Pest Control Part 1: What is a Pest?

posted in: Science | 11

Since I’m the resident entomologist on Biofortified, and because the main pests in almost all agricultural systems are insects it only makes sense for me to write something about pests and how they’re managed in agricultural situations. My role here on Biofortified is to write about the basic biology of pests, but I will be discussing management from time to time.

Ellipsidion australe: Cockroach? Yes Pest? No.

To say that insects are pests would be far too simplistic because of their sheer diversity. The two families of parasitoid wasps I’ve been discussing on the insects and pests information site, the Braconids and Ichneumonids consist of about 180,000 species together. If you want something to compare this to, there are roughly 10,000 mammalian species. There are a lot of insects around us, and they all have different ecological roles.

While some insects feed on crops, others feed exclusively on other insects which makes them the enemies of our enemies and thus…our friends. Even in a monoculture system, there are interactions between pest animals, their environment and people. Understanding these interactions is key to understanding things like why we need pesticides or why your town is inundated with ladybugs every year.

So…what, exactly constitutes a pest?

The most simple definition of a pest is an organism which pisses us off. That’s really it; the term is completely anthropocentric. Pests are creatures which interfere with our activities in any way, shape or form. In agricultural settings, insects cause damage in a variety of ways. The most common are the direct or indirect consumption of our goods such as a corn earworm or corn borer feeding on corn. There’s also the transmission of disease to livestock, plants and people. Some such as bed bugs feed on us directly and others like cockroaches share our dwellings and offend our sense of cleanliness. Others such as wasps or yellow jackets will inject us with harmful substances. Some like mosquitoes or aphids transmit diseases to us or our plants.

There are three very broad categories of pests which overlap: Medical/veterinary, urban and agricultural. Veterinary/medical and agricultural pests are fairly self explanatory; respectively they are insects which harm livestock and humans while agricultural pests harm crops. Urban pests are generally pests which infest our dwellings, although as I mentioned earlier there’s quite a bit of overlap between the categories.

Blattella germianica. Cockroach? Yes. Pest? One of the hardest to get rid of…

There are also natural enemies, the insects which feed on pests. A good example of natural enemies are the parasitoid wasps I’ve written about because they kill caterpillars which would normally eat our crops. There are several families of flies which feed on insects in a similar manner that parasitoid wasps do. There are also predatory insects which will help keep pest populations down.

There are also pollinators. Bees are a textbook example of this. Without bees, about 80% of the food we eat wouldn’t exist because they pollinate crops. No pollination, no fruit, nuts and other food crops. Even pollinators we don’t raise contribute millions of dollars to the economy every year.

Bear in mind, though, that there’s a lot of overlap between all of these categories. Whether an insect is a pest or whether it’s beneficial will depend solely on where it currently is, what it’s feeding on and what it’s interacting with.

Let’s use the example of the common insect family Meloidae as an example of how the term ‘pest’ and ‘natural enemy’ can almost paradoxically overlap. Some Meloid beetle larvae feed on grasshopper egg cases, which helps keep grasshopper populations down and reduces the amount of alfalfa lost through grasshopper damage. You’d think they’re a good thing to have around…and in some ways they are.

The problem comes when the adult beetles emerge. Meloid beetles are popularly known as ‘blister beetles’ because they produce a chemical called cantharadin which destroys skin and creates large, characteristic blisters. You can imagine how eating them would cause problems because cantharadin is incredibly toxic when ingested.

Herein lies the problem. Blister beetles are pests of alfalfa fields because alfalfa is fed to farm animals. You get a handful of beetles into a racehorse’s food and you’re out a multimillion dollar horse. Although they’re a good thing to have around as larvae, the adults are quite capable of killing animals as large as a horse. Some horse owners even take extreme measures to ensure the safety of their animals, sometimes buying feed from across the country to avoid any potential problems.

Ladybird beetles are another great example where this paradox comes into play. Ladybird beetles are prized in most agricultural situations because they consume aphids, which suck plants dry and transmit disease. They’re not prized in vineyards because they, too, secrete their own defensive compound in the form of bitter tasting alkaloids. They won’t kill you but even a small amount of ladybug can ruin very expensive wine by making it taste bad, which is known in the industry as Ladybug Taint.

They can also be urban pests, as anyone who lives in areas where asian ladybird beetles can be found. At the end of the growing season when food is scarce, the beetles look for places to overwinter. The best places are small cracks that allow high densities of ladybird beetles to congregate. Unfortunately for homeowners, they tend to find their way inside dwellings and become annoying uninvited houseguests.

Pest is a word that’s very simply defined. The problem is that a lot of the time, lines can be blurred depending on what you’re growing and the insect in question. Furthermore, a lot of the animals we consider pests play important roles in the environment. In the coming weeks, I’ll be talking more about the science of pest control and how it relates to agricultural settings.

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Joe Ballenger is an entomologist who works in the biotech industry as a contractor. In his spare time, he helps answer questions about bugs at Ask an Entomologist.