What to watch for in 2010: Thrips

Over the next few weeks (0r until the field season gets busy), I will be posting pest watches for 2010. These are designed to provide updates on pests that may be problematic or for which management strategies may differ in 2010.

First up is a group that has become near and dear to me in the last 2 years: Thrips!

Western flower thrips (Frankliniella occidentalis), www.bugwood.org. Photo: J.T. Reed

Thrips as pests

Thrips are of concern to southeastern growers for 2 reasons: 1. They vector potentially devastating plant viruses, and 2. Thrips can directly damage crops in their own right. In the cropping systems I work in thrips are both virus vectors (tobacco and caneberries) and cause direct fruit or plant injury (strawberries, blueberries, and grapes).

In tobacco, thrips vector Tomato Spotted Wilt Virus (TSWV) which can result in significant losses. Colleagues in the NC State Entomology Department developed a degree day model which predicts tobacco thrips (Frankliniella fusca, the most important TSWV vector in the southeast) flights. We have adapted these models for a website that will be launched for use by county agents and select growers this Thursday, April 1st.

Flue cured tobacco plant infected with TSWV. Photo: www.bugwood.org.

Thrips also vector Impatiens Necrotic Spot Virus (INSV), which infects many plants, including caneberries. Caneberry viruses are the topic of a multi state USDA Specialty Crops Research Initiative (SCRI) project with which I am involved. Lead by the University of Arkansas, this project is designed to identify and development management strategies for virus complexes in caneberries. Unlike many other plant-virus pathosystems, caneberries are often infected with 2 to 3 virus before they show symptoms. The virus complex appears to vary regionally, as do the likely vectors.

Blueberry fruit damage caused by thrips oviposition (egg laying) and feeding. Photo: Insect Management in Blueberries in the Eastern United States, Tuner & Liburd.

In strawberries, grapes, and blueberries, fruit injury by thrips is of greatest concern. Thrips feeding on blueberry can also damage blooms. Blueberry injury by thrips is weather and variety dependant in NC. Late blooming varieties (rabbiteye and northern highbush) are more prone to damage, but growers generally do not need to treat these varieties every year. Thrips injury to strawberries is rare in NC, and other injury is often misdiagnosed as thrips-caused.

Thrips can cause bronzing near the calyx in strawberries. Photo: Jack Kelly Clark, UC IPM Program.

Thrips do not cause malformed fruit. The damage in this image is due to lygus bug injury, but malformed fruit can also be caused be poor pollination or dried calyx disorder. Lygus bug injury is also rare in NC and has only been observed on late season or day neutral strawberries. Photo: UC IPM Program.

Thrips injury to grapes is more common in NC than injury to blueberries and strawberries. Grapes bloom and develop later in the spring, when thrips populations have built to higher numbers and are less susceptible to spring weather conditions.

What to watch in 2010

Thrips management in 2010 will be different than in a typically year, because we will likely have fewer thrips than normal and they may move into crops later. If this holds true, we may have fewer problems with thrips in tobacco and small fruits in 2010 than in a typical year.

Tobacco thrips models currently indicate that this spring will have low numbers of thrips. This is not surprising because cool, wet weather negatively impacts thrips during March and early April. This spring, much like last year, has been both cool and wet, until recently. It is possible that weather patterns could become more favorable for thrips, but unless the weather changes dramatically (rapidly warming and drying), I suspect that thrips movements will occur later this year and that populations will be lower than normal.

What does this mean practically? I anticipate that thrips treatments in blueberries may not be needed, and that strawberry injury may also be rare. As always, SCOUT and SAMPLE before making any treatment decision!

We continue to track and model tobacco thrips populations, and we will not have concrete predictions until the end of April, but I will continue to update as this picture becomes clearer.