CONTROL OF PAINTED LADY CATERPILLARS, VANESSA-CARDUI L (LEP, NYMPHALIDAE), ON BORAGE BY BACILLUS-THURINGIENSIS VAR KURSTAKI

In the summer of 1991 in the Saskatoon and surrounding area, borage (B orago officinalis), a speciality crop of Saskatchewan, was found heavi ly infested by Painted Lady caterpillar, Vanessa cardui L. Laboratory and field trials with Bacillus thuringiensis var. kurstaki Berliner (B tk; Dipel wettable powder) at 0.625 g/l showed efficacy against differ ent developmental stages of Painted Lady caterpillars. Treatment effec ts were assessed in terms of arrested feeding of the larvae, survival of caterpillars of different size cohorts (I, II, III according to siz e distribution), pupae and adult emergence. Efficacy against caterpill ars was assessed using four methods of treatment: 1. individual or; 2. a group of caterpillars in the laboratory; .3. caterpillars in the fi eld contained with a mesh enclosure; ana 4. field plot treatment. In l aboratory bioassays with borage leaf discs or while plants treated wit h Btk, the mortality of the smallest (< 1.25 cm, I) caterpillars was h igher than that of II (< 2 cm) or III (> 2 cm) caterpillars. Whereas u ntreated borage leaf discs fed to caterpillars were usually totally co nsumed within 2-4 h, 95 % Btk-treated discs were left uneaten even aft er 24 h. No adult emergence was observed from the pupae collected from caterpillars which were fed Btk-treated discs. Field application of B tk on borage resulted in a 4 to 8.5-fold decline in the Painted Lady c aterpillar population from 8 to 15 days post treatment. As borage plan t seeds are a rich source for gamma-linolenic acid which is used thera peutically for treatment of essential fatty acid deficiency in humans, it cannot be sprayed with chemical-pesticides when fully blossomed. W e therefore conclude that the use of Btk offers an acceptable, effecti ve non-chemical method to control Painted Lady caterpillar.