Biopesticides as tools in resistance management. Fruit Grower News, May 2019.

Michael Dimock, PhD, VP of Field Development, Certis USA

Scott Ockey, Field Development Manager, Certis USA

         The single most contributing factor to pesticide resistance is repeated exposure to the same or related ingredients, or modes of action. When populations of insects, mites, disease-causing fungi and bacteria are continuously exposed to pesticide active ingredients (AI) targeting the same or very similar metabolic target sites, there is a high probability that resistance will develop to that type of product.

         Natural populations of pests or pathogens include a few individuals that, through natural mutation, carry genes that confer some capability of tolerating the effects of a particular AI or mode of action (MoA). Repeated exposure to that AI exerts an artificial selection pressure that favors survival and reproduction of these individuals. Under continuous selection, resistance becomes more prevalent in each generation, and can eventually result in complete control failure. Often, resistance is due to biochemical changes in the metabolic target site of the pesticide MoA which may confer cross-resistance to other AI’s targeting the same or very similar target sites. Thus, resistance to one AI may place an entire class of pesticides at risk of failure due to resistance.

         Most biopesticides have multiple or nonspecific modes of action that don’t target a single metabolic site or gene. Even if pests are resistant to a single MoA, it is unlikely they will have cross resistance to a biopesticide that has multiple MoAs or target sites.

         One example is the naturally occurring fungus Beauveria bassiana, which infects and kills insects and mites. Spores of this fungus act as a contact mycoinsecticide with a MoA unlike chemical or botanical (extracts or oils) insecticides. After spores germinate upon contact with a host, the fungus penetrates the exoskeleton through a combination of enzymatic action, hydrostatic pressure and other biochemical mechanisms. The fungus grows within the host’s body and kills it within a few days. Because it acts by infection rather than targeting a single biochemical process, B. bassiana is able to control insects resistant to chemical insecticides with little or no risk of cross-resistance. In some cases, fungus and chemical insecticide may actually act synergistically as insects weakened by one AI become more susceptible to the other.

         Some microbial control agents are “plant activators,” stimulating a plant to increase its own natural defenses against disease in a process known as induced resistance or IR. These defenses include elevated levels of enzymes and antimicrobial proteins with nonspecific effects on basic functions such as growth and cell wall formation or pathogenic fungi and bacteria. Fungicides can be more effective when the crop to which they are applied has some level of resistance to infection, whether due to selective breeding or inducible defenses. Research at Montana State University has shown that the microbial plant activator Bacillus mycoides isolate J applied to sugar beets boots plant resistance to Cercospora leaf spot in a way that also renders the pathogenic fungus more easily controlled by triazole class of fungicides to which it has developed resistance. Two very different modes of actions deliver a “one-two punch” that slows further development of pathogen resistance to a single target site MoA.

         Bacillus thuringiensis (Bt) remains a stalwart microbial insecticide, with more than 60 years of commercial use in the USA for control of larvae of Lepidoptera (caterpillars), mosquitoes, blackflies, and several other types of insect pests. In this time, Bts have not shown cross-resistance with synthetic insecticides and are frequently used in programs with these products as a resistance management tactic.

         Because biopesticides rely on multiple, unique, and nonspecific modes of action, they should be considered essential components of effective resistance management programs.