There are claims that phytohemagglutinin (PHA), the lectin of common bean, Phaseolus vulgaris, is toxic when fed to the cowpea weevil, Callosobruchus maculatus, and that PHA serves as the chemical defense against this seed-feeding bruchid beetle (DH Janzen, HB Juster, IE Liener [1976] (1,4,8,9), one role was proposed by Janzen et al. (12), who showed that 1% (w/w) Phaseolus vulgaris lectin (PHA) from black bean caused mortality when incorporated into the diet of the cowpea weevil, Callosobruchus maculatus F. (Coleoptera: Bruchidae). They concluded that "a major part ofthe adaptive significance of phytohemagglutinins (PHA) in black bean and other legume seeds is to protect them from attack by insect seed predators" (12). Subsequently, the PHA-mediated cowpea/ common bean interaction has become a prime example of protein-mediated plant-insect interactions.Gatehouse et al. (6) (also P. vulgaris) was toxic to the cowpea weevil. Curiously, they noted that a highly purified preparation (>95%) of PHA was less toxic than a less pure commercial preparation obtained from Sigma Chemical Co., London (6). PHA is a tetrameric protein composed of two different subunit types: one erythrocyte-binding (E-type) and one lymphocyte-binding (L-type) (9). Gatehouse et al. (6) concluded that the Etype subunit was the major antimetabolite. However, subsequent studies by Boulter (2) indicated that purified E-type or L-type isolectins were both ineffective when tested alone against the cowpea weevil. To account for the biological activity of the mixture described in the earlier paper (6), Boulter (2) concluded that there must be a synergistic effect between the two different lectin subunits.These provocative results and conclusions led us to determine ifplant lectins represent a general category ofinsecticidal proteins (18). We screened 17 different commercially available plant lectins in our artificial seed bioassay system (21) using the cowpea weevil (18). Included in the study were a PHA lectin mixture (PHA isolectins composed of both the E and L subunits), as well as purified PHA-E and PHA-L. Our results showed that none ofthe three PHA lectin preparations were effective against the cowpea weevil when fed at 1 % (w/ w) in the diet (18), a level claimed effective in the published studies (6,12). Since we had shown that other lectins show great promise as biotechnology based plant chemical defenses (18,22) and because PHA is being advanced as a potential candidate for this role by some investigators, we decided to investigate the putative toxic qualities of PHA further.
MATERIALS AND METHODS Artificial SeedsTo test the effects ofvarious potential toxins on seed feeding insects, we used an artificial seed system previously developed in our laboratory (21). Briefly, the system involved milling decorticated cowpea seeds (cultivar California Blackeye No. 5) into flour, adding aqueous solutions of test compounds to the flour to make a paste, injecting the paste into Teflon molds, freezing the paste on dry ice and then liquid N2, ...