TOXICITY OF MICROCYSTIN LR, ISOLATED FROM MICROCYSTIS-AERUGINOSA, AGAINST VARIOUS INSECT SPECIES

Microcystin-LR (MC-LR), isolated from the cyanobacterium Microcystis aeruginosa Kuetzing emend. Elenkin strain CCAP 1450/4 was tested for biological activity against four species of insect and the invertebrate Artemia salina. The efficacy of pesticidal activity was compared with various insecticides. The 24 hr LD(50) value for third instar diamond-backed moth, Plutella xylostella, on ingestion from a treated leaf surface was 1.0 mu g cm(2), compared with a 72 hr LD(50) value for rotenone of 2.0 mu g cm(-2). The 24 hr LD(50) values of MC-LR and malathion on intrathoracic injection into adult house flies (Musca domestica) were 0.5 and 3.7 mg kg(-1), respectively. MC-LR had no effect on M. domestica when applied topically at dosages up to 32 mg kg(-1). MC-LR and malathion gave 24 hr LD(50) values of 4.7 and 13.1 mg kg(-1), respectively when injected into third instar cotton leafworm (Spodoptera littoralis). In fourth instar cabbage white butterfly larvae (Pieris brassicae) MC-LR injected gave 24 and 48 hr LD(50) values of 3.9 and 1.9 mg kg(-1), respectively, whilst the 24 and 48 hr LD(50) values for carbofuran were 0.4 and 0.3 mg kg(-1), respectively. An immersion bioassay with 1-day-old brine shrimp larvae (Artemia salina) gave 24 hr LD(50) values of 3.8 mu g ml(-1) for MC-LR and 1.8 mu g ml(-1) for carbofuran. MC-LR has appreciable insect toxicity, comparable to the three insecticides tested. The toxin look 24-48 hr to exert its full lethal effect in insects, much longer than the 1-3 hr it takes in mammals. The potential use of MC-LR as an insecticide is discussed.