Citrus Thrips

Citrus thrips are not just a pest of orange trees and tomatoes - now they feed on blueberries, too!

As true bugs, thrips (Thysanoptera) are cousins to lice, both of which evolved from sap-sucking aphids. Citrus thrips  are small, narrow-bodied insects that use rasping and sucking mouthparts to take nutrient rich sap from host plants. These pests are also vectors for disease. What puts citrus thrips in the news today is the way they are modifying their diet to coincide with human efforts at plant breeding.

Citrus thrips (Katja Schulz) CC BY 2.0

Citrus thrips description
​
First, citrus thrips are very difficult to see. They generally hide underneath leaves and leap away, flying to another hiding place when they feel threatened. Since their wings are really not designed for actual flight, thrips use a method called “clap and fling” to become airborne. Basically, a thrips claps its wings together, above its back, and then flings them apart, over and over. This creates enough lift to get somewhere else, but that’s about it. If you actually get ahold of one, you will see that they have 2 pairs of very skinny, fringed wings and a narrow, cigar-shaped body. Newly emerged adults are pale yellowish-green, while adults are black with red markings.

​Citrus thrips life cycle

Citrus thrips go through an incomplete metamorphosis called hemimetabolism. This means they have three distinct life stages: egg, nymph, and adult. Thrips are able to reproduce sexually, asexually, or bisexually. Dozens of eggs are inserted into leaf tissue. When they hatch, generally around March, the oval-shaped, clear yellow larvae start feeding on new growth, called “flush.” This causes distorted leaves and buds. Once the larvae have eaten their fill, they enter a resting pupal stage before becoming adults. With California’s mild climate, there can be as many as 8 generations of citrus thrips each year. That’s a lot of thrips!

Damage caused by citrus thrips

Thrips are weird. To understand how thrips damage plants, you need to know that they have a lop-sided jaw that is, in some cases, completely absent on the righthand side. They use the left side to cut into a plant part. Then, they inject saliva, which partially digests the plant material. This goo is then sucked out with tubes called maxillary stylets. [Imagine eating your burger that way!] Anyway, this is what causes the telltale distortions and silvery or bronze scarring of thrips feeding. Citrus skin scarring generally occurs in May and June, but it can continue throughout the summer months.

Citrus fruit damage to fruit (Image from "Citrus fruits; an account..., 1915) Public Domain

As disease vectors, citrus thrips can carry tomato spotted wilt and up to 20 other plant viruses to your otherwise healthy plants! The real kicker is that, with the advent of heat-tolerant blueberry plants becoming popular in California, thrips have discovered a rich new food source! Citrus thrips damage to blueberry plants includes deformed leaves, shortened internodes (spaces between leaves), and stem scarring. While the fruit is not directly affected, the overall health of the plants is compromised, which leads to reduced plant growth and lower fruit production.

Controlling citrus thrips
​
To initially check for citrus thrips, simply go out in the morning and hold a piece of dark paper (or your other hand) under affected areas and shake the stem. Since adults can escape, count them first. Then count the nymphs. You may need a hand lens. Since this can also shake loose fruit before it has ripened, you have to sort through the pros and cons. Some varieties of blueberry, specifically Jubilee, Misty, and O’Neal are less appealing to citrus thrips than, say, Star, so choose your blueberry variety with that in mind.

Once bushes are installed, pheromone traps and yellow sticky traps are used successfully to monitor for adult citrus thrips populations. These populations tend to peak from mid-August through mid-September. Citrus thrips eggs are present year round, but there is little that can be done to eliminate them at this stage. Some people claim that using a high pressure spray from the hose can help, but research does not back up those claims. Commercial growers spray blueberries with industrial grade insecticides. Unfortunately, thrips are notorious for developing resistance to chemicals that are used repeatedly.

Organic growers must rely on spinosad and lacewing larvae to control citrus thrips. Currently, research is being conducted on an entomopathogenic pathogen (fungi that cause disease), called Beauveria bassiana, is being studied as a possible treatment. [I will keep you posted, as I learn more about that particular option.]