Mites and Thrips

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).

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.

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.

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.

Over the last three months a few early season high tunnel operations on the Eastern Shore were having problems with some of their seedlings and leaf crops. Crops like spinach would have ‘whitening’ and then browning and eventually dead margins of their leaves while seedlings would collapse. The problem was found to be ‘red legged winter mites’ Penthaleus dorsalis, which is a new pest in vegetables and herbs for us (fig 1).

We have had onion and garlic bulb mites and now this same mite group is being found in spinach crowns where they are called--wait for it--spinach crown mites Rhizoglyphus sp. County educator Ben Beale in southern Maryland found crown mites after he inspected damaged spinach plants and mites also were found in northern Maryland spinach fields.

The Red legged winter mite, Penthaleus dorsalis has been found in more high tunnels (HTs) growing leafy greens this past winter for the 3rd year in a row. Most of the reports I have gotten from HTs are from the Eastern seaboard, but now I am receiving reports from locations further inland, such as Baltimore and Howard counties. Red legged winter mites have dark purple/black bodies and bright red legs (fig. 1). They thrive in what we would normally consider conditions too cold for an arthropod to cause problems.

While visiting some strawberry fields over the last few weeks, I ran into a few areas that had two spotted spider mite (TSSM) feeding. These were mostly in fields that had been using row cover. Spider mites Tetranychus urticae are well adapted to high-temperatures and can complete their life cycle in as little as 7 days when temperatures are > 80º F. The temperatures under row covers in the early part of our season were much above normal and rainfall was below normal leaving conditions hot and dry, which permitted spider mites (if present) to develop quickly. Warm dry conditions along with the nitrogen content of leaves greatly influence TSSM reproduction rate. Applying excessive nitrogen favors spider mite outbreaks.