Striking the two-spotted spider mite

Two-spotted spider mite (Tetranychus urticae) feeding damage on tomato leaf

Mites, in particular the two-spotted spider mite (Tetranychus urticae), are a severe problem of greenhouse and high-tunnel grown vegetables, including tomato, eggplant, pepper and cucumber. Two-spotted spider mite populations are difficult to regulate because of their wide host range, multiple generations that occur simultaneously, high reproductive potential, rapid population growth rate, short lifecycle, and propensity to develop resistance to miticides.

Biology and damage
Two-spotted spider mite adults are 1/50 to 1/60 of an inch (0.3 to 0.45 mm) long, oval-shaped and may vary in color from green-yellow to red-orange. However, this varies depending on the host plant fed upon. Adult females live about 30 days and may produce up to 200 eggs during a two-week period. Eggs hatch into yellow-green, six-legged larvae, which mature into eight-legged nymphs and then adults.

The lifecycle of the two-spotted spider mite, which includes an egg, larva, two nymphal stages and adult, generally takes one to three weeks to complete. However, this is dependent on the ambient air temperature.

Two-spotted spider mites prefer hot (greater than 80° F), dry (less that 50% relative humidity) environmental conditions. For example, the lifecycle from egg to adult takes only seven days at 84° F.

Two-spotted spider mites prefer to feed on leaf undersides. They use their stylet-like mouthparts to feed within individual plant cells, damaging the spongy mesophyll, palisade parenchyma and chloroplasts.

This decreases the chlorophyll content and the plant’s ability to manufacture food via photosynthesis. Damaged leaves appear bleached and stippled with small silvery-gray to yellowish speckles. Extensive two-spotted spider mite feeding may cause leaf drop.

Abundant populations of the two-spotted spider mite may result in the production of webbing on leaf undersides or on plant stems where all the life stages (egg, larva, nymph, adult) are located.

The webbing provides protection from watering and sprays of miticide applications, and it also allows mites to move among plants, especially when plants are spaced close together and leaves are touching each other.

Managing mites
Two-spotted spider mite management involves scouting and implementing appropriate cultural practices as well as properly applying miticides. Scouting vegetable crops may be achieved by tapping plant leaves over a white sheet of paper and then using a 10X hand lens to look for mites. If conducted routinely, this will help determine the level of infestation. Factors associated with cultural practices and applying miticides include the following:

1. Avoid over-fertilizing vegetables, especially with nitrogen-based fertilizers, which enhance the nutritional value of plants to two-spotted spider mites. This is primarily due to the higher levels of amino acids, which are essential for development and reproduction. In addition, over-fertilizing vegetables may result in the production of soft, succulent tissue that is easier for mites to penetrate with their mouthparts.

2. Don’t allow vegetables to become moisture-stressed, as this increases susceptibility to mites. In general, moisture-stressed plants tend to accumulate higher concentrations of soluble salts and amino acids, thus increasing their nutritional value to two-spotted spider mites.

3. Discard old plant material that may serve as a source of two-spotted spider mite populations when the next crop is started.

4. Remove weeds and heavily infested plants from both within the greenhouse or high tunnel and around the perimeter. Many weeds, including those in the nightshade family and creeping woodsorrel (Oxalis corniculata), may serve as alternate hosts or overwintering sites for two-spotted spider mites.

5. Irrigation practices such as overhead watering may reduce two-spotted spider mite populations by washing them off the tops of leaves. In addition, the increased relative humidity may suppress or sustain two-spotted spider mite populations below damaging levels. Even forceful water sprays applied to the underside of leaves, where two-spotted spider mite populations are typically located, may reduce populations and prevent damage from occurring.

Miticides
Miticides are another option in regulating populations of two-spotted spider mites in greenhouses and high tunnels, but it is important to abide by the following guidelines:

1. Use the recommended label rate or rate designated for two-spotted spider mites to obtain sufficient regulation.

2. Since most miticides have contact activity, thorough coverage of all plant parts, particularly leaf undersides, is important.

Macro view of Adult two-spotted spider mite

3. Spray applications must be performed routinely at intervals between six to seven days to kill life stages such as eggs that were not susceptible to previous applications.

4. Apply miticides when the susceptible life stages are predominantly present. A petroleum-based oil pesticide (insecticide or miticide) or ovicide (pesticide that is active on the egg stage) may need to be tank-mixed with a contact miticide in order to kill any eggs.

5. Two-spotted spider mite populations are usually localized, so spot-spraying specific areas within the greenhouse or high tunnel where mite populations are most prevalent may be effective. This can also reduce labor costs and indirect costs associated with mitigating the potential for resistance.

6. Always rotate miticides with different modes of action regularly (in general, every two weeks) to reduce the potential of two-spotted spider mite populations developing resistance. Furthermore, try to use pesticides with broad modes of activity such as insecticidal soaps (potassium salts of fatty acids) and petroleum or neem-based oils in rotation programs with narrow modes of action to mitigate the prospects of resistance development.

Biological controls
Biological control involves either releasing or preserving existing natural enemies. Two-spotted spider mites are preyed upon by a diverse array of natural enemies, such as predatory thrips, minute pirate bugs and predatory mites. Predatory mites may be purchased from commercial suppliers and then released into greenhouses or high tunnels. However, it is critical to establish a reliable scouting program to help time releases before two-spotted spider mite populations reach outbreak levels.

Be careful when using pesticides including insecticides miticides, and/or fungicides to regulate populations of other pests (insects and diseases), as some of these pesticides may negatively impact populations of natural enemies, thus resulting in an inadvertent increase in two-spotted spider mite populations.

For example, pyrethroid-based pesticides may actually increase the reproductive rate and dispersal of two-spotted spider mites. Furthermore, carbaryl (Sevin) and certain organophosphate insecticides may actually favor mite development by increasing the nitrogen levels in plant leaves.

Two-spotted spider mites can be managed in greenhouse and high-tunnel vegetable production systems, but proper implementation of cultural practices, proper use of miticides, and/or release of predatory mites is important in order to avoid having to deal with outbreaks. In the end, this will be less costly and result in maximizing yields.

Raymond Cloyd is professor and extension specialist in horticultural entomology/integrated pest management at Kansas State University. He is also a regular columnist for Greenhouse Management magazine.

Photos Courtesy Raymond Cloyd