Northeast North Carolina and southeast Virginia have been aptly dubbed ‘thrips central’ and getting ahead of these pesky, yield-robbing little insects is a must for high yielding crops from cotton to vegetables.
There are three primary species of thrips that attack crops in the Upper Southeast: tobacco thrips, western flower thrips and flower thrips.
One of the keys to managing thrips, says long-time thrips fighter and North Carolina State Entomologist George Kennedy is to know which species is causing the problem. Sounds simple, but even with a compound microscope and a trained eye it’s not easy. And, from the cab of a tractor or pickup truck it’s impossible.
Sometimes ‘when’ a species of thrips is found in a field can be a good way to identify the species. Typically, tobacco thrips are earlier season problems and are often responsible for early season transmission of tomato spotted wilt virus. Often this occurs when thrips feed on earlier planted vegetable crops and then move into peanuts and other crops in the region, Kennedy says.
Tobacco thrips are black, or at least darker than the other two primary thrips species and can sometimes be distinguished from western flower and flower thrips, if a grower is really skilled at scouting and identifying these pests.
Western flower thrips usually occur later in the season. It causes fruit and foliage damage, though usually fruit damage from direct feeding. It can be a significant cause of in-season spread of tomato wilt virus.
Flower thrips are usually the most abundant species of thrips found. Rarely does this species cause significant problems in crops.
Occasionally, when they occur in very high numbers, flower thrips can be a problem, but typically they are not a big enough threat to warrant an insecticide application.
Tobacco thrips are primarily a foliage feeder and western flower and flower thrips are primarily a flower feeder, but can feed on foliage.
Damage from flower and western flower thrips occurs when fruit is small and may be on the developing fruit within the flower shortly after pollination. As the fruit expands, dimpling can be seen throughout the cotton plant.
Damage from spider mites is very similar to damage done by western flower thrips.
Clearly, it is important to know whether the damage comes from mites or thrips, Kennedy stresses.
Different management techniques
“The way mites and thrips are managed is very different and too many growers waste insecticide because they are treating for the wrong critter,” he adds.
In seedling cotton, thrips damage tender leaves and terminal buds with their sharp mouthparts and feed on the escaping juices. Leaves may turn brown on the edges, develop a silvery color, or may become distorted and curl upward.
Light thrips infestations tend to delay plant growth and retard maturity. Once cotton plants are 4 to 6 weeks old, they usually outgrow thrips damage and begin to recover.
In most areas, plants recover and suffer no yield loss, if infestations remain light throughout the first few weeks after cotton emergence.
Heavy infestations may kill terminal buds or even entire plants. Damaged terminal buds result in abnormal branching patterns.
If late infestation persists, plant maturity may be delayed with resulting yield loss. In some areas of northeast North Carolina and southeast Virginia, this is almost an annual occurrence.
In this area, often called ‘thrips central’, thrips over-winter as hibernating adults in sheltered areas, as larvae on plants, or as pupae in the soil.
The larvae usually feed for approximately 6 days before pupating in the soil. Approximately 4 days later, new adults emerge which soon feed and lay eggs.
Most thrips species complete five or more generations per year in the area.
Persistent heavy populations of thrips in the area, which is a heavy production area for both cotton and peanuts, has created a relatively new problem with tomato spotted wilt virus, which is vectored by thrips.
Managing tomato spotted wilt virus (TSWV) starts with the understanding of how thrips spread the virus. If an adult lands on a plant that is infected with TSWV, that insect cannot spread TSWV. The only way it can be spread it is when thrips feeds on an infected plant immediately after the insect hatches from an egg.
Kennedy stresses that only tobacco thrips and western flower thrips can transmit TSWV from one plant to another. Once the insect is infected, it can spread the disease from plant to plant for the rest of the pest’s life.
“What that means in practical terms, is that the grower doesn’t have to worry about TSWV being spread within a field, unless there is a reproducing population of western flower thrips or tobacco thrips in that field,”Kennedy says.
Tobacco thrips tend to move around from the field to border areas and they reproduce in either, seemingly with little preference of one over the other. Western flower thrips tend to stay in a field, especially if the crop is flowering.
Since TSWV can only be transmitted when emerging larvae feed on plants, the real threat to movement of the virus in a field comes from populations of western flower thrips that stay in a cotton field or a vegetable field and reproduce there.
Good control options available
Growers have some good options for thrips control, including two seed treatment options and seed treatments plus a foliar application.
With the recent approval by the EPA of a new formulation of aldicarb to be sold as Meymik, growers will have the once standard option of using aldicarb (Temik) for early season thrips control.
Meymik will not likely be available to growers for the first part of the 2012 growing season, but should be available in plentiful supply by the 2013 season. It will most likely takeover the spot once held by Temik as the standard thrips treatment in North Carolina and Virginia.
North Carolina State University Entomologist Jack Bachelor notes that even though aldicarb is considered the standard treatment for thrips, under dry conditions, uptake of aldicarb by the plant can be reduced to the point that a foliar treatment may be needed.
It is not clear at this point what the use rates will be for Meymik, but if it is similar to the old three and five pound per acre rates of Temik, the product often provides early grow-off and fruit set in cotton and other crops. In cotton planted after May 15-20, the lower rate has traditionally been adequate to control thrips.
For the past five years or more both of the commonly used cottonseed treatments for thrips have performed well in field tests in all categories, except for persistence. Especially in early planted cotton, these materials may be gone before thrips populations reach their seasonal peak.
Bachelor says growers should plan on only three weeks or so of protection against thrips from either of the commonly used seed treatments.
In some cases, when using the seed treatments, a foliar application of Orthene or other similar insecticide may be needed to provide adequate protection against thirps.
Both cotton and peanut acreage is expected to be up in Tidewater area of North Carolina and Virginia in 2012.
To maximize production and reduce loss to thrips damage and to TSWV, growers will benefit from knowing their thrips and by managing these destructive little insects wisely.