Insecticides

INSECTICIDES

Like animals, insects are susceptible to infections, which may be caused by viruses, fungi, bacteria or protozoa. The use of microorganisms to spread diseases to particular insect pests offers an attractive method of biological control, particularly in view of the ever-increasing incidence of resistance to chemical insecticides. However, any microorganism used in this way must be highly virulent, specific for the target pest but non-pathogenic to animals, humans or plants. It must be economical to produce, stable on storage and preferably rapidly acting. Bacterial and viral pathogens have so far shown the most promise. Perhaps the best studied, commercially available insecticidal agent is B. thuringiensis. This insect pathogen contains two toxins of major importance. The δ-endotoxin is a protein present inside the bacterial cell as a crystalline inclusion within the spore case. This toxin is primarily active against the larvae of lepidopteran insects (moths and butterflies). Its mechanism of action is summarized in Figure 26.11. Commercially available preparations of B. thuringiensis are spore–crystal mixtures prepared as dusting powders. They are used primarily to protect commercial crops from destruction by caterpillars and are surprisingly non-toxic to humans and animals. Although the currently available preparation has a rather narrow spectrum of activity, a variant B. thuringiensis strain has been isolated and found to produce a different δ-endotoxin with activity against coleopteran insects (beetles) rather than lepidopteran or dipteran (flies and mosquitoes) insects.

The second B. thuringiensis toxin, the β-exotoxin, has a much broader spectrum encompassing the Lepidoptera, Coleoptera and Diptera. It is an adenine nucleotide, probably an ATP analogue that acts by competitively inhibiting enzymes that catalyse the hydrolysis of ATP and pyrophosphate. However, this compound is toxic when administered to mammals, so commercial preparations of the B. thuringiensis δ-endotoxin are obtained from strains that do not produce the β-exotoxin.

Strains of B. sphaericus pathogenic to mosquitoes were isolated several years ago. Strains of this organism with increased toxicity to mosquitoes have been isolated and might have considerable potential as control agents. Commercially available formulations of B. sphaericus are now available as larvicides.

Other insect pathogens are being evaluated for activity against insects that are vectors for diseases such as sleeping sickness, as well as those that cause damage to crops. Viruses may well have the greatest potential for insect control as they are host-specific and highly virulent, and one infected insect can release vast numbers of virus particles into the environment. They have already been used with considerable success against the spruce sawfly and pine moth.