agriculture, home gardens, IPM, ornamentals, vegetables

But does it work? Efficacy of biological control

yellow sticky card for monitoring insect pests in a greenhouse
Biocontrol should be preventative. This yellow sticky card lets you monitor which pests are present and when so that the correct biocontrol (or other management tool) can be used at the right time. It will also help you assess whether your management strategy is working.

If you are thinking about trying biological control, of course you want to know if it is effective. The short answer is, “Yes!” But of course it depends on which biocontrol organism you want to use (and how), which pest you want to manage, and where.

First, you should ask yourself a question: What do I hope to achieve? Some great reasons to use biocontrol for pest management include:

  • Protecting the environment and human health by using more environmentally-friendly pest management strategies
  • Reducing the number of chemical pesticide applications to a crop
  • Preventing (or dealing with) pesticide resistance
  • Meeting a need for a short REI (re-entry interval) or PHI (pre-harvest interval) on the crop
  • Biocontrols are the most effective (and cost-effective) management strategy (definitely true for some pests and settings!)

Second, in what context are you using biocontrol? Biocontrol is best used within a larger integrated pest management strategy. Are you using good sanitation and cultural practices (e.g., adequate but not excessive nutrition and water) that promote healthy plants? Are you regularly checking your plants so that you will notice pests when they are still infrequent (scouting)?

peppers in flooded field
Biocontrol should be used as part of an IPM plan. The excess rain and poorly-drained soil in this field make it unlikely that a biocontrol will be able to protect these peppers from a soil-borne disease.

Biocontrol should also be preventative (before pest pressure becomes high). If you are expecting to use only biological control to solve an already out-of-control pest problem, you will probably be disappointed. Similarly, if environmental conditions are very favorable for a pest, a biocontrol solution will probably be insufficient.

Each year, university researchers, extension staff, and private companies conduct efficacy trials to quantify how well pest management strategies work. Knowing how a biocontrol product/organism performed in these trials can help you decide if you want to try it on your farm or in your yard. It helps to know a little about how these trials are structured. Efficacy trials typically include some combination of the following types of control treatments:

  • non-treated control – plants are exposed to pests (either naturally, or deliberately by the researcher), but no pest management strategy is used; disease/damage severity should be highest in this treatment
  • chemical control – plants are exposed to pests, and a chemical pesticide is applied to manage the pest; sometimes an “industry standard” (what is typically used to manage that pest in that crop and setting) is designated by the author of the study; if no industry standard is designated, comparisons can still be made to the chemical treatment that worked best; disease/damage severity should be very low in this treatment
  • non-inoculated control – no pest pressure (i.e., plants were not deliberately exposed to the pest); sometimes disease or damage still occurs because of natural pest pressure, or because disease or insects spread from other treatments in the trial; disease/damage severity should be lowest in this treatment

Efficacy trials also include statistical analysis. In a nutshell, this analysis tells you whether two values are really different (often described as “statistically different”), or not. If two numbers are not statistically different from each other, it means that only by chance is one larger or smaller than the other. If you did the same experiment again, you might see a different relationship. One common way of expressing these differences is by using letters. If two treatments are assigned the same letter, then they are not statistically different. So in the example below, Bio1 is statistically different from Bio3 but neither Bio1 nor Bio3 is different from Bio2.

graph of sample efficacy trial results
An efficacy trial may compare biocontrol products (Bio1, Bio2, Bio3, and Bio4) to non-treated (high disease/damage), chemical (low disease/damage), and non-inoculated (low disease/damage) controls.

 

When interpreting an efficacy trial, you should compare a biocontrol of interest to the control treatments. Of course, it would be great to see biocontrol products that are just as effective as the chemical control (like Bio4), and sometimes they are. Sometimes, a biocontrol may be less effective than the chemical control, but more effective than taking no pest control action (like Bio3). Sometimes there’s so much variability (represented by the lines extending above and below the blue bars on the chart, called error bars), that a biocontrol product is not statistically different from either the non-treated control, or the chemical control (like Bio2). This makes it difficult to draw conclusions about how well the product worked.

But, it’s not always quite that simple. For example, in these efficacy trials, researchers deliberately expose plants to pests, and often they manipulate the environment to favor pest populations. For example, they might over-water plants to promote a soil-borne disease like damping off. While there can be value in assessing product efficacy in a “worst case scenario”, this may be much higher pest pressure than you are likely to encounter on your farm or in your yard. When looking at efficacy trials, you should consider:

  • How much disease/damage was observed on plants that were not protected in any way (non-treated control)? If it’s too low, it’s hard to be confident that the biocontrols being tested were effective, since even unprotected plants were pretty healthy.
  • How much did the most successful treatment (chemical control) reduce disease/damage? If even the “best” pest management strategy in the trial was not very effective, then pest pressure may have been too high, and it’s not surprising that the biocontrol was ineffective. If you practice good IPM, you likely won’t experience such high pest pressure.
  • How was the biocontrol applied (alone, or as part of a spray program with other products)? Applying single products in an efficacy trial can simplify interpretation, but may not mimic how you plan to use a biocontrol product. If a biocontrol was applied in combination with other products, you should compare the “biocontrol + other products” treatment to the “other products alone”  treatment to see what the biocontrol added to pest management.
  • What was your goal, again? For example, if you are hoping to replace one or two chemical applications in a larger spray program with a biocontrol, then a moderately effective biocontrol product (like Bio3) may meet this goal.

Because the efficacy of a biocontrol can depend a lot on the environment in which it is used (temperature, humidity, soil conditions, etc.), it’s also a good idea to initially try a new biocontrol in a small area of your farm or yard, and keep notes on what you did and how well it worked for you. You can modify your plan to find what works best for you. The manufacturer or distributor should be able to provide you with important details on how (and for how long) the biocontrol should be stored, and exactly how and when to apply it. And (as always!) if you are using a biocontrol that is also a pesticide (see previous post), make sure that you read, understand, and follow the label.

The following resources summarize efficacy results for biocontrol of plant diseases. As I find efficacy summaries of insect and mite pest biocontrol, I will add them. Or, feel free to suggest efficacy resources you know of in the comments!