Twospotted pest pressure

The twospotted spider mite, Tetranychus urticae, is a major pest of horticultural crops grown in greenhouses and nurseries because they: 1) feed on many different host plants, 2) have multiple generations during the growing season, 3) have short generation time, 4) females have high reproductive potential, 5) survive under varying climatic conditions, and 6) can develop resistance to miticides. This article provides a cursory overview of the biology and management of the twospotted spider mite.

Biology

The life cycle from egg to adult can be completed in seven days when temperatures are greater than 80 degrees Fahrenheit (26 degrees Celsius). Hot, dry conditions favor twospotted spider mite outbreaks. In addition, high temperatures and extended day-lengths may increase activity and feeding. High female reproductive capacity along with high temperatures and low relative humidity can lead to the development of “high” populations of twospotted spider mites.

Many plant varieties are susceptible to feeding by twospotted spider mites. In fact, plant type can influence reproduction (fecundity = number of eggs produced) by females during their lifespan. Table 1 shows the differences in eggs produced per female and their lifespan over specific time periods.

Twospotted spider mite populations are influenced by the nutritional quality (nitrogen content) of host plants, which may be associated with seasonal changes in plant physiology and fertilizer practices that cause changes in leaf tissue chemical composition. Furthermore, nutritional quality or nutrient content may be responsible for enhancing development and reproduction of twospotted spider mites. For instance, population growth and female reproduction may be positively correlated with the concentration of amino acids in plants or leaf nitrogen levels. Studies have reported that plants receiving high rates of nitrogen-based fertilizers are more susceptible to twospotted spider mites — a factor primarily affiliated with an increase in leaf nitrogen content or higher concentration of amino acids.

Management

The tolerance level for twospotted spider mites is “low” because the feeding damage caused by twospotted spider mite can substantially impact aesthetic quality and consequently crop marketability. Therefore, miticides are primarily used to suppress twospotted spider mite populations, which may increase selection pressure placed on populations due to the frequency of applying miticides. Consequently, an increase in the frequency of miticide applications can enhance the rate that mite populations develop resistance to miticides. The speed or rate of resistance depends on selection pressure or frequency of applications and the miticide application rate used. Twospotted spider mites have the ability to develop resistance to miticides in a short period of time because multiple generations can be present simultaneously. Furthermore, genetically isolated or localized populations may increase the potential for developing resistance. The reason for this is that these isolated or localized populations are less prone to immigrants or susceptible individuals from outside the greenhouse integrating with existing populations in the greenhouse, which can dilute the potential for resistance developing because susceptible individuals can breed with resistant individuals.

Avoiding resistance

Therefore, in order to alleviate the prospect of miticide resistance development, always rotate miticides with different modes of action within a generation. The idea is that the frequency of resistant individuals declines in subsequent generations in which the initial miticide is not applied. Thus, when the first miticide is reapplied, there is a high frequency of susceptible individuals to the initial miticide used compared to miticides that are applied afterward. Furthermore, be careful when mixing different pesticides as mixtures of miticides, insecticides and fungicides may increase the risk of resistance and also the occurrence of phytotoxicity (plant injury). In addition, cross and multiple resistance may be problematic in some situations. Cross resistance is based on a single resistance mechanism conferring resistance to pesticides (insecticides or miticides) in the same chemical class and/or having similar modes of action. Multiple resistance occurs when insect or mite pest populations develop resistance to more than one pesticide by means of more than one mechanism of resistance. Populations of twospotted spider mites may vary in their ability to develop miticide resistance due to variations in selection pressure, which is again associated with the frequency of miticide applications.

There are three factors (direct and indirect) affiliated with the use of miticides (and even insecticides) that can result in enhancing outbreaks of twospotted spider mites: 1) stimulating egg production (direct), 2) improving nutritional quality of the host plant (indirect), and 3) removing natural enemies that regulate twospotted spider mite populations (indirect). The removal or inhibition of natural enemies by pesticides, in fact, is one of the main factors responsible for promoting outbreaks of twospotted spider mites. Moreover, some pesticides, including insecticides and fungicides, can accelerate the dispersal behavior of twospotted spider mite populations by inducing irritation or agitation of the mites. In addition, certain pesticides can increase female reproduction and longevity of twospotted spider mite adults.

Types of miticides

The two types of miticides used against twospotted spider mite populations are contact and translaminar. Contact miticides kill mites directly or when the mite pest walks over a treated surface and any miticide residues enter the body and move to the site of action. Translaminar miticides work by penetrating leaf tissues and forming a reservoir of its active ingredient within the leaf, which provides residual activity against mites that may feed later on after residues dry. Also, miticides may differ in regard to the life stages (egg, larva, nymph and adult) that they are most active against. Table 2 lists the commercially available miticides registered for use in greenhouses and/or nurseries, and the susceptible life stages of the twospotted spider mite.

Insecticides that are used to suppress populations of other pests may have no effects on twospotted spider mite populations. However, applications of insecticides may actually lead to outbreaks of twospotted spider mites. In fact, development and reproduction of twospotted spider mites can be stimulated when exposed to certain insecticides. Some insecticides, including carbaryl may stimulate twospotted spider mite outbreaks by killing the natural enemies (e.g. predatory mites and predatory bugs) of the twospotted spider mite.

Applications of insecticides may actually lead to outbreaks of twospotted spider mites. In fact, development and reproduction of twospotted spider mites can be stimulated when exposed to certain insecticides.

Some cultural practices can influence twospotted spider mite populations. For instance, water-stressed plants are more susceptible to these pests, which can lead to outbreaks; whereas, overhead irrigation reduces twospotted spider mite populations. High-volume, high-pressure water sprays will dislodge mites from plant leaves, thus suppressing twospotted spider mite populations. The removal of heavily infested plants (greater than 50 percent infested) will help to alleviate future problems with twospotted spider mites.

Biological control agents, such as predatory mites, including Phytoseilus persimilis, can be used to regulate populations of the twospotted spider mite. However, predatory mites must be released early in the production cycle in order to maintain twospotted spider mite populations below damaging levels. The population growth of any predatory mite depends on prey density, temperature and relative humidity. In addition, plant type can impact effectiveness by decreasing searching ability, especially plants with trichomes (hairs).

In conclusion, twospotted spider mites are a seasonal problem on many plants grown in greenhouses and nurseries. However, there are a number of plant protection strategies that can be implemented in order to avoid outbreaks of twospotted spider mites, such as rotating miticides with different modes of action, irrigating plants overhead, avoiding over-fertilizing plants with nitrogen-based fertilizers, and using biological control when feasible.

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.