Citrus Thrips

Adult citrus thrips are 1/25 inch (1 mm) long, orange-yellow insects with fringed wings. Citrus thrips move actively on new blueberry foliage during the spring and summer. The first and second generation are usually found near the tips of new sucker growth at the base of the plant; subsequent generations are found on new growth at the top of the canopy during the last few weeks of harvest through fall.

Female thripslay about 25 eggs in new leaf tissue or green shoots. Eggs laid during the spring and summer hatch within a few days; eggs laid during October and November overwinter and hatch the following April.

First-instar larvae are very small, whereas second-instar larvae are about the size of adults, spindle-shaped, and wingless. Third- and fourth-instar (propupa and pupa) thripsdo not feed and complete development on the ground.

Citrus thrips do not develop below 58°F (14°C). They can produce up to eight generations during the year if the weather is favorable.

Do not confuse citrus thrips with western flower thrips, Frankliniella occidentalis. Western flower thrips are very common in blueberry fields during bloom and migrate out of blueberry fields shortly after the end of petal fall. Western flower thrips damage to blueberries is very rare, even when thrips numbers are very high, and usually only consists of a few random berries with oviposition scars.

Citrus thrips are primarily a pest of blueberries grown in the San Joaquin Valley; damage in coastal blueberry production areas is rare. Citrus thrips feed at the growth tip of developing shoots and leaves, causing stunting and scarring of new shoots coupled with curling and discoloration of new leaves. In severe cases feeding can cause terminal growth to die causing new lateral shoot growth.

Damage to new growth is most common after harvest, from late June through early October. Stunting of fruiting wood during the previous summer and fall can cause a reduction in yield the following spring. Citrus thrips do not cause any reductions in fruit quality.

In areas where citrus thrips are a problem, carefully monitor numbers. If monitoring indicates insecticide control is needed, apply an insecticide shortly after harvest. In areas with high pest pressure or more vigorous plants, one or more additional applications may be necessary.

All varieties of blueberries can be damaged by citrus thrips. However, the variety Star is especially attractive to citrus thrips. Avoid planting this variety in areas close to established citrus production orchards or otherwise prone to citrus thrips problems. Some research suggests that the use of overhead sprinklers can reduce thrips populations.

While a number of natural enemies attack citrus thrips in citrus, natural enemies are rarely found in blueberries and do not provide economic control.

Application of the Entrust formulation of spinosad is the only reliable organic option to reduce citrus thrips numbers. Other organically-acceptable methods have not proven successful in field studies. These include lacewing larvae releases, twice per week applications of water at high pressure, and use of entomopathogenic pathogens (such as Beauveria bassiana).

Citrus thrips has a history of rapidly developing resistance to chemicals that are used repeatedly and frequently for its control. For example, resistance to beta-cyfluthrin (Baythroid) and fenpropathrin (Danitol) have been documented in several citrus groves in Kern County. With the limited number of pesticides available for citrus thrips now and in the foreseeable future, monitor citrus thrips levels carefully, and limit treatments only to populations that are causing economic damage.

Monitor for citrus thrips weekly from the last 2 weeks of harvest through September using beat samples. Sample 10 shoots to get a general idea of pest density in any particular area of a field. Initially, sample all varieties to determine which variety has the highest thrips density. From then on, sample only the variety with the most thrips. There is no need to sample fruit.

1. Sample in the morning when it is cool; in the afternoon adults quickly fly away when disturbed.
2. Observe the quality of new growth by noting the prevalence of scarring to new leaves and shoots.
3. Tap the terminal 6 inches of new growth onto a clipboard or other flat surface (preferably black).
4. Count the thrips.
5. Repeat this process on 10 shoots per each region of the field to be sampled.
6. Calculate the average number of thrips per beat sample.

Apply insecticides after harvest when there are an average of 25 to 30 thrips per beat sample in the variety with the highest thrips numbers. At this pest density, scarring of the stems and curling of leaves will begin to be present. If this threshold is reached or exceeded before the end of harvest, wait and treat after harvest because

• Citrus thrips cause no direct damage to fruit.
• There is still time for plants to recover prior to the development of fruiting buds.
• A treatment may affect logistics related to picking crews, preharvest intervals, and residues on fruit.

The exception to this rule is when SPOTTED WING DROSOPHILA is present. In this case preharvest treatments of spinosad, spinetoram, or fenpropathrin will control citrus thrips.

Re-treatments may be needed in some areas of high citrus thrips pressure. In these cases, re-treat if new scarring becomes evident and thrips numbers have returned to 25 to 30 thrips per beat sample. When re-treating, rotate the insecticide mode of action to avoid the development of resistance.

Do not apply insecticides after late September. At this time eggs stop hatching until the following spring, and adult thrips die off during the winter. Also, by early October blueberry plants have already produced fruiting wood, and fruiting bud differentiation has already occurred, such that fall thrips populations have little to no impact on the next year's crop.