While growers can potentially find any type of insect pest in the high tunnel, whiteflies and thrips are the most common… and the most troublesome. In addition to insects, several species of mites are serious and common pests. These pests have several characteristics in common that makes them difficult to manage, including multiple generations per cropping cycle and small body size (thus, they are hard to find when populations are low). In addition, there are relatively few pesticides that are effective against these pests. Because of this, high-tunnel vegetable growers need to begin managing these pests when crops are started and long before pests become a problem.
Species & Description
Several whitefly species can be pests in the greenhouse, including sweetpotato whitefly (SPWF), greenhouse whitefly (GHWF), and banded wing whitefly (BWWF), but it is the sweet potato whitefly (also called the silverleaf whitefly) that we see most often in samples sent to the diagnostic labs. Growers should be able to identify whiteflies, but they will need a hand lens with a 10x or higher magnification. SPWF (Figure 1) generally hold their wings more vertically than GHWF, so there is often a space between the wings above the abdomen. With GHWF (Figure 2), the wings touch or overlap slightly above the abdomen. The nymphal and pupal stages of the GHWF have more than a dozen long filaments and many much shorter filaments that ring the edge of the body. While the SPWF nymph and pupa have filaments, they are less noticeable and fewer in number. The SPWF can be much more difficult to control with insecticides. SPWF can also vector tomato yellow leaf curl virus in Kentucky.
Monitoring Pest Level
Whiteflies are only found on the undersides of leaves. Simply tapping a plant will cause the adults to flutter around the plants, but growers need to turn over and examine mature leaves to look for the immature stages. Healthy whitefly immatures appear yellow; those that are whitish and transparent have already emerged as adults; and those that are darkened are dead or have been parasitized by wasps. It is important for growers to monitor whiteflies weekly and record the number of adult and immature whiteflies per leaflet. Examining five leaflets per plant from five plants on a weekly basis can be sufficient for a small high tunnel. It is important to search various plants in the high tunnel in order to identify hot spots. These records enable growers to see changes in whitefly populations from week to week and over the cropping cycle. Using records, we can determine the effectiveness and residual activity of insecticides or biological control agents. Yellow sticky cards can be an effective sampling tool for assessing whitefly populations.
There are several natural enemies of whiteflies that can be purchased and released in greenhouses or high tunnels. While natural enemies do not eliminate their hosts, they can significantly slow the rate of growth of the pest populations and can keep populations at levels below those that cause economic damage. Wasps belonging to the Encarsia and Eretmocerus (Figure 3) groups attack whitefly immatures, but it is important to identify the whitefly species and match it with the corresponding wasp species. It is also important to release wasps when whitefly densities are low and immature life stages are present.
Many of the field insecticides used to control the same pests on the same crop are prohibited in the greenhouse. With respect to pesticide use in the high tunnel, the high tunnel is considered as a greenhouse. A list of the greenhouse prohibited insecticides for vegetables is listed at the end (page 129) of Vegetable Production guide for Commercial Growers (ID-36). There are several insect growth regulators that are used against immature stages (IRAC 7C, 15, 16) and a few neonicotinoids (IRAC 4A) and sulfoxaflor (IRAC 4C) that target immature and adult whiteflies. It is important to rotate among modes of action (IRAC groups) with each whitefly generation. Some producers are using Neem for maintenance sprays when pest populations are low. If you are planning to release wasps, it is critical to avoid spraying insecticides after the release is made.
Species, Description & Damage
There are several species of thrips in high tunnels that are very difficult to manage, particularly when populations are allowed to get large. Two common thrips species in the greenhouse are the western flower thrips and onion thrips. Both of these cause stipling of tissue, scarring of plants, and distortion of plant growth. Thrips can also vector tomato spotted wilt virus and impatiens necrotic spot virus.
Thrips are tiny (only 1/20 of an inch when full grown) and often hide in protected parts of plants. This can make detection of early infestations a bit more challenging, but because controls for thrips are limited, early detection of infestations is critical for good management.
Monitoring Pest Level
We recommend two methods to monitor for thrips: brightly colored yellow or blue stick cards, and tapping flower or bud clusters over a white piece of paper. Sticky cards are hung just above the tops of the plants with a minimum of 1 per 2000 sqare feet or 3 in a tunnel, whichever is greater. They can be placed near entrances, ventilators, or other potential entry points. Because thrips may be hidden in flower clusters or buds, it is helpful during scouting to tap these structures over a white piece of paper to record thrips numbers. Growers should examine at least five flower clusters or buds per plant on at least five plants on a weekly basis. As with whiteflies, record the numbers to track changes over time and potential hot spots in the high tunnel.
It is important to sample for thrips when purchasing plants, cuttings, or seedlings and before bringing them into your high tunnel. If multiple high tunnels are available, plants from different sources should be isolated until you determine if they are infested.
In terms of biological control, there are several species of predaceous mites and minute pirate bugs that are available commercially to release and attack thrips. These are most often more effective against the immature stages. Biocontrol agents are not compatible with many common greenhouse insecticides, so insecticides need to be selected carefully and used judiciously. Biological control is most effective when started when thrips first begin to buildup in the high tunnel. Two commercial formulations of entomopathogenic fungi can also contribute to suppression of thrips.
Species & Description
There are three species of mites that can be common on high tunnel vegetables: two-spotted spider mite (TSSM), broad mite (BM), and tomato russet mite (TRM). While the first two species have a wide host range, TRM is only a problem on tomatoes, but it can be a devastating pest. Traditionally TSSM has been the most common mite in protected production systems in Kentucky, but recently BM (Figure 5) and TRM (Figure 6) have become much more common. Mites are not insects, so growers should not be surprised when insecticides have no effect on mite populations. In fact, some insecticides may increase mite populations by eliminating their natural enemies.
Each of these mite species can damage plants in different ways. TSSM causes stippling and bronzing of leaves, and a telltale webbing is apparent when populations are high. TSSM is also the largest of these three pests. The enzymes injected by BM while feeding is toxic to peppers and can cause symptoms that are easily mistaken for herbicide injury or virus infection. TRM, smallest of the three species, causes tomato leaves to turn brown and die, and results in russetting of fruit (Figure 7). This too can be mistaken for disease or herbicide damage.
Monitoring Pest Level
As these mites are very small, it is often more effective to scout for their damage than for the mites themselves. With TSSM, look for stippling or bronzing of leaves, webbing between leaves and or fruit, and gold flecking on tomatoes (the latter may also be due to thrips). With TRM, look for stem bronzing, premature leaf death low on the plant, or fruit russetting. TRM advance upwards on the plant and are not found on dead leaves. Mite problems often begin near windows, ventilators, or doors to the high tunnel/greenhouse.
In terms of miticides, there are several field miticides that are prohibited in the greenhouse/high tunnel (e.g. Oberon, Portal, and Movento), but Acramite, Agri-Mek, Kanemite, and Nealta are all approved miticides for tomatoes without greenhouse restrictions. These are effective against spider mites but not necessarily BM and TRM. The only tomato miticide that lists these mites on the label and does not have a greenhouse prohibition is Agri-Mek and similar generics.
With biological control of mites, there are several predatory mites, a predatory bug, and a gall midge that are sold commercially to control spider mites in the greenhouse/high tunnel. There are also several predatory mites that control broad mites and cyclamen mites. There is one predatory mite that can help to control TRM if released ahead of the advancing infestation. Again, the compatibility of miticide sprays with releases of predatory mites can be a major management concern.
Ric Bessin, Extension Entomologist and John Obrycki, State Entomologist