Insecticides with novel modes of action: Mechanism, selectivity and cross-resistance

Efforts have been made during the past two decades to develop insecticides with selective properties that act specifically on biochemical sites present in particular insect groups, but whose properties differ from other insecticides. This approach has led to the discovery of compounds that affect the hormonal regulation of molting and developmental processes in insects; for example, ecdysone agonists, juvenile hormone mimics and chitin synthesis inhibitors. In addition, compounds that selectively interact with the insect nicotinic acetylcholine receptor, such as imidacloprid, acetamiprid and thiamethoxam, have been introduced for the control of aphids, whiteflies and other insect species. Natural products acting selectively on insect pests, such as avermectins, spinosad and azadirachtin, have been introduced for controlling selected groups of insect pests. Compounds acting on the nervous site that controls the sucking pump of aphids and whiteflies, such as pymetrozine, or respiration, such as diafenthiuron, have been introduced for controlling sucking pests. All the above compounds are important components in pest and resistance management programs.     

Oxidative Enzymes, Ribonuclease, and Amylase in Lemon Buds Infested with Aceria sheldoni (Ewing) (Acarina: Eriophyidae)

In lemon buds the optimum for peroxidase activity was a reactionmixture of 0.03 M guiacol, 0.1 per cent enzyme (acetone powder),and 0.0086 per cent H²O², in 0.015 M acetate buffer pH 4.0.For polyphenoloxidase activity the optimum was a reaction mixtureof 0.3 per cent catechol and 0.025 per cent enzyme (acetonepowder), in 0.05 M phosphate buffer pH 7.0. Both enzymes werestrongly affected by mite infestation. The activity of polyphenoloxidaseand peroxidase in infested buds reaches respectively two andthree times that in uninfested buds. There are indications thatthe enhanced oxidative activity and corresponding increase inphenol level in the infested buds is part of a defence systemwhich may develop in the plant after infestation. The optimum for RNAase activity was a reaction mixture of 0.06per cent RNA and 0.02 per cent purified enzyme, in 0.035 M sodiumpotassium-buffer pH 5.6. An increase of about 30 per cent inRNAase activity occurred after mite infestation. Amylase activitywas affected slightly only after a heavy infestation.     

Growth Accelerators and Inhibitors in Lemon Buds Infested by Aceria sheldoni (Ewing) (Acarina: Eriophyidae)

The biological activities of growth accelerators and inhibitorsin lemon buds both infested and uninfested with the citrus budmite, Aceria sheldoni (Ewing), were examined by the Avena test.The growth-promoting activity of the purified ether extractcontaining the auxin was diminished in proportion to the degreeof infestation. Paper chromatography fractions of extracts obtainedfrom infested buds exhibited a strong inhibiting activity onAvena coleoptile elongation in the presence of 0.1 ppm indol-3yl-aceticacid. This inhibiting activity seems to be due to the presenceof a high level of phenolic compounds. The phenol level andits biological activity in both infested and uninfested budswere examined. Infested buds exhibited a higher amount of phenolsthan uninfested ones, the level of the phenols increasing withbud age. The phenols from infested buds inhibited Avena coleoptileelongation to a much greater extent than those from uninfestedbuds. Our results suggest that the presence of the citrus budmite in lemon buds can increase the level of phenols and possiblyother growth depressors in the buds, and that they, in turn,may cause the observed inhibition of growth and developmentof the infested citrus plant.