Toxins, insect control

Insects have evolved resistance to Bt toxins in the laboratory, yet only one crop pest, the diamondback moth (Plutella xylostella), has evolved resistance to Bt toxins under open field conditions (Tabashnik et al. 2003). But this resistance was not caused by Bt crops, rather it occurred in response to repeated foliar sprays of Bt toxins to control this pest on conventional (non-GE) vegetable crops (Tabashnik 1994). Based partly on the experience with diamondback moth and because Bt crops cause season-long exposure of target insects to Bt toxins, some scientists predicted that pest resistance to Bt crops would occur in a few years. 

F. Maggio B. L. Sollod H. W. Tedford G. F. King, Spider Toxins and their Potential for Insect Control. In Comprehensive Molecular Insect Science L. I. Gilbert, K. latrou, S. S. Gill, Eds. Elsevier Oxford, 2005 Vol. 5, pp 221-238. 

This protein has a billion-fold safety factor for humans and is acceptable for engineering into plants for insect control. The Farm Chemicals Handbook contains this statement about the B.t. toxin "Harmless to humans, animals and useful insects. Safe for the environment. Exempt from requirements for a tolerance on all raw agricultural commodities when applied to growing crops, for both preharvest and postharvest uses" (8). 

Similar tests have been conducted with a coleopterous-active MCap product (MYX 1806) targeted against the CPB on potatoes, hi these tests, potato foliar protection and yields were compared for MYX 1806 and its naturally occurring counterpart, Bt variety son diego (M-One insecticide) (Figures 4 and 5). The results confirm that when equivalent toxin rates were applied for each product, the persistence conferred by the MCap system resulted in superior CPB control. Additionally, it was demonstrated that the protected toxin of MYX 1806 could be applied at two thirds the toxin rate, and yet still maintain higher levels of insect control than the full rate delivered by M-One. 

A new class of insecticidal toxins called vegetative insecticidal proteins has recently been isolated from Bt. They are produced during the vegetative growth stage. The proteins are different from other known proteins, and their function and versatility for insect control have yet to be elucidated. 

The natural ecosystem maintains a delicate balance between pests and predators. Pest insects can be controlled by the artificial release of predators. One example is a parasitic wasp, Diadegma insulare. The adult female wasp lays eggs in a Plutella xylostella larva and pupates inside the cocoon of the mature larva. This and other insect predators are available commercially, but the usage is limited. Protozoa and nematodes are also used in insect pest control. One example of a protozoan that effectively infects locusts and controls the population is Nosema locustae. A commercially available nematode insect control agent is Steinemema carpocapsae. This nematode parasitizes scarab larvae with a symbiotic Photorhabdus bacterium that produces insecticidal toxins. 

The surfaces of insects are also covered by a layer of wax. Insect cuticular waxes are also involved in various types of chemical communication between individuals of a species and reduce the penetration of chemicals and toxins as well as infectious microorganisms. Analyses and identification of insect waxes is the first step towards developing methods of insect control. 

Certainly, in the case of the insecticides, past experience has shown this approach to be successful, with synthetic pyrethroids as the best example. Other commercially useful botanical pesticides include nicotine, pyrethrum, rotenone and several other alkaloids. Similarly, other natural compounds modifying feeding behavior or inhibiting the growth of insect larvae, are considered viable alternatives to acute toxins, for insect control (3). One of the most successful examples so far is Neem, extracted from the seeds of the tree Azadirachta indica. 

An immense amount of research on representative 8-endotoxins has been devoted to understanding the mode of action on susceptible insects. In general, following ingestion, the crystalline inclusions are dissolved and then converted to active toxins by insect proteases. The active toxins bind to specific receptor sites and produce pores in the insect gut which results in loss of homeostasis and septicemia, which are lethal to the insect (Broderick et al., 2006). In addition, there may be other less-characterised insect control functions of these toxins such as avoidance of the toxins and feeding paralysis prior to completion of the full lethal pore-formation process (Aronson and Shai, 2001). In many cases, larvae become less susceptible to 8-endotoxins as they age due to fewer binding sites in the older larvae (Gilliland et al., 2002). 

B. thurigiensis is a common Gram-positive, spore-forming soil bacterium that produces inclusion bodies, microcrystalline clusters of many different proteins. These crystalline proteins, called 5-endotoxins, are the ion channel toxins that are sold commercially for pest control. Most such endotoxins are protoxins, which are inactive until cleaved to smaller, active proteins by proteases in the gut of a susceptible insect. One such crystalline protoxin. 

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