Selective Chemical Pesticides and Biological Control
Selectivity is a major concept in pesticide application and efficacy. The primary aim of pesticide selectivity is to reduce the negative impacts of the application on non-targets and the environment. In other words, if you're going to spray a pesticide, wouldn't it be nice if it killed only the organisms that are causing a problem and did no other environmental harm? That's a far cry from the history of pesticides in general. Typically, pesticides - or more specifically - insecticides, have been "broad spectrum". A broad spectrum insecticide can be viewed as a good thing, and here's why: 1. broad spectrum means that it will kill most insects that it contacts, thus you are able to exert considerable mortality to most, if not all, pest populations in a field with just one application. 2. One application means you save on application costs which includes fuel, chemicals, water (it takes a lot of water to make an application), and of course, labor. 3. In regards to labor, one application also reduces harmful pesticide exposure potential to your workforce. 4. One application also means less soil compaction from heavy equipment running up and down the rows of your field, and reduces negative impacts on air and water quality.
Having all of these benefits may make it seem like a broad spectrum approach is really the best way to go. However, there are significant problems with broad spectrum insecticides. Big problems.
First, most of the broad spectrum insecticides are classified as nerve poisons. Turns out insects and other animals, including humans, have very similar nerve anatomy and physiology. So, insecticides that negatively affect an insect's nervous system, also negatively affects ours. Meaning, insecticides can kill us too, if used improperly or through accidental exposure. The benefit mentioned in point #1. above, about controlling multiple pest problems with just one application has a hidden detriment. The broad spectrum insecticide also will kill off any upper trophic beneficial predators and parasitoids as well. That means whatever control the beneficial insects provided will be eliminated. The outcome will be secondary pest outbreaks --- pest populations kept under control by beneficial predators and parasitoids, will now be released from those natural controlling effects resulting in pest populations that can explode causing more even more headaches. |
Another issue relates to point #3, about one application reducing the risks of insecticide exposure. The hidden problem here is that many of the broad spectrum insecticides also have a very long residual. That means that once the application is made, the toxin is environmentally stable and remains in place and keeps working for several weeks, not just several days. That residual activity means that exposure hazards aren't necessarily minimized. It also means that the insects are exposed for a long time to the same toxin, maybe even through more than one generation. That becomes a part of the resistance puzzle. Insects readily adapt to environmental selection pressures. Researchers have noted through the decades that insect populations easily become resistance to insecticides.
And resistance is one of the biggest problems facing a secure food future that we've had in a long time. Insects have developed resistance to nearly every class of insecticide we have available and it's become such a problem that we've had to resort to a new pesticide classification system to help prevent the use of the same kinds of chemical classes over and over within a crop; it is referred to as Resistance Management. Please see the following website for more detailed information: http://www.irac-online.org/ |
Be Careful What You Ask For
Biological control workers were some of the first to request growers start using more selective insecticides. The thrust of the idea was: use insecticides that did not harm predators and parasitoids. If you preserve their populations, they will continue to thrive and provide control, thus reducing the need for additional insecticide applications. It also was requested that growers consider applying some kinds of insecticides at a particular time or in a particular way to reduce the contact between insecticides and beneficial insects - that is also selectivity. It's a great idea. It works wonders when it's done properly. But there are pitfalls. Not everything that appears selective is selective in a harmless way. Take the case of the product referred to as Esteem or as it's known by its generic name: pyriproxifen.
Pyriproxifen is classified as an Insect Growth Regulator. These types of chemicals mimic certain hormones in insects. When applied as a pesticide, they change the normal concentration or timing of the natural hormone which disables the insect's ability to grow and molt properly, thus killing it. The benefit of these types of chemicals is that they are highly specific to insects and typically do not cause environmental harm. They are exactly the type of selective chemical that biological control workers clamored for, for decades. Esteem was even more selective in that it worked very well against scale insects and in citrus the one scale that needed to be controlled was California Red Scale (below infesting fruit). So, the Esteem was applied with the idea that it would selectively kill the Red Scale and leave everything else alone.
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The history of biological control usually starts with the story of the invasion of California by the citrus killing pest, the cottony cushion scale, and the procurement and release of its predator the Vedalia beetle (above) in 1889. By 1890, the cottony cushion scale was controlled and citrus growers never had to worry about spraying for it again. Awesome story, click here for the details. With regards to this seminal biological control program and selective chemicals, things take a sinister turn. The iconic Vedalia beetle, cleanser of California's citrus orchards, holding the scale scourge at bay for a century, ends up being extirpated by a "selective" chemical. Oh the irony.
When California growers began to use Esteem in citrus in 1998 for California Red Scale control, Vedalia beetles ended up exterminated from all San Joaquin Valley citrus orchards for a period of 9 months. In the case of Esteem, Vedalia beetle turns out to be highly sensitive and succumbs to its effects even if it isn't directly sprayed. It was noted that Vedalia beetles were wiped out for several miles around spray areas. The result of Vedalia beetle's extermination was secondary outbreaks of cottony cushion scale in dozens of areas that then could only be controlled only with very harsh, broad spectrum insecticides. Since this disastrous episode, growers decreased their use of pyriproxifen and the beetles seem to be a bit more tolerant. However, problems with cottony cushion scale and other pests will continue if we don't use caution and conduct the needed research on insect growth regulators and other "selective" insecticides before their use. |
The ability of Vedalia beetle to suppress and maintain cottony cushion scale populations to below damaging levels again and again is evidence that the biological control method works. See the following website for a project on the famed Galapagos Islands that were invaded by cottony cushion scale: http://biocontrol.ucr.edu/rodolia/rodolia_icerya_biocontrol_galapagos.html.