Good Bug, Bad Bug: Using Beneficial Insects in the Greenhouse

Ladybug on leaf

When I walk into a greenhouse—any greenhouse—I’m immediately hit with the smell of green and earth. The smell is so pungent, it transports me to another place, somewhere that work and stress and responsibilities don’t exist. You can imagine how unsettled that calm becomes the moment I realize that thrips or whiteflies have managed to penetrate the defenses and are sneakily destroying those plants that the greenhouse is supposed to be protecting.

Although planting in a greenhouse makes a clean break from the many problems common to outdoor planting, that doesn’t mean that these issues won’t find their way inside. You’d need a battery of miniature sniper towers to prevent all unauthorized entries through the tiny cracks and unscreened vents, let alone the ninja pests that hitch rides into the greenhouse on new plants.

Luckily for all of us, there are mercenaries available for just this sort of thing. Predatory and parasitic insects have caught the attention of research and commercial greenhouses across the country—this is great news for the rest of us, because the increasing demand for these insects is making them easier to order through catalogs and websites.

Bugs in the greenhouse

I had never considered the benefits of using predatory insects in a controlled setting like a greenhouse until a few lacewings wandered into my greenhouse one day. Well, I say they wandered, but what they really did was lay some eggs on my African violets. It took a few minutes before I realized what I was looking at (if you’ve never seen them, lacewings lay unusual eggs that hang from long, delicate stalks). Being the curious type, I left them alone, just to see if they’d hatch—after all, it’s been a terrible year for flower thrips (I accidentally introduced some on an infected plan—quarantine, quarantine, quarantine!).

A few weeks later I started seeing teensy green nymphs wandering around on my plants and I noticed that the flower thrips were becoming less and less of a threat. Within a few weeks, the colony of thrips was completely gone, which was a great relief to both myself and the African violets. Now, I’m not one to ever put all my eggs in one basket, but so far, so good. Knowing the green lacewings are on patrol makes me worry less about thrips and mites; when I accidentally uncover one of these little hunters, I’m pretty sure they’re wiggling their antennae at me as if to say, “No worries, boss, I’ve got this.”

Beneficial insects suited to greenhouse life

Not all bugs are created equal, especially when it comes to working for a living. If your greenhouse is reasonably small with manual temperature and humidity controls, you’re going to need much hardier stock than the commercial producer who has the ability to create precise climate zones. Small producers often order new bugs each year instead of attempting to establish a breeding colony of beneficials, just to simplify the process.

Generalist hunters like ladybugs, mantids and my green lacewings are excellent choices for starting a greenhouse biological control program. These insects are readily available, inexpensive to purchase and familiar to almost any gardener. Heck, if you can find some mantids running around in your garden or ladybugs overwintering in your attic, they’ll transplant well into a tightly screened greenhouse.

For other growers, more specific help might be needed. If you’ve got a particularly noxious case of mites, it may benefit you to set species-specific predators on them. You can fight mites with mites—predatory mites like Galendromus occidentalis and Amblyseius californicus are hungry hunters who are able to roll with the punches as conditions change in the greenhouse. Predatory mites are often used as a short-term control for chronic problems with their plant-feeding cousins.

When caterpillars, sawflies or beetles are the problem, parasitic wasps may be the answer. Unlike their cousins the social wasps, parasitic wasps are very small and usually lack a stinger. During the reproductive phase of their lives, these wasps seek out soft-bodied larvae as food sources for their young.  When the wasps find a suitable host, they lay a series of eggs on or inside the nuisance insect. Upon hatching, the new wasps feed on the parasitized host and the cycle of life continues.

Other jobs for beneficials

So far, there’s been a lot of talk about killing other bugs with bug on bug violence, but I haven’t gotten to the very best part of beneficial insects that I discovered while reading about commercial greenhouse tomato production a while back. As it turns out, bumblebees are making a huge splash on the beneficial insect scene; we’ve heard a lot lately about the value of native bees to our gardens, especially with the decline of the European honey bee.

If you’re growing vegetables indoors in a medium to large greenhouse, you may want to invest in a bumblebee box stuffed with these nifty pollinators. Like honey bees, bumblebees are docile creatures and not prone to stinging, but unlike honey bees, they don’t make a lot of honey or wax that creates yet another chore. Bumblebees are the primary pollinators of tomatoes, the perennial greenhouse favorite, and do a smashing job with other plants that require labor-intensive hand-pollination like peppers and strawberries.

Since discovering the lacewings in my greenhouse, I’ve developed a healthy appreciation for what they can do. Not only do I use considerably fewer chemicals in my plants (after carefully checking that the lacewings are elsewhere), my little greenhouse helpers are more than ready to scoop up trouble pests before I’m even aware of their presence. You may not be ready to bring bumblebees or parasitic wasps on board just yet, but adding a few friendly ladybugs to your greenhouse might just enhance that sense of calm your greenhouse bestows every time you open the door.

Kristi Waterworth
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