Spraying pesticides is what I do for a living. For me, this occurs mostly with potatoes. Because many of the pesticides I test are experimental, additional safety precautions may be necessary. If you drive by Miller Research on a summer morning, you may see my crew dressed in Tyvek suits driving our research sprayer back and forth across the rows of potatoes. We are usually dressed head to toe with protective gear designed to prevent us from being exposed to any unwanted chemical drift. This does not mean these sprays pose a danger to the consumer. The pesticides break down over time and we conduct research to ensure that residues are not present when it comes time to consume the crop.

Miller jeff
Owner and Researcher / Miller Research LLC / Rupert, Idaho

However, it would be nice to apply products that don’t require all the protective gear. Such products would make it safer for applicators. Additionally, it would be nice to apply products that only target the pathogens I want to limit, reducing the potential adverse effects of the pesticide on non-target organisms. Biological pesticides have the potential to do just that. Biological pesticides, or biocontrol agents, are generally defined as products that contain microbes that are designed to reduce the ability of pests to attack plants. For brevity’s sake, this article will focus on biological pesticides that cause disease.

Biological pesticides are living organisms themselves and are best used to prevent a pest from causing problems rather than to rescue a plant from a problem that has already started. Biological pesticides work in a number of ways.

  1. Direct competition – Organisms that cause disease in plants (pathogens) first need a spot to infect the plant. If all those spots are already filled, then the pathogens may be excluded. It’s similar to one person coming late to dinner and finding that all the seats are taken. Because the organisms in the biological product have taken all the seats, there are no chairs available for the pathogen. Some biological organisms can colonize root or leaf tissue, making it more difficult for the pathogen to find a seat at the table.
  2. Antibiosis – Some biological pesticides can be mean. They don’t just take away seats at the dinner table, they attempt to knock out the pathogen as it comes in the door. The organisms in the biological pesticide produce compounds that kill or inhibit the growth of pathogens.
  3. Predation or parasitism – The organisms in some biological pesticides simply eat the pathogens. These organisms aren’t so concerned about the plant – they just want to eat the pathogen.
  4. Induction of host resistance – Plants have evolved methods of fending off pathogens. For example, when a pathogen attacks a plant, the plant can produce compounds that are like warning sirens. These “sirens” cause the plant to use resources to ward off the infection. Biological pesticides can prime the system by ringing the alarm without the pathogen being present. That way, when the pathogen does show up, the defenses have already been primed.
  5. Plant growth promotion – Some biological pesticides can indirectly act like vitamins for the plant. The organisms in the biologicals may make nutrients in the soil more readily available to the plant. Since the plant can now extract more nutrients from the soil, it is healthier and better able to naturally fight off pathogens.

Reading this list could lead one to ask, “Why don’t we just use biologicals and toss out conventional pesticides?” There are several reasons. In field trials, biologicals are generally not as effective as conventional products. Biological pesticides suffer from higher variability in performance, can be slow to act, have a limited shelf life and may not persist very long after an application. Generally, using biologicals requires more work and more cost for the producer than using conventional pesticides.

In our research here in southern Idaho, we have tested many biological pesticides through the years. At first, our goal was to compare the biological and conventional products head to head. In these tests, the conventional products were always more effective. However, a “beauty pageant” type approach of comparing products on a stage is not completely fair. A better approach is to simply measure what the biological product can do compared to no action. With this approach, we have observed some benefits with biological pesticides.


As I have discussed in previous columns, too often agricultural products are oversold on their abilities, and I have seen this with some biological pesticides. Producers who want to use biologicals should try them on a limited basis to see if they can provide value to their operation. The potential benefits of lower toxicity and reduced impact on non-target organisms are very attractive. I would love to ditch the Tyvek suit on those summer days. I just need to be sure that doing so doesn’t leave me hanging with less disease control as a result.

Information in this article was adapted from “Biological Fungicides” produced by the Integrated Pest Management program in the department of plant science and landscape architecture at the University of Connecticut Extension.