flies. Nevertheless, based on these studies, we conclude that the walnut husk fly is neither a weak flier nor likely to be confined to a single orchard if other hosts are available within 1⁄2 mile.