GUT PROTEINASE ACTIVITY IN INSECT PESTS OF CANOLA

The digestive proteinases of three important pests of canola, Brassica napus L. and B. rapa L., in western Canada were characterized by assessing the proteolytic activity of homogenates of their midguts against azocasein or azoalbumin at various pH levels and in the presence of diagnostic proteinase inhibitors. The midgut of larvae of the bertha armyworm, Mamestra configurata Wlk., had maximum proteolytic activity at pH 10.5 which was inhibited 45–60% by serine proteinase inhibitors such as the soybean trypsin inhibitor. The midgut of larvae of the diamondback moth, Plutella xylostella L., had maximum proteolytic activity at pH 10 which was inhibited 56–75% by serine proteinase inhibitors. The two lepidopterans thus use a serine-like proteinase in digestion. The midgut of adults of the flea beetle, Phyllotreta cruciferae Goeze, exhibited maximum proteolytic activity at pH 5 which was inhibited 33–61% by specific cysteine proteinase inhibitors such as cystatin and trans-epoxysuccinyl-L-leucylamido (4-guanidino)-butane (E-64) and was activated strongly by L-cysteine. Aspartic proteinase inhibitors such as pepstatin A also decreased proteolytic activity by 21–50%. Serine proteinase inhibitors were without effect. Therefore, P. cruciferae appears to use both cysteine- and aspartic-like proteinases in digestion. Cotyledons and first true leaves of canola, B. napus cv. Westar, contained inhibitory activity against serine, cysteine, and aspartic proteinases when tested against bovine trypsin, papain, or porcine pepsin, but the level of antiproteinase activity is insufficient to provide significant resistance against any of these pests.