Abstract-The effect of aphid population size on host-plant chemical defense expression and the effect of plant defense on aphid population dynamics were investigated in a milkweed-specialist herbivore system. Density effects of the aposematic oleander aphid, Aphis nerii, on cardenolide expression were measured in two milkweed species, Asclepias curassavica and A. incarnata. These plants vary in constitutive chemical investment with high mean cardenolide concentration in A. curassavica and low to zero in A. incarnata. The second objective was to determine whether cardenolide expression in these two host plants impacts mean A. nerii colony biomass (mg) and density. Cardenolide concentration (µg/g) of A. curassavica in both aphid-treated leaves and opposite, herbivore-free leaves decreased initially in comparison with aphid-free controls, and then increased significantly with A. nerii density. Thus, A. curassavica responds to aphid herbivory initially with density-dependent phytochemical reduction, followed by induction of cardenolides to concentrations above aphid-free controls. In addition, mean cardenolide concentration of aphid-treated leaves was significantly higher than that of opposite, herbivore-free leaves. Therefore, A. curassavica induction is strongest in herbivore-damaged tissue. Conversely, A. incarnata exhibited no such chemical response to aphid herbivory. Furthermore, neither host plant responded chemically to herbivore feeding duration time (days) or to the interaction between herbivore initial density and feeding duration time. There were also no significant differences in mean colony biomass or population density of A. nerii reared on high cardenolide (A. curassavica) and low cardenolide (A. incarnata) hosts.

1 The attractiveness of pitfall traps baited with a synthetic host volatile attractant to colonizing adult Colorado potato beetle, Leptinotarsa decemlineata (Say) was evaluated in a field setting. 2 Significantly more postdiapause, colonizing adult L. decemlineata were captured in baited than unbaited pitfall traps. 3 The potential for this synthetic kairomone to enhance the efficacy of trap cropping as a management tool was evaluated by comparing conventionally managed plots with like-sized plots bordered by either attractant-treated trap crop or untreated trap crop. 4 More postdiapause, colonizing adults, egg masses and small larvae were present in attractant-treated trap crops than in untreated trap crops. 5 There were no significant differences in egg mass and small larvae densities between plots bordered by attractant-treated trap crops and conventionally managed plots, but there were significantly fewer large larvae and adult beetles in conventionally managed plots. 6 Plant canopy area of conventionally managed plots was significantly greater than in plots bordered by either type of trap crop. 7 Yields for conventionally managed plots and plots bordered by attractanttreated trap crops did not differ, and less insecticide (44%) was applied to plots bordered by attractant-treated trap crops.

1 The attractiveness of potato plants treated with a synthetic host volatile blend [(Z)-3-hexenyl acetate (þ/-)-linalool, and methyl salicylate] to newly emerged and 5-day-old adult Colorado potato beetle, Leptinotarsa decemlineata (Say), was compared at four doses against untreated control plants and plants treated with an azadirachtin-based antifeedant in greenhouse cage arenas. 2 Attractant-treated plants (derived release rates of 0, 5.7, 17.1 or 57 mg/h) were significantly more attractive than untreated control plants to newly emerged and 5-day-old adults only at 57 mg/h. 3 Attractant-treated plants were significantly more attractive than antifeedanttreated plants to newly emerged and 5-day-old adults at the 5.7 mg/h treatment level and higher. Mean insect density on attractant-treated plants in the attractant/antifeedant study was significantly higher than in the attractant/control study. 4 Habituation to the synthetic attractant was evaluated by exposing adult beetles to the synthetic attractant for 0, 1, 2.5, 4, 8, 12 or 16 h, before release into a wind tunnel in which an attractant-baited plant model was placed at the upwind end. Insects exposed to the synthetic host attractant for 8 h moved to the synthetic attractant-baited plant model whereas insects exposed to the synthetic host attractant for 12 and 16 h did not. Furthermore, beetles exposed to the synthetic attractant for 0 and 1 h moved at rates greater than, or equal to, the median whereas beetles exposed for longer time periods moved at rates significantly less than the median. 5 These results demonstrate the potential for using the synthetic plant attractant and an antifeedant as components in a stimulo-deterrent strategy for management of the Colorado potato beetle as shown by us in another study.

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