Asian Bean Thrips

Asian bean thrips were first seen on US soil in March 2020, in Florida. Native to tropical Asia, this pest now ranges from Japan to Australia. And in March of 2021, it was found in Central America, in Belize. It’s probably only a matter of time until it spreads to our gardens, so let’s see what we’re up against.

Asian bean thrips male (left), female (right). Note white antennae bands. (Felipe N. Soto-Adames, FDACS-DPI) Public Domain

Asian bean thrips damage

Asian bean thrips (Megalurothrips usitatus) suck the sap out of snap bean, cowpea, lima bean, peanut, and soybean plants. And potatoes. They also feed directly on host plants. This feeding usually begins in the flowers. Pods and growing tips become deformed and twisted, with streaks of russeted reddish-brown where thrips feeding has occurred. Heavy infestations can cause stunting, wilting, and poor pod set. In China, Asian bean thrips are responsible for crop losses of 30% to 100%.

Thrips feeding damage to bean leaf (Scot Nelson) Public Domain

Asian bean thrips identification

To me, Asian bean thrips look like hoverflies with pointy rear ends. They are very small. You could almost fit 10 females across the face of an American dime. Males are only half that size. Size isn’t the only difference. Females are black, with yellow or white bands. Their wings are banded, too. Males are yellow, with a similar but less defined banding pattern.  Both males and females have black antennae with white bands. Larvae start yellow but turn red as they near adulthood.

Asian bean thrips lifecycle

These pests love warm weather and they can complete their lifecycle in as little as 10 days. Eggs are inserted into flowers, leaves, and pods. After hatching, larvae go through two molts over 2 or 3 days before dropping to the ground to pupate. Pupae seem to prefer moist, sandy soil, so they probably won’t feel welcome on my sun-scorched clay, but I’ll still keep a lookout. Just in case.

Adult Asian bean thrips feed on pollen, mate, and lay eggs. If no mate is available, they can reproduce without them. This ability is called parthenogenesis and it is why aphid populations can seem to explode overnight. I can only imagine that Asian bean thrips populations can do the same thing.

Asian bean thrips management

Since this invasive pest is new to North America, we’re not really sure what we’re up against. Many beneficial insects eat thrips, but we don’t know their efforts will be enough. We’re not even sure if those garden helpers like the taste of Asian bean thrips. All we can do is hope.

Actually, we can also remove plant debris once a crop has finished its cycle. Adding that plant material to the compost pile may help reduce hiding places for this new pest. Since thrips, as a group, are known to develop resistance to pesticides rather quickly, chemical sprays are not recommended. My guess is that mechanical control methods, such as diatomaceous earth (DE) and sticky barriers may help. They can’t hurt.

If you think you have Asian bean thrips in your garden, try to catch one or take a picture and reach out to your local Master Gardeners, County Extension Office, or Department of Agriculture. Those groups work hard to protect our gardens from invasive pests, and we can help them with any information we might have.