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.
The chemical basis underlying orientation to fruit and fungal odors was investigated for the dried-fruit beetle,Carpophilus hemipterus (L.). In wind-tunnel bioassays of walking and flight response from 1.8 m, beetles were attracted to odors of the yeastSaccharomyces cerevisiae on agar, aseptic banana, or banana inoculated withS. cerevisiae, although both banana substrates elicited greater response than the yeast alone. When presented in a two-choice bioassay, the yeast-inoculated banana attracted approximately twice as many beetles as did the aseptic banana. GC-MS analysis of the headspace volatiles above these odor sources revealed a somewhat more complex and concentrated volatile profile for yeast-inoculated banana than for aseptic banana. The odor from yeast on agar had fewer components, and these were present at lower concentrations than the odors of either banana substrate. By blending mineral-oil or aqueous solutions of the 18 components of inoculated-banana odor in varying concentrations, it was possible to mimic closely the headspace profile of the natural odor. This synthetic odor also elicited beetle attraction in the wind tunnel at levels comparable to the inoculated banana. Through a series of bioassays in which individual components were subtracted from or added to a synthetic odor blend, it was determined that ethyl acetate, acetaldehyde, 2-pentanol, and 3-methylbutanol comprised the simplest blend of compounds evoking full behavioral response. However, 2-methylpropanol or butanol were apparently interchangeable with 3-methylbutanol in this blend, and comparable response could also be elicited by replacing acetaldehyde with a combination of both 2-pentanone and 3-hydroxy-2-butanone. Thus, our results suggest that this generalist insect herbivore locates its hosts by a long-range response to a variety of blends of common fruit volatiles, whose concentrations are enhanced by fungi.
Standard gravimetric technique was used to assess the effect of induced resistance in soybean on the consumption and utilization of food by the Mexican bean beetle (MBB), Epilachna varivestis, and the soybean looper (SBL), Pseudoplusia includens. Induced resistance had significant retarding effects on the development and growth of both SBL and MBB. SBL fed leaves of previously injured soybean plants had a 8.5% longer developmental time through the entire larval stage and a 10.4% lower pupal weight than larvae fed leaves from uninjured plants. MBB fed on leaves from previously injured plants through the first three instars had a 9.1 % longer developmental time and a 16.5% lower final weight than those fed on leaves from uninjured plants. Induced resistance showed no significant effect on total food consumed by either SBL or MBB. Nutritional indices including relative consumption rate (RCR), relative growth rate (RGR), efficiency of conversion of ingested food (ECI), approximate digestibility (AD), and efficiency of conversion of digested food (ECD) were calculated on dry weight measurements. In general, induced resistance had greater impacts on the performance of MBB than on that of SBL. Résumé Influence de la résistance induite chez le soja sur le développement et l'alimentation de Pseudoplusia includens et Epilachna varivestis Une technique gravimétrique uniforme a été utilisée pour mesurer l'effet de la résistance induite du soja et la consommation et l'utilisation d'aliments par E. varivestis (Coléop.: Coccinellidae) et P. includens (Lepid.: Noctuidae). La résistance induite a significativement retardé la croissance et le développement des 2 insectes. La consommation de feuilles déjà abîmées a prolongé de 8.5% la durée du développement de P. includens et a réduit de 10.4% le poids des chrysalides. La consommation de feuilles de soja abîmées pendant les 3 premiers stades d'E. varivestis a prolongé le développement de 9.1% et a réduit de 16.5% le poids des nymphes. La résistance induite n'a pas provoqué une modification significative de la quantité de feuilles de soja abîmées chez les 2 insectes. Les indices de nutrition, comprenant le taux de consommation relative (RCR), le taux de croissance relative (RGR), l'efficacité de transformation de l'aliment ingéré (ECI), la digestibilité approchée (AD) et l'efficacité de conversion de l'aliment digéré (ECD), ont été calculés à partir des poids secs. En général, la résistance induite a eu des effets plus nets sur les performances d'E. varivestis que de P. includens.
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