Expect to see two spotted spider mites on vegetables
With the hot and dry weather, we have been having expect to see two spotted spider mite (TSSM) Tetranychus urticae problems in tomatoes and cucumbers (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. They occur in the highest numbers on the undersides of leaves, but when their populations get very high they can move to the top-side of a leaf or onto fruit. Most high tunnel cucumber and tomato cultivars are susceptible to two spotted spider mite damage, but growers need to be sure to check all the different varieties of tomatoes and cucumbers they are growing for mite infestations.
Our present weather conditions favor rapid mite development and reproduction, which enables them to complete a generation in as little as 5-7 days which could lead to a 50X increase in the population in just one week. Heavily infested leaves may be covered with very fine, irregular webs in which mites and eggs can be found (fig. 1). Once the spider mites begin to make these webs they are even more difficult to control. This is because the mites hide under the protective layer of webbing making it that much more difficult to get spray coverage to the underside of the tomato leaf.
Early detection is the key for controlling two spotted spider mites. As soon as two spotted spider mites are detected, control efforts need to be taken. Infestations are highest on older leaves and at the base of the leaf near the petiole. Normally this is where I would recommend the use of insecticidal soaps or horticultural oils before mite populations build and damage occurs. Oils unfortunately should not be used under high temperatures (>90°F) and high humidities.
Biological control of TSSMs with predatory mites is used successfully in greenhouse tomato and cucumber production. How well the predatory mites work depends on the initial pest populations, temperature, humidity and cultural practices that are found in the GH. Usually, several releases are required for effective control. Unfortunately, we currently do not have enough good information about whether or not these predatory mites are effective in high tunnel situations. If used these predators are best utilized as are the organic controls—very early on in a mite infestation.
Because of the frequent harvest intervals for cucumbers and tomatoes the use of some of the synthetic miticides can be limited. Miticides that can be used on cucumbers and tomatoes that have a relatively short preharvest interval are Kanemite and Portal XLO and Acramite which has a 3-day phi. Other TSSM controls that work well but have a longer phi can be found in the 2024-2025 Mid-Atlantic Commercial Vegetable Production Recommendations guide. Thorough spray coverage is a must if you are going to achieve control of TSSM.
This article appears in Volume 15, Issue 5 of the Vegetable and Fruit News.