The efficacy of RH-7988 against green peach aphid (Myzus persicae (Sulzer)) and cabbage aphid (Brevicoryne brassicae (L.)) on cabbages.

Abstract

Green peach aphid (Myzus persicae (Sulzer))and cabbage aphid (Brevicoryne brassicae (L.)) are pests of brassicae due to their ability to reduce the quantity and quality of yield through direct and indirect damage. Problems with resistance to current aphicides and consumer demand for selective pesticides has led to the need for new aphicides. Towards this end, field and laboratory experiments were set up to compare a novel aphicide (RH-7988) with a currently registered aphicide (pirimicarb), for aphid control in cabbages.

Three field trials were conducted using a 5x5 latin square design. The number, size and species of aphid colonies per plant was recorded at intervals after treatment. In laboratory experiments the LC50 and temperature/toxicity relationships of RH-7988 and pirimicarb were determined using aphids treated on leaf discs. To investigate the residual activity of RH-7988 aphids were caged on to treated cabbages and mortality was assessed 24 and 48 hours after treatment. The toxicity of RH-7988 to several natural enemies was determined by exposing insects to fresh (wet) spray residues.

No phytotoxic effects were recorded on cabbages sprayed With RH-7988 plus surfactants (Triton B1956, Triton AG98, Sunspray 6E and Citowett). RH-7988 was equivalent to pirimicarb in controlling aphids on cabbages in the first 14 days after treatment (aphid populations were < 20% that of the control treatment). After this time (up to 35 days after treatment) aphid populations on RH-7988 treated plants were, on average, 38% that of populations on plants treated with pirimicarb.

There was no significant difference between the LC50 values of both aphicides when tested against each aphid species. The toxicity of RH -7988 to the two aphid species did not change significantly between 10 and 300C and were not significantly different to pirimicarb. The aphicides had a significant residual effect (cf. control treatment) on cabbage aphid and green peach aphid for 5 and 10 days after treatment, respectively. RH-7988 also reduced aphid numbers on leaves that emerged subsequent to aphicide application (between 18 and 23 days after treatment). The recommended field rate of RH-7988 (100 g a.i./ha) was more toxic towards the two aphid species (100% mortality) than to several natural enemy species and life stages (corrected means ranging from 0-59.3%). RH-7988 has good potential to be included as a aphicide for use on brassicas. Excellent control is achieved for periods longer than pirimicarb and RH-7988 is selectively more toxic to aphids than to some of their common natural enemies.