Inspecting and Sampling Thrips in Vegetables

Inspections monitoring thrips populations in vegetable crops require the utmost consistency and attention to detail. However, they are simple enough to be easily incorporated into a scouting routine with an economy of time and effort. The key to efficient thrips scouting lies in knowing which thrips are likely to infest a crop, and which plant parts are most likely to be infested. In the case of peppers, melon thrips infest growing points of plants of all ages, but can cause severe foliar damage in pre-bloom crops. After blooming has started, melon trips quickly infest the blooms. As the petals fall, the melon thrips larvae move under the calyx of the small fruit and feed, causing economic damage. Scouting for melon thrips therefore requires that all these plant parts be inspected, with focus on each part during its most sensitive stage.

As other species of thrips may also infest these sites, it is imperative that the scout identify which species are present. Field identifications using the identification key included in this package are a good place to start. They provide immediate feedback on which thrips are present. However, they are also time consuming, and can limit the detail needed to fully assess an infestation. For these reasons, it is advisable to supplement field identifications with microscope examination of larger samples of insects. A good method for doing this is to collect samples of blooms or foliage periodically throughout the course of a crop.

Samples should consist of enough plant parts to insure that the range of resident thrips will be included. Based on our experience, samples should include at least 20-25 blooms or growing points. Samples can be taken from representative areas within a farm, or amassed, a few plant parts at a time, from the entire farm. Collecting the plant parts directly into sealable plastic bags with enough alcohol to wet them thoroughly will kill insect specimens instantly and keep then in good condition until they can be examined. To remove the insects from the plastic bags, follow the same steps described for working with water pan traps. Be sure to agitate the bag contents well before pouring them through the coarse strainer. Returning the plant material to the bag, and washing it vigorously with water several times will dislodge insects that may remain. With practice, such samples can be strained and examined in 20-25 minutes.

Within-field Distribution Trends

A consistently pursued sampling program will establish for the scout the seasonal trends of the local thrips populations. This may seem like a lot of work, and it is! However, the payoff is the ability to anticipate what will happen in future crops. A good sampling program will also enable scouts and farm managers to evaluate changes in insecticide programs and the influence of befeicial insects. An axiom of good integrated pest management is that you can never know too much about your crop.

To establish thrips population trends in a cropping system, collect the data in an orderly fashion. Sampling intervals of once every seven to fourteen days are sufficient to define the broadest trends. For greater detail, such as in evaluating an insecticide program, sampling intervals should be shortened appropriately.

Once sampling has begun, practicing orderly record keeping is imperative. Maintaining a computerized database is an excellent way to achieve this. Be sure to include in the records such details as the crop, sample location, sample date, plant part samples, number of plant parts sampled, and the number of males, females and immature stages of each species collected. Keeping track of the beneficial insects that occur in samples will define the role they play in managing thrips numbers.