Protecting veggies from mites

The twospotted spider mite (Tetranychus urticae) is a severe pest problem for greenhouse- cucumber, eggplant, pepper, and tomato. The reasons they are difficult to suppress are many: wide-host plant range, multiple generations that occur simultaneously, high reproductive potential, rapid population growth rate, short life cycle, and ability to develop resistance to miticides.
 

Biology and damage

Twospotted spider mite adults are 1/50- to 1/60-inch (0.3- to 0.45-mm) long, oval-shaped, and may vary in color from yellow-green to red-orange (Figure 1). However, this varies depending on the host plant. Adult females live approximately 30 days, producing up to 200 eggs during a two-week time period. Eggs hatch into yellow-green, six-legged larvae, which mature into eight-legged nymphs, and then adults. The twospotted spider mite life cycle, which includes an egg, larva, two nymphal stages, and adult, takes generally one to three weeks to complete; however, this depends on the ambient air temperature. Twospotted spider mite prefers hot (>80°F or 26°C), dry (<50% relative humidity) environmental conditions. For example, the life cycle from egg to adult may be completed in seven days at 84°F (29°C).

Twospotted spider mites prefer to feed on leaf undersides because they do not like exposure to sunlight. They use stylet-like mouthparts to feed within individual plant cells, resulting in damage to the spongy mesophyll, palisade parenchyma, and chloroplasts. This decreases chlorophyll content and the plant’s ability to manufacture food by means of photosynthesis. Damaged leaves by twospotted spider mite feeding appear bleached and stippled with small silvery-gray to yellowish speckles (Figure 2). Extensive twospotted spider mite feeding may cause leaf drop.

High populations of the twospotted spider mite may lead to the production of webbing on leaf undersides or plant stems where all the life stages—egg, larva, nymph, and adult—are located. Webbing provides protection from watering and sprays of miticide applications, and also allows mites to move among plants, especially when plants are spaced close together and leaves are touching each other.
 

Managing mites

The effective management of twospotted spider mite involves scouting, the implementation of appropriate cultural practices, and proper application of 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 infestation levels – especially localized infestations.

Here are the practices that need to be followed in order to manage twospotted spider mite populations:

• Avoid over-fertilizing vegetables, especially with nitrogen-based fertilizers, as this enhances the nutritional quality of plants to twospotted spider mites, resulting in greater feeding rates and more extensive plant damage. This is due to higher amino acid levels, which are essential for development and reproduction. Furthermore, over-fertilizing vegetables may result in the production of soft, succulent tissue that is easier for mites to penetrate with their stylet-like mouthparts.
• Do not allow vegetables to become moisture stressed as this increases susceptibility to mites. Moisture stressed plants, in general, tend to accumulate higher concentrations of soluble salts and amino acids, which increases their nutritional value to twospotted spider mites.
• Discard old plant material that serves as a source of twospotted spider mite populations prior to starting the next vegetable crop.
• 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 (Solanaceae) and creeping woodsorrel (Oxalis corniculata), may serve as alternate hosts or overwintering sites for twospotted spider mites.
• Use irrigation practices such as overhead watering, which may reduce twospotted spider mite populations by washing off any that are present on the tops of leaves. In addition, the increased relative humidity may suppress or sustain twospotted spider mite populations below damaging levels. Even forceful water sprays applied to the underside of leaves, where twospotted spider mite populations are typically located, may reduce twospotted spider mite populations and thus prevent damage from occurring.
   

The use of miticides is another option in suppressing populations of twospotted spider mites in greenhouses and high tunnels producing vegetable crops. However, it is important to abide by the following guidelines:

• Use the recommended label rate or rate designated for twospotted spider mites in order to obtain sufficient suppression.
• Most miticides have contact activity only, so thorough coverage of all plant parts, particularly leaf undersides is important.
• Spray applications must be performed at frequent enough intervals (between six to seven days) in order to kill life stages such as eggs that were not susceptible to previous applications.
• Apply miticides when the susceptible life stages (e.g., larva, nymph, and adult) are predominantly present. However, 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.
• Since twospotted spider mites cannot fly, populations are usually localized, so spot spraying specific areas within the greenhouse or high tunnel where mite populations are most concentrated may be more effective and less disruptive than “blanket applications.” This can also reduce labor costs, and any indirect costs associated with mitigating the potential for resistance.
• Rotate miticides with different modes of action regularly (about every two weeks) to reduce twospotted spider mite populations developing resistance. In addition, 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 reduce the prospects of resistance developing.
   

Biological control primarily involves releasing specific natural enemies. Twospotted spider mites are preyed upon by many natural enemies such as predatory thrips, minute pirate bugs, and especially predatory mites including Phytoseiulus persimilis. 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 twospotted spider mite populations reach outbreak proportions. It is important to note that when using pesticides, including insecticides, miticides, and/or fungicides to suppress populations of other pests (insects and diseases), that some pesticides may either directly or indirectly impact populations of natural enemies thus resulting in an inadvertent increase in twospotted spider mite populations. For example, pyrethroid-based pesticides may actually increase the reproductive rate and dispersal of twospotted spider mites, and carbaryl (Sevin), and certain organophosphate insecticides, may actually enhance mite development by increasing nitrogen levels in plant leaves.
 

Conclusion

Twospotted spider mites can be managed in greenhouse vegetable production systems through a holistic program that includes proper implementation of cultural practices, proper use of miticides, and/or timely releases of predatory mites. Growers who follow such practices will avoid having to deal with twospotted spider mite outbreaks and the disposal of infested plants.

In other words, the effort you put forth to protect your veggies from mites will be well worth the time and money.

Raymond Cloyd is professor and extension specialist in Horticultural Entomology/Integrated Pest Management at Kansas State University in Manhattan, Kan.