Bean Thrips

Bean thrips (Caliothrips fasciatus) were first found in the 1800s on orange leaves, but they prefer eating beans and other legumes. That being said, these California natives are also found on avocado, pear, and walnut trees, in the navels of navel oranges, mandarins, and other citrus, as well as cantaloupes and other melons. They don’t eat all of these plants, but they do like using them for shelter because of how close they are to your legume crops.

Known as California bean thrips and North American bean thrips, these pests are also called citrus thrips, which is incorrect. Citrus thrips (Scirtothrips citri) are another species. These are also not Asian bean thrips (Megalurothrips mucanae), another entirely different species that we will explore another day. While different, these species have a lot in common with our California bean thrips.

​Bean thrips identification

Female bean thrips (Mr Thrips) CC BY-SA 3.0

Adult bean thrips have dark, greyish-black bodies with white wing bands. The legs have light and dark bands and both genders have fringed wings that they fold over their backs when at rest. Of course, these pests are about the size of any letter on an American penny, so you probably won’t see all that. If you use a hand lens, you can. You would also be able to see that larvae are yellowish to orange. Those larvae are about 1 mm long at their biggest. Eggs are even smaller, at only 0.008 inches. If you could see them, you’d notice that they are banana-shaped.

What you are more likely to see is the tiny black frass (bug poop) deposits which are often found on the top and underside of leaves, in and around citrus navels, and where clusters of fruit touch. You will also see the damage they cause.

Bean thrips on the underside of a leaf (Whitney Cranshaw, Colorado State University, Bugwood.org) CC BY 3.0 US

Bean thrips damage

Both larvae and adults feed on leaves and growing tips. As they feed, they cause leaves and growing tips to turn brown and distorted. Silvering and premature defoliation can also occur, reducing plant vigor and resulting in sunscald. Bean thrips also feed on immature beans.

Bean thrips lifecycle

Female thrips lay eggs in the leaves and fruit of legumes. Legume fruits are also known as pulses. Pulses are the beans and lentils we enjoy eating. When bean thrips eggs hatch, first and second instar larvae feed heavily on leaves, growing tips, and immature pulses before they fall to the ground and go through two-stage pupation. Winged adults emerge and the cycle starts again. Like most other thrips, mating is not needed for reproduction to occur: unfertilized eggs produce male offspring and fertilized eggs produced females, but there’s more to it than that.

Research has found that females generally only live 20 to 55 days. At the 10-day mark, short-lived females stop producing female offspring. If they happen to live for more than 50 days, they will produce female offspring until the 30-day mark. How does this stuff evolve? And how do they know? It baffles me.

Bean thrips control

Bean thrips are very bad fliers. They can only fly a few feet in a crazy zig-zag or spiral pattern. But they can catch rides on the wind, so you have to monitor plants for signs of thrips during warm and hot weather.

Bean thrips thrive in hot weather and there isn’t anything you can do about that. But there are steps you can take to make your garden less appealing to bean thrips. First, they like to hide out in weeds, such as prickly lettuce (Lactuca serriola) and wild blue indigo (Baptisia australis) so keep weeds, especially those in the legume family, several feet away from your garden beds.

Severe bean thrips infestation of wild blue indigo (Whitney Cranshaw, CSU, Bugwood.org) CC BY 3.0 US

Yellow sticky sheets attract and capture all sorts of thrips and other pests and they are inexpensive. While research has shown that green sticky sheets are more effective at capturing bean thrips in particular than other colors, you’ll probably do fine with the yellow.

So, what’s going on (and under) your plants’ leaves?