Effects of Cultivar and Insecticide Choice on Oleander Aphid Management and Arthropod Dynamics on Asclepias Species1

Performance of oleander aphid (Aphis nerii Boyer de Fonscolmbe) and large milkweed bug (Oncopeltus fasciatus Dallas) on 21 species of Asclepias was evaluated in a two-year field study. Season-long pest and beneficial insect population growth was monitored. Plant quality ratings also were obtained. Relative numbers of Monarch butterfly (Danaus plexippus (Linnaeus)) larvae were also recorded. All milkweed species supported growth and development of oleander aphid except A. vestita, which failed to establish in this study. The only species that did not become infested with milkweed bugs were A. syriaca and A. sullivantii. The plant species with the lowest number of aphids, the highest plant quality ratings and the highest number of Monarch larvae (which is desirable as a larval food plant for the butterflies) was gooseplant, A. physocarpa. A. tuberosa cultivars also ranked high among all species tested. Five insecticides were tested for efficacy against the Oleander aphid. All insecticide products evaluated: Endeavor (pymetrozine), Orthene (acephate), Merit (imidacloprid), Tempo (cyfluthrin) and Mpede (insecticidal soap) resulted in short-term reductions in aphids in field plots during 1999. Reinfestation occurred within two to three weeks. Parasitoids and predators were also suppressed to varying degrees by materials applied. Index words: oleander aphid, Aphis nerii, milkweed bug, Oncopeltus fasciatus, Monarch butterfly, Danaus plexippus, milkweed, butterfly weed, Asclepias spp., parasitoids, predators, host plant resistance. Species used in this study: Asclepias asperula (Decne.) — antelope horn; Asclepias curassavica L. — bloodflower, ‘Silky Gold’ and ‘Red Butterflies’; Asclepias emoryi Tidestr; Asclepias eriocarpa Benth. — indian milkweed; Asclepias erosa Torrey — desert milkweed; Asclepias exaltata L.; Asclepias fascicularis Decne — narrowleaf milkweed; Asclepias fruticosa L. — swan plant; Asclepias hirtella RE Woodson — tall green milkweed; Asclepias incarnata L. — swamp milkweed, ‘Soul Mate’, ‘Cinderella’, ‘Ice Ballet’; Asclepias oenotheroides, Cham & Schelecht ‘Ice Follies; Asclepias physocarpa (E.H. Mey) Schlechter — gooseplant; Asclepias solanoana RE Woodson; Asclepias speciosa Torr — showy milkweed; Asclepias sp. ‘Davis, CA’, Asclepias sullivantii Engelm. — prairie milkweed; Asclepias syriaca L. — common milkweed; Asclepias tuberosa L. — butterfly weed, ‘Gay Butterflies’, ‘Hello Yellow’; Asclepias verticillata L. — whorled milkweed; Asclepias vestita Hook & Arn. Per.; Asclepias viridis Walt. Fl. Carol — green milkweed. Insecticides used in this study: acephate (Orthene T T & O), O, S dimethyl acetylphosphoramidothioate; cyfluthrin (Tempo 20 WP), cyano (4-fluoro-3-phenoxyphenyl) methyl 3-(2,2 -dichloroethenyl)-2,2-dimethyl-cyclopropanecarboxylate; imidacloprid (Merit 75 WSP), 1-[(6-chloro-3-Pyridinyl) methyl]-N-nitro-2-imidazolidinimine; insecticidal soap (Mpede 2%); pymetrozine (Endeavor WDG). Significance to the Nursery Industry Asclepias species and cultivars were evaluated for differences in susceptibility to oleander aphid and large milkweed bug. Insecticides were evaluated for their degree of aphid suppression and for non-target effects on natural enemies of the oleander aphid. The only species that did not become infested with milkweed bugs in this two-year study were A. syriaca and A. sullivantii. Plants with the lowest number of aphids, the highest plant quality ratings and the highest number of Monarch larvae (which is desirable, since this is a larval food plant for the butterflies) were gooseplants, A. physocarpa. A. tuberosa cultivars also ranked high among all species tested. All products evaluated: Endeavor (pymetrozine), Orthene (acephate), Merit (imidacloprid), Tempo (cyfluthrin) and Mpede (insecticidal soap) effected short-term reductions in aphids in field plots during 1999. Reinfestation occurred within two to three weeks. Parasitoids and predators were also suppressed to varying degrees by materials applied.