Are Biocontrols at Risk for Resistance? By Kerstin P. Ouellet

As biocontrols increase in popularity and frequency of use beyond organic crops, some growers may be wondering if resistance is a possibility.

Antibiotic resistance is consistently in the news, and new instances of crop chemical resistance are reported almost as often. But the difference, experts say, is in how microbial biopesticides work — and that’s what makes resistance so unlikely.

“Resistance is not an issue with microbial pesticides because of the multiple modes of action [they use against] the pest/pathogen,” says Kevin Ong, professor and Extension plant pathologist, Texas A&M AgriLife Extension Service, and director, Texas Plant Disease Diagnostic Laboratory, College Station, Texas.

Microbial pesticides use a microorganism (bacterium, fungus, virus, protozoan) as the active ingredient. Conventional synthetic pesticides are generally man-made chemicals. Resistance develops as a factor of a product’s mode or modes of action — how it attacks a pathogen or insect. While most synthetic pesticides act via a single mode of action, most microbial biopesticides offer multiple modes of action, and they are biological.

“Resistance results from an alteration at the site of action in the target pathogen. Chemicals with a single site of action are characterized as having higher risk than those with multiple sites of action. Most microbial biopesticides offer multiple modes of action,” explains Anissa Poleatewich, assistant professor of plant pathology and host-microbe interactions, University of New Hampshire. “Microbial biopesticides are generally considered to have low risk for developing resistance.” In other words, when a pest adapts to find a way around one mode of action, microbial biopesticides still have other modes of action to disrupt the pathogen.

Modes of Action Can Include:

  • Competition: the biocontrol literally blocks the offender out, leaving no space for it to develop.
  • Plant defense activation: the biocontrol activates the plant’s internal defense system.
  • Parasitism or predation: the biocontrol consumes or parasitizes detrimental organisms.
  • Secondary metabolites: the biocontrol releases antagonistic metabolites that work against detrimental organisms.

“Microbial biopesticide organisms are selected from thousands for their modes of action,” says Matthew Krause, product development manager at Bioworks. “Trichoderma and Bacillus are good examples. You can find them anywhere. They’re primary decomposers of soil, but very few actually break down disease. Manufacturers have to select from thousands of strains to find the ones that are the most effective.”

The interaction between host and biocontrol is very broad and complex, not simple lock and key. In contrast, synthetic chemicals have molecules that work by targeting specific receptor sites in offending organisms. The interaction of a chemical molecule is very much a lock-and-key system — part of the molecule has to fit into a very specific receptor and then act, and a small change in that lock would change the fit of the chemical molecules.
“Such products, when used repeatedly, can lead to resistance problems, as seen with chemical pesticides,” says Surendra Dara, Extension entomologist, UC Cooperative Extension, San Luis Obispo, California. “Microbial pesticides on the other hand, do not have such a risk, as the infection process is longer and more complex than the activity of a [chemical] molecule.”

How Do Pests Develop Resistance to Pesticides

Two major factors determine whether a pathogen or insect develops resistance to a product: selection pressure and adaptiveness. Like many living organisms, pathogens constantly change and evolve, adapting to survive in their current environment. In the same way, “mutations in an insect can alter the target site, rendering the toxic [chemical] protein not effective or less harmful,” Dara says.

Different pathogens and insects in a population have different levels of sensitivity to a chemical. The stronger, resistant individuals will survive after each application. Weaker individuals will eventually be killed off, leaving
a largely resistant population. After a few generations, a grower may see declining control.

When growers use the same chemical pesticides repeatedly, normal pests die, leaving the adapted, mutant individuals to thrive and increase their population. “This is why it is important to use chemical and non-chemical control options and rotate [pesticides] that have different modes of action to reduce the selection pressure and limit the buildup of resistant populations,” Dara explains.

With biopesticides the approach is different. “In nature an insect usually encounters only a few infectious spores, and its defenses have evolved to combat those few microbes,” says Stefan Jaronski, research entomologist, USDA ARS Northern Plains Agricultural Research Laboratory, Sidney, Montana. “In biopesticide use, we overwhelm the insect with thousands of spores per insect either from direct spray or from accumulation of spores as the insect moves and feeds on treated foliage. Enough troops, and you overwhelm the defenses.”

What Can Mimic Resistance?

If a grower using biocontrols experiences a perceived decline in control after successful use, it’s important to examine other potential factors in play. Dara suggests a few to consider include compatibility with adjuvants, other tank mix materials, shelf life, time of application, application strategies and using against a non-target pest.

For example, environment can impact the activity of both the pathogen and the microbial biopesticide. “If growing outdoors, how has the weather changed regarding temperature and rainfall? In the greenhouse, seasonal variations in humidity, ambient temperature, irrigation water temperature and substrate temperature will affect microbial activity,” says Poleatewich.

Increasing soil moisture can also increase disease pressure. Overwatering stresses plants by decreasing available oxygen in the root zone, and stress is a well-documented risk for disease development. “Personnel changes occur in the greenhouse,” says John Francis, technical services manager at Bioworks. “Newer applicators may not understand biopesticides well enough to ensure proper spray coverage, resulting in less than optimum control.”

Also, because microbial biopesticides are living organisms, they tend to have a shorter shelf life than other pesticides. Microbial biopesticides have specific storage requirements; for example, some need to be kept cold until used. It’s essential to check expiration dates, experts caution.

Two additional factors affecting efficacy: location and contact. Many biologicals, in particular the fungi, are contact agents. For example, fungus spores especially must land on insect cuticles to be effective, so applying a mycoinsecticide from above plants does no good, when the whitefly immatures are on leaf undersides. Viruses and bacteria have to be eaten by an insect, but still enough have to be placed “under the insect’s mouth” to kill that insect. So location is still important.

As for contact, continuing with our whitefly example, adults oviposit on younger leaves, which means their location constantly shifts, so multiple applications should be made to different locations.

“Identification is also very important — a very accurate diagnosis is essential,” says Debbie Palumbo-Sanders, technical services specialist at Bioworks. “For green peach aphid you may bring in a banker plant system with parasitic wasps, but if you get a foxglove aphid population, they may not be able to parasitize it. It’s very specific predator-prey.”

For all these reasons, simply put, “resistance is unlikely,” says Jaronski. With multiple modes of action and the physical nature of how they control a pest/pathogen, biopesticides will continue to prevent and control given proper use.

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