Comparison of the potential of organophosphorous insecticides and soaps in conifer aphid control

A comparison of the toxicity to two aphids, Elatobium abietinum and Adelges cooleyi, and a coccinellid predator, A. obliterata, of dimethoate, fenitrothion and malathion with Safer's insecticidal soap (SIS) and saponified corn oil and sunflower oil confirmed that the organophosphates were more toxic. This did not detract from the effectiveness of the soaps under field conditions provided that appropriately higher concentrations were applied. SIS was not more toxic to the insects than the saponified oils with corn oil soap, which is rich in oleic and linoleic acid, frequently producing higher insect mortality levels. In E. abietinum a linear relationship was demonstrated between insecticide susceptibility and aphid instar, with late instars apparently becoming progressively more susceptible. This relationship disappeared when LC₅₀ values were corrected for size differences. Larval Aphidecta obliterata were more susceptible than adults and field applications of these chemicals should be timed to avoid the larval feeding period. Phytotoxic symptoms appeared on Sitka spruce needles after application of sunflower oil soap, current year needles being particularly susceptible to damage. Corn oil soap or SIS could be substituted during this susceptible period.