This past year there have been hundreds of new high tunnels constructed in Kentucky through the NRCS cost-share program. While nearly any crop can be produced in a high tunnel, a number of growers have indicated they intend on producing tomatoes, peppers, cucumbers, greens, strawberries, and raspberries. High tunnel vegetable production has some pest problems that, while they are not unique, are uncommon with outdoor production. Like greenhouses, high tunnels provide an environment for rapid plant growth and season extension. This is also can also be an optimal environment for some plant pests.
Mites, whiteflies, thrips, aphids, and fungus gnats tend to be more common in high tunnel production. These arthropod pests have several characteristics in common including relatively short lifecycles which contributes to their ability to increase in numbers quickly. They are also tiny and some are hidden on plants which allows there early buildup to go unnoticed unless growers are actively scouting for them. Besides damaging plants through feeding, whiteflies, thrips, and aphids are also vectors of a number of viruses.
Two mite species, tomato russet mite and broad mite, produce symptoms that are often mistaken for disease or herbicide injury which slows their diagnosis and management.
Tomato russet mite is becoming one of the more common problems with high tunnel tomato production. Damage by these mites is often mistaken for disease. These are tiny mites relative to other mites, but can be seen with a 20x hand lens. They initiate their damage to the lower foliage of plants and work their way upward. Growers often describe the damage as the lower leaves being burned up or fired up. With heavy infestations the developing fruit become darkened and russeted. The damage moves rapidly up the plant and from plant to plant. The mites are easily moved from plant to plant by workers handling plants in the high tunnel. If this is suspected, growers need to look for the mites 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. Tomato russet mite feeds on plants in the Solenaceous family.
Broad mites, also known as tropical mites, are also common in high tunnels and have a very wide host range. Like tomato russet mite, the broad mite is very small and easily overlooked. The broad mite feeds in the fruit and leaf buds of the plants, but are not found on the expanded leaves. Broad mites inject saliva into plants that is toxic. Peppers are very susceptible to damage by broad mites, but they can also impact tomatoes. They cause distortion of the new foliage and stems. Damage results in a hardening of tissue and downward cupping to the young leaves. This damage can be mistaken for virus or herbicide damage. Growers with symptoms should monitor for mites with a 20x hand lens.
Among the mite pests, two-spotted spider mites are still the most common of the mite pests of high-tunnel tomatoes. They are called spider mites as the mites produce silk and webbing in apparent with large infestations. They also produce stippling of the leaves, noticeable tiny spots of the leaves where they remove cell contents with their piercing-sucking mouthparts. Mites can be found anywhere on the plant, but are most noticeable on the upper part of leaves. As with russet mites, these are moved easily by workers in the high tunnel.
There are several cultural controls that are very important to help prevent these mite problems. Growers need to begin the production cycle with a weed-free high tunnel. Outside of the high tunnel there should be a weed free zone, the wider the better, but 6 to 10 feet would be a minimum. Young plants need to be inspected to ensure they are pest free. During the production cycle, growers need to monitor for symptoms of mites and mites themselves at least weekly. If light infestations are noted, insecticidal soap and horticultural oils (be sure to follow precautions on the label to avoid injury) can be used to reduce numbers. In terms of miticides, the high tunnel is treated as a greenhouse with respect to pesticides. Pesticides prohibited in the greenhouse are also prohibited in high tunnels. There are two options for miticides to control two-spotted spider mite, Portal and Agri-Mek. For broad and tomato russet mites only Agri-Mek is available in high tunnels.
Whiteflies are common in high tunnels on tomatoes. These often go unnoticed at first as they feed only on the undersides of leaves. As populations build workers begin to notice the adults that fly when plants are disturbed. There are two species, the greenhouse silverleaf whiteflies. Of the two, the sliver leaf is more problematic in that there are biotypes that can be resistant to some insecticide IRAC groups. Silverleaf whitefly is also a vector of tomato yellow leaf curl virus that is very damaging to tomato. Silverleaf whitefly can also produce some ripening disorders on some heirloom tomatoes. Insecticides for whitefly control with high-tunnel tomatoes can be grouped into two classes, those that control only the immature stages as insect growth regulators and those that target both the immatures and adults. Because they only infest the lower surface of leaves, in is critical to get thorough coverage with foliar sprays. The same cultural controls with mites also help to prevent whitefly problems. There are commercially available wasps that can keep whiteflies at low levels, these minute wasps are first released when whitefly levels are low and rereleased periodically through the production cycle.
Thrips and aphids can be problematic in the high tunnel when their numbers are excessive, but this is relatively uncommon. More commonly they are problems as vectors of viruses, tomato spotted wilt with thrips and several other non-persistently by various species of aphids. Unfortunately, it is very difficult to control virus spread by aphids and thrips with insecticides as transmission occurs so fast. However, well timed applications of stylet oils can reduce the ability of aphids to spread virus.
There are available tomato hybrids that have resistance to some of these viruses. In some cases insecticides can help to reduce secondary spread of viruses with subsequent generations of vectors developing on infected plants. As with problems already mentioned, weed-free high tunnels, a weed-free zone around the high tunnel, pest free transplants, and regular scouting are critical to managing these problems.