EFFICACY OF THE PESTICIDE CARBARYL FOR THALASSINID SHRIMP CONTROL IN WASHINGTON-STATE OYSTER (CRASSOSTREA-GIGAS, THUNBERG, 1793) AQUACULTURE

The pesticide carbaryl is applied to intertidal oyster beds in Washing ton State to control burrowing thalassinid shrimp. We studied efficacy and found a significant dose response relationship for both ghost shr imp, Neotrypaea californiensis, and mud shrimp, Upogebia pugettensis. A threshold response was observed, suggesting that reducing the commer cial application rate below 5.6 kg ha(-1) would decrease efficacy and increase variability of resulting kill, particularly for Upogebia. Exp osure time (time between application at low tide and flood tide) signi ficantly affected the relationship, suggesting that even lower rates ( e.g., 2.5 kg ha(-1)) could be effective when exposure lime is sufficie ntly long (>2-3 h). Typical oyster beds are exposed from 2 to 6 h duri ng minus spring tides. Carbaryl persisted slightly longer (40-45 days) in muddier substrate where Upogebia is present than in the well-drain ed sand inhabited by Neotrypaea. However, given rapid initial decline after application (<1 ppm in 24 h), reducing application rate would no r greatly influence persistence at levels toxic to nontarget species. This study suggests that growers should be aware of the species of shr imp present on individual oyster beds, because Neotrypaea causes much higher initial oyster seed mortality than does Upogebia. No oysters su rvived beyond 300 days on untreated or treated plots where Neotrypaea was present. Because of seasonal recruitment of postlarvae to the estu ary in late summer and early autumn, Neotrypaea is also able to reinfe st treated plots immediately, suggesting that long-term control for th is species is more problematic.