Effectiveness of larval defenses against spider predation in cranberry ecosystems

Two laboratory experiments, a choice test and observations of individual spider-larva interactions, were conducted to determine how the morphological or behavioral defenses, or both, of larvae may affect spider predation success for 2 genera of spiders [Pardosa (Lycosidae), and Habronattus (Salticidae)] commonly found in Massachusetts cranberry, Vaccinium macrocarpon Aiton, ecosystems. Four species of pest larvae with different potential de fenses were offered as prey: a spanworm, Ematurga amitaria Guenee (Lepicloptera: Geometridae); a sawfly, Pristophora idiota Norton (Hymenoptera: Tenthridinae); gypsy moth, Lymantria dispar L. (Lepidoptera: Lymantriidae); and a fruitworm, Sparganothis sulfureana Clemens (Lepidoptera: Tortricidae). In the choice tests, E. amitaria larvae were killed significantly more often than either P. idiota or S. sulfureana larvae. No L. dispar larvae were killed in either experiment, and observations suggested that the long setae of the L. dispar larvae provided defense against spider attack. Observations revealed that behavioral and morphological crypsis was a more effective defense than thrashing and regurgitating when E. amitaria larvae were encountered by either genera of spider and also suggested the presence of a chemical defense(s) in P. idiota larvae. Construction of a retreat in cranberry foliage and wriggling movements by S. sulfureana larvae were highly effective in deterring salticid predation. We conclude that the defensive mechanisms possessed by these larvae may effectively influence prey selection and significantly limit predation by spiders in the cranberry agroecosystem.