TO PENETRATE OR NOT TO PENETRATE - A BEHAVIORAL CHOICE BY BEAN BEETLE1ST INSTAR LARVAE IN RESPONSE TO PHASEOLUS-VULGARIS SEED SURFACE QUALITY

Survival of Acanthoscelides obtectus larvae depends on the ability of the first instar to pierce the seed coat of Phaseolus vulgaris, which represents a critical sequence because of physical characteristics and toxicity. We have investigated the influence of seed surface quality on larval boring behavior by the usual method of surface washing with different solvents, or by removing the testa, or by coating the testa with a polymer spray. Observations were made on isolated larvae in no- choice and dual-choice bioassays. In the no-choice situation, larval p enetration was reduced after seed coats were soaked with chloroform, w hereas water, diethyl ether, or methanol had no significant effect. Th e ratio of boring attempts to successful penetrations was increased on seeds washed with chloroform. In the dual-choice situation, larvae av oided artificially coated seeds and preferred untreated seeds over tho se washed with chloroform or methanol. These results indicate that bor ing stimulants exist on the seed coat and that they are removed by chl oroform and methanol or made inaccessible by artificial coating. The h igh mortality of first instars on seeds washed with these two solvents is attributed to a lack of chemicals necessary to initiate and sustai n boring behavior, causing larval stress, possibly due to prolonged lo comotory activity and starvation. Thus, seed coat quality may influenc e the population dynamics of A. obtectus.