The effect of timing of application on the incompatibility of three fungicides (metalaxyl, mancozeb, copper oxide) and isolate MK2001 of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin was assessed in vitro. Isolate MK 2001 at 1 × 107 conidia/ml concentration was highly pathogenous to adults of Lygus lineolaris (Hemiptera) (84% mortality at day 4 post‐treatment). The fungicides at the manufacturer recommended rate (1X) were strongly fungistatic and inhibited the fungal isolate radial growth on Sabouraud dextrose agar (SDA). Mancozeb, metalaxyl and copper oxide exhibited insecticidal activity of 24, 40 and 48% respectively, on L. lineolaris adults. It is clear that application of the fungal isolate MK2001 2–4 days before applying fungicides metalaxyl, mancozeb or copper oxide synergized the insecticidal effect of the isolate. On the contrary, application of metalaxyl, mancozeb, copper oxide 2–4 days before applying isolate MK2001 antagonized the insecticidal effect. The simultaneous use of each fungicide (metalaxyl, mancozeb or copper oxide) and the isolate gave lesser insect mortality. This study revealed that although some fungicides are incompatible for use alongside fungal isolates, the proper evaluation of their time of use could be beneficial in biological control or IPM programmes. Furthermore, the application of B. bassiana isolate MK2001 followed by fungicide application could synergize insecticidal activity.

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Experiments were conducted under laboratory and field conditions to assess the effect of the fungicide Zineb (zinc dimethyl-dithiocarbamate) and the insecticide Cygon (dimethoate) on the pathogenicity of the isolate MK 2001 of Beauveria bassiana (Balsamo) Vuillemin, a potential microbial agent for biological control of the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois). The isolate was highly pathogenic to adults yielding an LC50 of 7.2 × 103 conidia/mL at 6 d post treatment and an LT50 of 3.07 d at a concentration of 1 × 108 conidia/mL. Zineb inhibited the in vitro growth of the fungus in Sabouraud's dextrose broth and on Sabouraud's dextrose agar. It had no insecticidal activity against adult L. lineolaris in laboratory bioassays. A field test with caged L. lineolaris adults on lettuce Lactuca sativa L. var. longifolia, at a phenological stage of 50 leaves further demonstrated that Zineb and Dimethoate had no effect on B. bassiana against L. lineolaris. Adult mortality 10 d after application did not differ significantly in treatments with B. bassiana alone from treatments with B. bassiana + Zineb, dimethoate alone, B. bassiana + dimethoate, B. bassiana + Zineb, and B. bassiana + Zineb + dimethoate. No larva was observed in cages treated with B. bassiana alone or in combination with Zineb or dimethoate. On the contrary, 11 larvae/plant were recorded in the untreated cages and 5 larvae/plant in the dimethoate-treated cages.