Potato aphid (PA); Macrosiphum euphorbiae (Thomas)

Frank Zalom, Douglas Walsh, Eileen Cullen
and Christine Tobia

Department of Entomology
University of California
Davis, CA 95616
Phone: (530) 752-8350

Gene Miayo

UC Cooperative Extension
70 Cottonwood
Woodland, CA 95695

POTATO APHID CONTROL WITH NARROW RANGE OILS, 1998: Eleven insecticides were evaluated to determine the efficacy of narrow range oils against PA. Processing tomatoes were seeded on the UC Davis campus in late May and were later infested with PA collected from nearby commercial fields in early Jul. The plots were treated on 29 Jul, when PA populations exceeded the UCIPM recommended treatment threshold (50% of sampled leaves infested). Treatments included Orchex 796 oil at two concentrations, Safe-t-cide oil, Sunspray oil, two neem treatments (Trilogy neem oil and Neemix), three pyrethroid treatments (Warrior at two rates and Danitol), and dimethoate. Each treatment and an untreated check were replicated five times each in a RCB design. All applications were applied with an Echo D-9 Duster/Mister at a vol of 50 gal per acre. PA and aphid mummy (parasitized aphids) densities were determined by sampling 4 leaves from each plot prior to application on 29 Jul, and following application on 5 and 12 Aug. Percent reduction in PA densities between the pre-treatment and post-treatment counts were determined. ANOVA was performed on the raw data (arcsine transformed), and on the data corrected for the reduction in PA densities observed in the untreated plots. Least square means were calculated, and treatment means compared with the untreated check for raw data, and with the conventional treated check dimethoate for the corrected data.

PA densities declined in all plots after the treatments were applied. The percent reduction in densities in the untreated plots relative to pre-treatment densities was 13.2% for the 5-Aug sample, and 61.0% for the 12-Aug sample (making interpretation of the analysis of data from 2 wk post-treatment somewhat suspect). PA densities in treatments with Orchex 796 at both concentrations and a Trilogy neem oil were significantly different from those in the untreated check (P < 0.05). Efficacy of the narrow range oils Safe-t-cide and Sunspray were not significantly (P > 0.05) relative to the untreated check. Dimethoate and both pyrethroids reduced PA densities significantly (P < 0.01) relative to the untreated check. Neemix did not significantly (P > 0.05) reduce densities relative to the untreated check. Corrected PA densities from 5 Aug did not differ significantly (P > 0.05) from those observed for dimethoate or the pyrethroids for all treatments except for Neemix and Sunspray which provided significantly (P < 0.05) less control. Corrected values for the 12 Aug PA counts are also presented, but again these are suspect due to the substantial decline in untreated check densities. No significant difference (P > 0.05) was found in the number of aphid mummies between treatments on either 5 Aug or 12 Aug. However, parasitism was low (> 20% in all of the plots), possibly influencing the results of our analysis. On 5 Aug, few aphid mummies were found in the plots treated with the high rate of the pyrethroid insecticide Warrior, and none were found in the plots treated with dimethoate. Moderately fewer parasitized PA were found in the plots treated with Trilogy, Neemix, Danitol, the lower rate of Warrior and the 2% rate Orchex 796. Parasitism appeared to recover in the plots treated with the oils (with the possible exception of Safe-t-cide) by 12 Aug, but remained lower in the other treatments. Efficacy of the narrow range oils was lower than that observed for dimethoate or the pyrethroids that we evaluated. It was our observation that inadequate coverage was an issue. Leaves within the canopy which were not adequately contacted by the spray supported relatively high densities of aphids after the treatments were applied. Laboratory observations of Orchex 796 efficacy at 1% and 2% concentrations indicated that all PA were killed when contacted by the spray.