Induced resistance in soybean was investigated using mechanical injury and herbivory by the soybean looper, Pseudoplusia includens (Walker), as inducing factors. Dual-choice feeding-preference tests with the Mexican bean beetles, Epilachna varivestis Mulsant, were used to assess the induced resistance. Comparisons of leaves from plants treated by mechanical injury, soybean looper herbivory, and application of soybean looper larval regurgitate on mechanically wounded leaf surfaces revealed that herbivore-feeding injury was a better inducer than mechanical injury. The regurgitate of soybean looper larvae may contain factors that enhance induction of resistance. Tests using various types of mechanical injury as inducing factors showed that the level of induced resistance depended on the total number of wounded cells in contact with healthy cells and not on the total amount of plant tissue lost. We suggest that there is a positive correlation between the intensity of the inducing factor and the level of resulting resistance in soybean. Interplant transfer of inducing signals was tested, but no evidence was found that such transfer occurred.

Five species of diabroticites with different host-plant preferences produced an essentially identical array of metabolites when fed radiolabeled cucurbitacin B synthesized in vivo and purified fromCucurbita maxima Duchesne seedlings. All species excreted the bulk of the cucurbitacin (67,17-94.59% total dpm), permanently sequestered a small proportion of a cucurbitacin conjugate in the hemolymph (0.98-2.76%), and apportioned the remainder between the gut, the body, and the eggs (when present). Markedly different ratios between the excretory metabolites (i.e., polar vs. unmetabolized cuc) suggest thatDibrotica virgifera virgifera, a grass specialist, andAcalymma vittatum, a cucurbit specialist, have lower rates of metabolic alteration than the polyphagousD. undecimpunctata howardi, D. balteata, andD. cristata, which is associated with relict prairies. Mean life-spans ofD. balteata and D. v. virgifera and maleA. vittatum decreased significantly with continuous feeding onCucurbita fruit containing cucurbitacins (vs. fruit devoid of cucs). The longevity of femaleA. vittatum, however, was unaffected by the presence of cucurbitacins.

Laboratory studies showed that the soybean phytoalexin glyceollin, an isoflavonoid previously investigated as an inhibitor of fungal pathogens, is an effective antifeedant for some insect species. Glyceollins extracted from soybean, Glycine max Merrill, cotyledons were applied to the surface of common bean leaves, Phaseolus vulgaris L., in five concentrations, including physiological concentration, for feeding preference tests. Leaves treated with glyceollin at concentrations below physiological levels were less acceptable to the southern corn root worm, Diabrotica undecimpunctata howardi Barber, and the Mexican bean beetle, Epilachna varivestis Mulsant, than untreated leaves. Feeding deterrence was positively correlated with increasing concentrations. The bean leaf beetle, Cerotoma trifurcata Forster, was not affected even by very high doses. An ethological concentration (EC ••) was computed based on the log dose-reduction of acceptance of treated disks. The EC •• for the Mexican bean beetle was 6.1~g/mg leaf dry weight and 3.5~g/mg for the southern corn rootworm. Results indicate that soybean phytoalexins may represent a common defense against microorganisms and insect herbivores.

The Mexican bean beetle, Epilachna varivestis Mulsant (Coleoptera; Coccinellidae), is an oligophagous insect that accepts leaves of soybean, Glycine max (L.) Merrill, as food but exhibits distinct preferences among varieties. It is believed that the basis for discrimination between the resistant PI 229358 and the susceptible cv. 'Williams' is chemical. By the ablation of chemosensilla on antennae, maxillary palpi, labial palpi, and foretarsi, followed with leaf disc preference tests, we found antennal and maxillary palp chemosensilla to be almost entirely responsible for Mexican bean beetles' discrimination between foliage of the resistant and susceptible genotypes. All appendages, and the epipharynx, were observed by scanning electron microscopy and, except for foretarsi, were found to bear several types of presumably olfactory and gustatory sensilla, which are figured and described. Since behavioral observations show that olfaction is usually sufficient to cause rejection of PI 229358, we conclude that olfactory sensilla mainly on antennae and maxillary palpi are critical in food discrimination in this system.

We investigated the role of simple phenolic acid precursors of isoflavonoid phytoalexins, as free compounds, in food selection by the Mexican bean beetle, Epilachna varivestis Mulsant, a pest of soybean, Glycine max Merrill, and other legumes. An aerosol spray method was developed to apply compounds in a uniform and reproducible manner to leaf surfaces of common bean, Phaseolus vulgaris L., a preferred host. Twenty-three compounds were tested in dual choice preference tests. Included were benzoic acid, cinnamic acid, simple phenolic acids (hydroxylated benzoic and cinnamic acids), and their methoxylated analogs. Six compounds were found to cause rejection of leaves. All of these were hydroxylated acids with the hydroxyl group adjacent to the carboxylic acid group on the benzene ring. Two of these six (gentisic acid and salicylic acid) are reportedly produced in soybean leaves.

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