Mites can be challenging pests to manage in greenhouses and high tunnels. This is partly due to their small size; mites can go undetected until their numbers build and damage becomes apparent. We are fortunate to have several miticides available for many of our high tunnel vegetable crops. However, these materials need to be managed appropriately because there are limitations on the number of uses in greenhouses, mite populations have the ability to become resistant to miticides that are over-used, and some miticides are effective against only certain species of mites. As a result, regular weekly monitoring becomes the backbone for mite management.
Scouting for Mites and Their Damage
In the case of vegetable crops, there are three primary mite species to scout for routinely: (1) two-spotted spider mite, (2) tomato russet mite, and (3) broad mite.
Two-spotted Spider Mite
The two spotted spider mite (Figure 1) is the largest of these mites when fully mature, and it usually has a very noticeable dark spot on each side. Body color is variable from yellow to tan to green.
They are spider mites because they produce webbing and the webbing is noticeable between plant structures when populations are large (Figure 2). Generally, producers should look for the spider mites and the stippling (small light-colored spots) they cause on leaves.
Tomato Russet Mite
Prior to 2009, we had not seen damage due to tomato russet mite, but since that time, these mites have become one of the more common pest problems in high tunnel tomato production in Kentucky. Tomato russet mite feeds on plants in the Solanaceous family.
While these are tiny mites relative to other mite species, they can be seen with a 20x hand lens (Figure 3). Because russet mites are difficult to find, growers should scout for their damage.
Early signs of damage are either appearance of small russeted fruit or bronzing to stems of plants. Problems often begin near bases of plants, and then work upward. Damage by this mite is often mistaken for disease. Incredible numbers of minute mites will develop and cause leaves to turn brown and die. The mites move from these leaves in search of new foliage. Growers often describe the damage to the lower leaves as being “burned” or “fired up.”
Feeding on fruit causes it to become darkened and russeted (Figure 4). The damage moves rapidly up the plant and from plant to plant; the mites can be blown between plants or carried by workers handling plants in the high tunnel.
If russet mite is suspected, growers need to look for them on the upper and lower sides of green foliage, just above the damaged leaves. Growers are often surprised how rapidly tomato russet mite problems move through the high tunnel.
This mite is not in the same family as spider mites, so miticides that just control spider mites may not be effective.
The other mite we see occasionally on peppers and tomatoes in high tunnels is broad mite. It is also known as tropical mite, and it has a very wide distribution and host range. Like tomato russet mite, the broad mite is minute, but also hidden near the bud of plants. It feeds in buds and is not found on expanded leaves. Because this mite is difficult to find, growers should scout for its damage.
Broad mites inject saliva, which can be toxic, into plants. Peppers are very susceptible to damage by broad mites, but these mites can also impact tomatoes. Broad mites cause distortion of new foliage and stems. Damage results in a hardening of tissue and downward cupping of young leaves (Figure 5). This damage can be mistaken for virus or herbicide damage. Growers observing symptoms should monitor for mites with a 20x hand lens. Broad mite eggs are characteristic: they are translucent and appear to be covered in jewels.
Each of these species is often kept under control in the field through predation by natural enemies. However, reliance on broad-spectrum insecticides can impact natural enemies and result in damaging populations. In these situations, miticides may be needed, but keep in mind that not all miticides control all mite species.
Each miticide has restrictions on maximum use per crop cycle, and some have restrictions against consecutive applications. Back-to-back applications with the same miticide may foster the development of resistance. Miticides are not use preventively, only when mite problems are first noticed. If a second application is needed, a miticide from a different IRAC group needs to be used. Generally, producers should wait a week after a miticide application to assess its effectiveness.
Available Greenhouse and High Tunnel Miticides
|Miticide||Active ingredient||IRAC Group||For use on these mites||For use on these crop groups|
|Spider mites||Broad mite||Russet mite||Fruiting veg.||Cucurbit veg.||Leafy greens|
*1 – Spinach and head lettuce only; *2 – Pepper and eggplant only
*3 – Greenhouse/high tunnel use only; *4 – Cucumber and melon only
*5 – Tomato only
By Ric Bessin, Extension Entomologist