PHYTOTOXICITY OF INSECTICIDES USED TO CONTROL APHIDS ON SITKA SPRUCE,PICEA-SITCHENSIS (BONG) CARR

Treatment of 2-4 year old Sitka spruce in the field with the insectici des dimethoate, malathion, pirimicarb, Pynosect (pyrethrum + resmethri n) and Savona (insecticidal soap) applied as foliage sprays, and diazi non, chlorpyrifos and malathion applied as soil drenches, at standard rates, revealed no adverse effects on tree growth or needle densities. However, Savona caused needle browning and trees treated with chlorpy rifos showed a 25% increase in height growth (P < 0.01) and a 13% incr ease in side shoot extension growth (P < 0.05) after 2 years compared with control trees. Trials on potted plants with insecticides applied at the standard rate (X1) or twice the standard rate (X2) revealed no effects on height growth for any compound by the end of the growing se ason, but showed that all the insecticides tested had some effect on p lant dry weight (DW) or needle density. Dimethoate and malathion at th e X1 rate increased shoot growth, whereas malathion at X2 and Pynosect at X1 and X2 were noticeably phytotoxic. Root DWs were not increased or were reduced more than shoot DW, which led to significant reduction s in root:shoot ratios. The low plant DWs in the Pynosect treatments w ere associated with a high rate of plant mortality (13-30%), whereas p lant survival in the other treatments was generally good. Needle densi ties on the new leader were reduced by the X2 pirimicarb treatment and by both rates of Pynosect and chlorpyrifos. Needle retention on the p revious year's leader was reduced by Pynosect acid chlorpyrifos. Indiv idual needles on the new leader were distinctly smaller on transplants sprayed with malathion or with Pynosect at the X2 rate, whereas needl es remaining on the previous year's leader were heavier on trees in th e X1 malathion treatment. Crown Copyright (C) 1996 Published by Elsevi er Science Ltd.