Page Intro: Perhaps the most invasive pest. Thrips are known for getting into the tightest of places. They are known for getting growers into tight places as well. On this page you will get a little background, get to know how they become a problem, find out that thrips are a serious pest, get some advice, all of it topped off with some help for the scout, and some solutions.
Some Background
Thrips are members of the order Thysanoptera. Perhaps, though, based on information I’ve obtained from growers who’ve had serious problems with these pests, they should change the name of the order to Nightmaroptera or something like that. There are over 4500 species throughout the world (get help with ID at the University of California), with several known to invade greenhouses and interiorscapes — especially greenhouses with floral crops. A thrips (yes, thrips, with an “s” is the singular and plural versions of this pest’s name) is not only known for the direct plant damage it causes, but also for the serious plant diseases it vectors. [Intro]
How They Become a Problem
Thrips usually invade greenhouses in myriad ways. First of all they reside in structures year ’round, hiding in the smallest of places. Case in point: I know a grower who once discovered great numbers of thrips hiding in rotted sections of wooden-framed benches. He picked at the mushy wood with a penknife and uncovered their lair. Wow. He was surprised, and so was I. They also enter greenhouses on plants, of course, which highlights, again, the need for serious plant inspections before plants are accepted and allowed to take up space in your greenhouse. The adult thrips are also flyers. Their wings are more like feathers having fringes of hairs, or setae, and are very difficult to see-especially since they keep them close to their yellowish to dark green to brown-black elongate bodies when not in use. They do, however, function well. Their wings carry them, combined with natural breezes, over great distances (and they’ll fly sorties right into your greenhouse or interiorscape, especially in the fall when their outdoor host plants are beginning to fade). One situation I was made aware of took place in a tomato grower’s greenhouse. His greenhouse was surrounded (at a distance, albeit) by alfalfa fields. The fields were loaded with thrips, but of no consequence to the alfalfa. However, when the alfalfa was harvested it left on the streets literally millions of homeless thrips. Can they fly? Oh, yeah. Just ask that tomato grower. He had millions of thrips show up in his greenhouse in a just couple of days. He needed to spray. No questions there; biocontrols simply cannot deal with that type of pest pressure. He did have an alternative though: he should’ve had no-thrips screens installed. Yet, for some reason he refused to go that route, and after three consecutive years of this situation, I could longer sympathize with him. He went out of business soon after — the sprays he relied on, unfortunately, were no longer effective. [Intro]
A Serious Pest
Female thrips can reproduce sexually or asexually without a mate — it is their option. Isn’t that just peachy? They lay their eggs in the soft tissue of plants, which is not terribly damaging to the plants, in itself, but it does protect the eggs from just about everything under the sun. That can’t be a good thing. The damage to plants and flowers is caused by the other end of these pests-the feeding end. Thrips have rasping and piercing-sucking mouthparts. They first rasp the cells causing a “wound.” The “wound” oozes plant sap. The thrips then insert their straw-like stylet into the damaged cells and begin to draw the juices. And if that isn’t enough for you in the damage department, thrips are famed for their transmission of two horrific plant diseases: impatiens necrotic spot virus (INSV) and tomato spotted-wilt virus (TSWV). I’ll give you information about these plant viruses, so read on. I’ll let know something right now, though: if you don’t want your plants to get these special tospoviruses, don’t let thrips into your facility no matter how loudly they knock. If they do get in, control them as quickly as possible. Feeding damage can pale in comparison to what these viruses can do.
Thrips can lay a number of eggs. Their life cycle can be fairly short, dependent upon the temperature. If it’s cool, it can take over a month’s time from egg to adult; if hot, it can take as little as fourteen days. Needless to say, thrips are more problematic in the hottest months of summer. The various larval stages-of which there are two — and the adult stages are typically well-placed in their host plants. They can often be found deep within the plant’s flowers. Moreover, they will also hide in closed buds making their control, especially with chemicals, difficult at best. Fortunately, some of the biocontrol agents available to thwart them will go in after them and drag ‘em out screaming. Many growers who do use chemical pesticides in their effort to control them will suit up during the hottest part of the day. This is when the thrips are most active, often venturing freely all over the plant instead of only hiding within its tight places. Fully-robed growers are very much a threat to thrips at high-noon in the heat of the summer months. For one thing, they’re really ticked off to begin with for having to be in such sweltering conditions swimming in their sweat when they’d rather be swimming in cool, refreshing mountain stream (or frolicking half naked, jumping from the walls of a flooded quarry while drinking Mountain Dew with their friends).
Some Advice
Somewhere between a complete and incomplete metamorphosis, thrips have a unique way of growing up. Two of their stages, the prepupal and pupal stages, at least for some species, are spent in the soil, growing medium and other friable material. In a nutshell, they will fall off the plant and drop to the ground. Good news, with the right tools they can easily be controlled down-under. Right tools like the predatory mite Hypoaspis miles. Not all thrips species, mind you, do this. The majority of which growers will be concerned with do do this, though. Some common species are flower thrips, echinothrips, greenhouse thrips, onion thrips and, by far the most prevalent and actually the star of this page, are western flower thrips. [Intro]
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