EFFECT OF THE SYSTEMIC FUNGICIDE BENOMYL ON THE SYMBIONTS AND MYCETOCYTES OF THE BIRD CHERRY-OAT APHID (RHOPALOSIPHUM-PADI) (HOMOPTERA, APHIDIDAE) REARED ON WHEAT PLANTS

Only a few symbionts and mycetocytes in the bird cherry-oat aphid Rhop alosiphum padi (Linnaeus) reared for three days were affected on plant s treated with 50 ppm benomyl. The symbionts of aphids reared for the same time on plants treated with 100 ppm benomyl, however, were abnorm al and at an early stage of degeneration. Mycetocytes contained more r esidual bodies than normal. The symbionts and mycetocytes of embryonic aphids on such plants were also abnormal. When aphids were reared for 3 days on plants treated with 200 ppm benomyl, the symbionts and myce tocytes were drastically affected. Most of the symbionts were very abn ormal and had degenerated. The mycetocytes were full of electron-lucen t and electron-dense structures intermixed with abnormal cell organell es. Symbionts and mycetocytes of embryonic aphids were abnormal. In co ntrast, when the systemic insecticide pirimicarb was used to kill aphi ds, it did not affect the symbionts and mycetocytes. When aphids were reared for 6 days on plants treated with 50 ppm benomyl, their symbion ts and mycetocytes and those of their offspring were also degenerate a nd abnormal, as were those of F1 generation aphids from mothers reared from birth on plants treated with 50 ppm benomyl. From a consideratio n of the known mode of action of benomyl on tubulin and the presence o f tubulin only in the mycetocyte cells, it is concluded that the degen eration of symbionts in benomyl-treated aphids is a secondary conseque nce of the degeneration of the aphid mycetocytes.