In the garden with Susan: thrips may be small, but they are very harmful

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It was bad enough that the cold, wet spring in our area affected the ability of our tomato plants to produce a normal crop. When an unusual insect problem landed on them as well, I started to take it personally.

About two months ago I started noticing small light colored spots on the lower leaves of tomato plants. At the time, there was no serious damage, so I didn’t feel the need to investigate further. It wasn’t long before the same signs appeared on our red bean leaves.

When gardeners see unusual damage to our plants but see no insects, it can be easy to suspect that a plant disease is the problem. Fortunately, I was able to use a magnifying glass and a microscope to spot the culprits: the thrips.

Chances are you haven’t seen the thrips themselves in your garden because they are really tiny at only 1/16 inch long. Like aphids, they have drill/sucking mouthparts which they use to extract sap from plant foliage. These small insects possess fringed wings, have a striped abdomen, and their color varies from yellow to brown to black.

There are several stages in the life cycle of thrips. Adults overwinter in plant debris. After mating, females lay eggs in the foliage or flowers of young plants. The larvae hatch, feed and go through two stages called ‘stages’. They then descend into the ground for two more instars, during which they develop wings and complete their transformation into adults.

Thrips are most commonly found on beans, peas, cucumbers, melons, pumpkins, squash, garlic, onions, peppers and tomatoes. In addition to stippled leaves, other signs of their damage are stunted plants, curled leaves, and scarred fruit. If you have ever noticed deformed rose buds, this is also due to thrips.

The good news is that thrips have many natural predators such as big-eyed plant bugs, green lacewings, parasitic wasps, predatory thrips (yes, there are actually good thrips), and hoverflies. The bad news is their abundance, as well as the fact that it’s so hard to see them. After doing some research, I learned that thrips tend to be more prevalent in hot, dry weather, which certainly matches what we’ve been experiencing for the past two months.

How to get rid of thrips? I wish there was a simple answer, but they are so difficult. First, monitor your garden daily and inspect the crops listed above for damage. It is possible to knock thrips off plants by spraying them with a stream of water from the hose. You can also buy or make blue sticky traps since thrips are attracted to this color; hang them near their host plants.

I’ve had good luck catching them inside sheets or rolls of corrugated cardboard. Lay your cardboard trap on the surface of a bed where thrips are causing problems. The next morning, pick up the cardboard and tap it on the inside edge of a bucket partially filled with water and a squirt of dish soap. You will soon see tiny insects doing the backstroke.

If you’ve had significant problems with thrips on some of your crops, it’s a good idea to clean up plant debris at the end of the season so you don’t give the thrips a place to hide over winter.

Organic products that will control thrips include diatomaceous earth, insecticidal soaps, neem, pyrethrins or spinosad. As always, follow label directions.

Susan Mulvihill is the author of “The Vegetable Garden Problem Solver Handbook” and “The Vegetable Garden Pest Handbook”. She can be contacted at [email protected] Watch this week’s video at youtube.com/susansinthegarden.

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