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July 1, 2011

Biopesticides Come of Age

Cliff Ohmart, Wines & Vines
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For as long as pesticides have been used—and particularly since the invention of synthetic pesticides—there has been great interest in developing active ingredients that have minimal impact on non-target organisms. An active ingredient in a pesticide is the material that kills the pest. The other materials, inert ingredients, do not affect the pest but are in the pesticide formula to make it stable in the environment, mix well with water for spraying, etc. Finding active ingredients that do not negatively impact non-target organisms has proven to be a real challenge.

Historically speaking, most of the synthetic pesticides that were very effective in killing pests have had significant negative impacts on non-target organisms. Examples are the organochlorine insecticides like DDT and the organophosphates like Lorsban and Guthion. Often their efficacy was due not only to their direct toxicity to the pest, but also their residual effect (they remained toxic in the environment for an extended period.) While this was great in terms of pest control, it was not so great for non-target organisms.

As the need and demand for reduced-risk pesticides grew, the chemical industry responded by developing some active ingredients with much lower impacts on non-target organisms. Scientists discovered that certain naturally derived chemicals also could control pests. This class of materials became known as biopesticides, and they are playing an increasingly important role in pest management. In some circles, however, their efficacy is viewed with skepticism. I thought I would devote a few paragraphs to this important class of pesticides in an attempt to clear up some misunderstandings about them.

Biopesticides are naturally occurring chemicals or micro-organisms that control pests through various modes of action. Some people use the term biorationals instead of biopesticides, but as far as I can determine this is not an officially recognized term. Biopesticides come in two basic classes: 1) microbials (pesticides derived from microbes), which are microorganisms such as fungi, bacteria, viruses, protozoans and nematodes; and 2) biochemicals such as pheromones, plant extracts that are not directly toxic to pests, or fatty acids and soaps.

Microbials and biochemicals
Microbials function as effective pesticides in different ways. Some microbials that are effective in controlling arthropods (e.g. insects, spiders and mites) produce a toxin such as Bacillus thuringiensis (Bt), which is lethal when ingested. Some bacteria and viruses are ingested by an arthropod and reproduce inside it, killing the pest through production of toxins such as nuclear polyhedrosis viruses (NPVs). Some fungi enter an arthropod and produce mycelia that literally fill up the body cavity of the host, killing it. Some microbials do not kill their host but render it ineffective as a pest. For example, there is a nematode parasite of a Monterey Pine wood wasp pest that enters the female wasp larva, migrates to where the ovaries will form when the larva pupates, then produces young nematodes that enter the wasp eggs forming in the ovaries of the adult wasp. So instead of laying viable eggs, the female wood wasp lays eggs that contain a nematode rather than a wasp embryo. The nematode emerges from the egg, finds new wood wasp larvae to infest and the cycle starts all over again. The nematode does not kill the wood wasp larva or adult but sterilizes the adult female.

Microbials effective in controlling fungal diseases do so using a range of modes of action that are specific to the microbial species. Some are effective in controlling pathogens by colonizing the plant surface first but without causing damage to the plant, preventing the pathogen from getting established on the plant surface. In other cases the microbial will produce compounds that interfere with germination of the spores or growth of the pathogen.

Pheromones are volatile compounds released by females of many insect species for the purpose of attracting their male counterparts so that mating can occur. Pheromones are effective at very low volumes and are species-specific, meaning the pheromone released by the female of one species is not attractive to the males of another species. Not all insect species use pheromones for mating. However, pheromones are very common in some groups such as moths. Commercially available pheromones are synthetic copies of the pheromone released by the females. A synthetic pheromone is released into the air in such quantities that it confuses the males so they cannot find the females, hence the use of the phrase “pheromone confusion” to describe this pest-management approach. Pheromone confusion is very convenient because it will only affect the target pest. Examples of commercially available pheromones for control of vineyard insect pests are vine mealybug, European grapevine moth and omnivorous leafroller.

Arthropods are small creatures, so they have a very high surface-to-volume ratio, making them prone to rapid desiccation. To prevent this from happening they evolved a waxy cuticle that helps keep their body moisture from escaping. Fatty acids and soaps kill arthropods by dissolving the cuticle on the outside of their bodies, causing them to lose moisture and desiccate. Since all arthropods depend on this cuticle to prevent desiccation, biopesticides that are fatty acids or soaps are broad spectrum, killing both pests and many non-target arthropods. One would therefore expect these biopesticides to be very disruptive, but they are not because they have very short residual activity, meaning they are only effective for a short time. Examples of this type of biopesticides are Kaligreen and M-pede.

Another relatively new group of naturally derived biochemicals is called SARs, which stands for systemic acquired resistance. When sprayed on a plant, SARs will stimulate it to produce biochemicals that reduce its susceptibility to pests, sort of like creating an immune response. An example of this group is the fungicide Regalia.

Not toxic to the pest?
One characteristic that a biochemical active ingredient must possess in order to be registered as a biopesticide is that it cannot be directly toxic to the pest. I am sure this statement got your attention. How can something be a pesticide and not be toxic to the pest? The previous three paragraphs described active ingredients that all have the capability of controlling pests without being directly toxic to the pest. There are some pesticide-active ingredients derived from natural products but have been altered and are directly toxic to the pest so they are not biopesticides. Some examples are Avermectin (e.g. Agri-Mek), Pyrethrins (e.g. Pyrenone), Spinosad (e.g. Success), Insect Growth Regulators (e.g. Confirm) and Azoxystrobin (e.g. Abound).

The $43 billion global conventional agrichemical market is mature, meaning it does not change much from year to year. Herbicides are about 44% of the market, insecticides are about 23%, and most of the rest are fungicides. The number of new, conventional active ingredients being launched and the number of new leads for modes of action are declining. The biopesticides market is currently valued at $2 billion, but it is growing more than 50% per year. Some of the reasons for this rapid growth are: They leave no problematic chemical residues on the crop; re-entry time after a spray is 24 hours or less; they have very little environmental impact; most are registered for use in organic production; often their modes of action are as such that development of resistance is not as likely as with conventional pesticides, and getting an active ingredient to market costs in the neighborhood of $3 million to $5 million compared to $250 million for a conventional pesticide.

In some circles biopesticides are thought of as being not very efficacious, sort of a pesticide “light.” There are several possible reasons for this view. First, in some cases they have been used improperly and therefore failed, leaving a lasting negative impression in the minds of growers. They cannot be used like the long residual, fuming-type pesticides characteristic of the older chemistries for which one could get away with poor coverage and timing. Because of their unique modes of action and/or their short residual effectiveness, coverage and timing of application of biopesticides are absolutely critical for effective control. In some cases biopesticides have been tried when all other materials have failed, which is using them in a no-win situation. They are not materials that can be used to clean up a problem that has gotten out of hand.

Used more in conventional farming
Biopesticides can be used successfully in a range of situations. Many biopesticides are compatible with conventional pesticides and can be tank mixed. They can also be used in rotation with conventional pesticides to reduce the possibility of the development of resistance of important conventional materials. Biopesticides are useful in situations where residue management of spray materials is an issue. Late-season problems with mildew or bunch rot can be treated with a biopesticides right up until harvest because their re-entry intervals are 24 hours or less.

If you associate biopesticides with organic farming, it might surprise you to find out that more are used in conventional fields than in organic fields. They are a great fit in many pest-management programs.

Dr. Cliff Ohmart is vice president of professional services for SureHarvest. Previously he served as research/IPM director at the Lodi-Woodbridge Winegrape Commission. He has been writing about sustainable winegrowing issues for Wines & Vines since 1998.