Crop Damage

Thrips inflict damage on vegetable crops when feeding and laying eggs. Damage from egg-laying is most common in species that infest blooms. When the eggs are inserted into the pistil walls, minute scars develop when the fruit expands. In some fruiting vegetables dimple scars develop when the fruit are fully matured. In tomatoes, such scars may result in uneven color development at maturity. In the case of western flower thrips, egg-laying scars develop into slightly raised, pale white bumps. Again, color development may be affected around such scars.

Feeding injuries occur on both fruit and foliage. Thrips infesting blooms typically lay their eggs in the pistil or other flower parts. By the time the larvae hatch, the petals and anther have often dried and fallen. Larvae in such circumstances seek shelter under the fruit's calyx. Several generations of thrips can feed and develop under the calyx of pepper fruit, damaging immature tissues which develop corky or leathery blemishes with maturity.

Disease Transmission

The principal vegetable disease transmitted by thrips is tomato spotted wilt virus (TSWV) (tomato, peanut). This disease has been known for many years, and occurs worldwide on over 200 host plants (Iwaki, et al. 1984, Cho, et al. 1986, Stobbs, et al., 1990). Symptoms are often dramatic. They include ringspotting and necrosis on fruits, foliage and in growing points. The ringspotting often involves intricate patterns of concentric bands. When young plants are infected death is often the result. TSWV can cause economic losses in several vegetable crops, especially tomatoes, peppers and lettuce.

Because TSWV infects not only vegetables, but ornamental plants and weeds as well, it poses a significant challenge to growers. This challenge is made greater by the fact that TSWV is transmitted by at least seven species of thrips (Mau, et al. 1990, Peters, et al. 1990, Webb, et al. 1997). Six of these species are included in the identification key. Unfortunately, these include some that are especially difficult to manage. The disease occurs in several strains, some of which are now considered separate viruses. Species known to transmit TSWV or one of its strains include:

• Frankliniella fusca (Hinds), the tobacco thrips
• Frankliniella occidentalis (Pergande), the western flower thrips
• Frankliniella schultzei (Trybom)
• Scirtothrips dorsalis Hood
• Thrips palmi Karny, the melon thrips
• Thrips tabaci Lindeman, the onion thrips
  
  

Although Frankliniella bispinosa has been shown to transmit TSWV in controlled experiments (Webb, et al. 1997), it is not an economically important vector. This is due to such factors as its seasonal abundance, its use of alternate hosts which are not TSWV reservoirs and the low level of TSWV inoculum in the native non-crop environment.

Thrips can only acquire the virus in the larval stage. However, they cannot transmit the disease until they reach the adult stage. This provide some window of opportunity for managing disease spread within a field through careful scouting, and timely application of effective insecticides. Unfortunately, as the mobile adult thrips often move from one host plant to another, insecticide applications cannot provide complete freedom from the disease. This threat is especially great where sensitive vegetable crops are grown interspersed among fields of TSWV reservoirs, such as peanuts. Planting vegetables sequentially through an entire season in adjacent fields can also result in disaster in late plantings, as virulifeous (virus-carrying) thrips will migrate from one field to the next.

Identifying the species of thrips that infest vegetable crops is important (Chamberlin, et al. 1993, Allen & Broadbent 1986, Paliwal 1975). It is equally important to be aware of the species that infest the surrounding crops, as well as their seasonal abundance. While TSWV may be known to occur in an area, it may rarely reach significant levels if crops are grown during periods when vector species are low in number.