Comparison of the potential of organophosphorus insecticides and soaps in conifer aphid control.
A laboratory comparison of the toxicity to the conifer aphids Elatobium abietinum and Adelges cooleyi [Gilletteella cooleyi] and to the coccinellid predator Aphidecta obliterata of dimethoate, fenitrothion and malathion with Safer's insecticial soap (SIS) and saponified maize oil and sunflower oil confirmed that the organophosphates were more effective. This did not detract from the effectiveness of the soaps under field conditions in Scotland, provided that appropriately higher concentrations were applied. The saponified oils with maize oil soap, which is rich in oleic and linoleic acid, frequetly produced higher mortality levels than SIS. In E. abietinum, a linear relationship was demonstrated between insecticide susceptibility and aphid instar, with later instars apparently becoming more susceptible. This relationship disappeared when LC50s were corrected for size differences. Larvae of A. obliterata were more susceptible than adults and field application of the chemical should be timed to avoid the larval feeding period. Phytotoxic symptoms appeared on Sitka spruce needles [Picea sitchensis] after application of sunflower oil soap, current-year needles being particularly susceptible to damage. Maize oil soap or SIS could be substituted during this susceptible period.