Insect Pest - True Bugs, Thrips and Mites

Because of the bright sunny days we have had in the past 3-4 weeks as well as higher temperatures lately vegetables in high tunnels (HT) are highly susceptible to thrips and two spotted spider mite infestations because of the hot dry conditions.

At times growers may be surprised to find a fairly large number (10-15 adults and 7-10 immatures/leaf) of thrips on the leaves of cantaloupe, watermelon and potato early in the season. However, no thrips feeding dam age was visible on the leaves.

Vegetable and bramble growers in Maryland have called me often over the last two years about fruit problems in their fields possibly caused by thrips. As an overall study of the possible impact thrips may be having on vegetable and fruit quality I have been conducting a two-year survey for their numbers and species.

Thrips are a major problem for vegetable and flower growers throughout the United States. In the past 10 years, some species (in particular, western flower thrips (WFT)) have caused increasing problems. Symptoms of thrips damage include scarring and/or deformation of the leaves and fruit (Figs. 1-3). Additionally, certain thrips species can damage vegetables by vectoring viruses such as Tomato Spotted Wilt Virus (Figs. 4-5). However, some symptoms that appear to be thrips-induced can be caused by environmental or varietal factors and this can lead to increased applications of pesticides when they were not necessary.

This week I saw several high tunnel tomatoes and cucumbers with moderate to severe two spotted spider mite (TSSM) Tetranychus urticae infestations (fig. 1). These pests vary in color from white to yellow to red. Mites feed by sucking chlorophyll from the plant, which at first appears as small white or yellow marks on the top surface of the leaf (fig. 2). This damage may look like other problems early on and because the mites are difficult to see on the underside of the leaf at this stage of damage their build-up can go unnoticed. The feeding damage then progresses to loss of chlorophyll, yellowing, browning (fig. 3) and eventual death of the leaves or whole plant.

This has been an odd season, like most of them I guess, with lots of rain when we normally have drought and cool temperatures when we normally have heat. We have had problems this year with striped cucumber beetles, hornworms, leafhoppers, squash bugs and surprisingly with twospotted spider mites, Tetranychus urticae (TSSM).

About two weeks ago I heard some reports of possible damage caused by broad mites Polyphagotarsonemus latus in caneberries in the mid-Atlantic region. So I ventured out and looked at several raspberry and blackberry growing operations in Maryland. I found one farm with a light to moderate infestation of broad mites in their raspberries. This operation was relatively small consisting of a little more than a ¼ acre or so of caneberries.

I visited some strawberry fields over the last few weeks in Maryland and while most of the fields were on plastic, some were matted row production. Overall I found low levels of mites in most of the fields, with a few hot spots.

Bulb mites (usually Rhizoglyphus spp) are a problem of garlic that easily go unrecognized. Usually growers notice a general yellowing of their garlic plants with the tips of leaves often turning brown (fig 1). If you examine the bulb it can have feeding marks on the outside of the bulb (fig. 2) or the basal plate can separate easily from the bulb (fig. 3).

With the cooler weather we have had of late I was surprised to find low levels of mites in strawberry fields, with a few hot spots of mites in some high tunnels. There were two species of mites found: the two spotted spider mite, Tetranychus urticae and the cyclamen mite Phytonemus pallidus.