Microbial insecticides

Fungi are broadly associated microbial pathogens of insects. Beauveria bassiana is extensively used as a microbial insecticide in China and Eastern Europe, especially for the control of the com borer Ostrinia spp. However, fungi have had no significant development as microbial insecticides in the United States. 

C. M. Ignoffo, ed.. Handbook of Naturally Occurring Pesticides Microbial Insecticides, CRC Press, Boca Raton, Fla., 1986. 

Microbial insecticides are very complex materials in their final formulation, because they are produced by fermentation of a variety of natural products. For growth, the bacteria must be provided with a source of carbon, nitrogen, and mineral salts. Sufficient nutrient is provided to take the strain of choice through its life cycle to complete sporulation with concomitant parasporal body formation. Certain crystalliferous bacilli require sources of preformed vitamins and/or amino acids for growth. Media for growing these bacilli may vary from completely soluble, defined formulations, usable for bench scale work, to rich media containing insoluble constituents for production situations (10,27). Complex natural materials such as cottonseed, soybean, and fish meal are commonly used. In fact, one such commercial production method (25) is based on use of a semisolid medium, a bran, which becomes part of the final product. 

The second type of material includes spores, which may or may not produce disease symptoms but which can germinate in the insect gut and give rise to vegetative bacterial cells which in turn may produce, and exoenzymes such as phospholipases (lecithinases) or hyaluronidase. The phospholipases may produce direct toxic symptoms owing to their action on nervous or other phospholipid-containing tissue. Hyaluronidase breaks down hyaluronic acid and produces effects on animal tissue which are morphologically similar to the breakdown of insect gut wall in the presence of microbial insecticide preparations. 

Effects of Sesquiterpene Lactones on Seed Germination. Sesquiterpene lactones are common constituents of the Asteraceae but are also found in other angiosperm families and in certain liverworts (31,32). These highly bitter substances exhibit a wide spectrum of biological activities (J 3) which include cytotoxicity, anti-tumor, anti-microbial, insecticidal (34) and molluscicidal (35) properties. Furthermore, they are known causes for livestock poisoning and contact dermatitis in humans (33). Structure-activity relationship studies on sesquiterpene lactones have demonstrated that biological activity frequently depend on the presence of the cr... 

Akhurst RJ. Safety to non-target invertebrates of nematodes of economically important pests. In Laird M, Lacey LA, Davidson EW, editors. Safety of microbial insecticides. Boca Raton, FL CRC Press 1990. pp. 233-240. 

Siegel, J.P. and Shadduck, J.A. 1990. Safety of microbial insecticides to vertebrates-humans. In Safety of Microbial Insecticides (M. Laird, L.A. Lacey, and E.W. Davidson, eds), pp. 101-113. CRC Press, Boca Raton, FL. 

Van Rie. J.. ci al. Mechanism ol Insecr Resistance to the Microbial insecticide Bacillus ihuringicmiv." Stiomr, 72 iJanuary 5. 19901. 

This paper will focus on the baculoviruses (family Baculoviridae), a group of viruses reported as pathogens of many destructive insect species affecting agricultural and forest production. 1 will emphasize the role of these pathogens as introduced microbial insecticides or "living insecticides" (1). 

The control of scale insects, whiteflies, plant hoppers, aphids, chinch bugs, phytophagous beetles, flies, caterpillars, masquitoes, and stored product pests costs more than one-half billion dollars annually in the United States alone. In the last decade great progress was made in the commercial production of microbial insecticides which can partially replace synthetically prepared insecticides. 

In the first part of this chapter, we deal with insecticides including miticides and nematocides, which include very useful compounds such as avermectins and milbemycins, produced by bacteria and fungi. We list out microbial insecticides of importance and review the works mainly on the mode of action and biosynthesis of each metabolite. In the next part, major mycotoxins are listed and recent topics on them, especially on their biosynthesis, are described. Since contamination of two major mycotoxin groups, aflatoxins (AFs) and trichothecenes, in food and feed is a worldwide problem, they are treated in detail in the last part of this chapter. Recent studies on their biosynthesis, regulatory mechanism for their production, and inhibitors of their production are described. 

Yu and Nguyen (1996) showed that selection of a strain of diamondback moth (Plu-tella xylostella) with permethrin for 21 generations resulted in over 600-fold resistance to permethrin in this strain. The resistant strain was also cross-resistant to all pyrethroids tested, including bifenthrin, fenvalerate, esfenvalerate, A.-cyhalothrin, fluvalinate, and tral-omethrin. However, it remained susceptible to organophosphate, carbamate, cyclodiene, neonicotinoid, avermectin, and microbial insecticides tested. Biochemical studies indicated that pyrethroid resistance observed in this strain was most likely due to decreased target site sensitivity. 

Although it was once thought that insects would not develop resistance to microbial insecticides, scientists have confirmed cases of diamondback moths in commercial farms that showed resistance. This is another argument for using BT with care, and only after trying cultural or physical control methods for which resistance is not a factor. 

Joung K-B and Cote J-C (2000) A review of the environmental impacts of the microbial insecticide Bacillus thuringiensis. Technical Bulletin No. 29. Horticulture... 

Figure 1. Foliar persistence of microbial insecticides on cabbage. Arlington, Wisconsin 1988. Dipel and Javelin are commercial products based on Bt var. kurstaki. Persistence was measured by bioassay of field treated foliage, removed at various times post-application against 3rd instar DBM larvae. Rates of product applied are expressed in terms of grams of delta endotoxin (as measured by polyacrilamide gel electrophoresis) applied per acre.

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