Suppress those twospotted spider mites

The twospotted spider mite, Tetranychus urticae, is one of the most destructive spider mite species. Not only does it feed on a wide-range of ornamental plants, including more than 300 species grown in greenhouses and nurseries, it also has ability to develop resistance to miticides. Therefore, to effectively battle these pests, growers must understand how to effectively use miticides and develop sound rotation programs.
 

How mites resist

Twospotted spider mites possess the ability to develop resistance to miticides within a short-period of time, due to three biological parameters: short life cycle, high reproductive potential, and haplo-diploid breeding system. The risk of resistance developing in greenhouses and nurseries is especially high due to the relative isolation of spider mite populations, a long growing season, and intensive use of miticides, which is associated with the “zero tolerance” mentality for nearly all insect and mite pests of ornamental crops. In fact, a number of spider mite populations have already developed resistance to commonly used miticides (such as abamectin) due to extensive use over the years.

It is important to understand that resistance develops at the population level, not within an individual, and that populations may differ widely in regards to resistance potential. As such, producers should realize that the genes for resistance may already be present in a twospotted spider mite population before a miticide is applied although these genes usually occur at very low frequencies in the population. An individual does not become resistant to a miticide, but multiple applications of the same miticide across several spider mite generations results in the removal of susceptible individuals from the population. This can lead to a twospotted spider mite population that has a high frequency/proportion of resistant individuals, which is more difficult to suppress. The extent of resistance may vary depending on the geographical location of the spider mite population (and strain) and amount of selection pressure exerted on the spider mite population.

Continual reliance on miticides increases the probability of twospotted spider mite populations developing resistance. Rapid reproductive rate and low dispersal capability are factors that contribute to the ability of twospotted spider mite populations to develop resistance to miticides. Because twospotted spider mites cannot fly, greenhouses tend to have isolated populations with limited movement of susceptible individuals into the spider mite population to dilute resistance levels. Some long-term crops, such as roses grown for cut flower production, remain in place for years, along with twospotted spider mite populations, which are continuously exposed to a variety of miticides, thus increasing the potential for resistance developing in the population.

The genetics and breeding system (haplo-diploid) of twospotted spider mite impacts the rate of resistance, allowing them to develop resistance to miticides rapidly. For example, twospotted spider mite reproduces by a combination of sexual and asexual means resulting in young (offspring) that develop from both fertilized and unfertilized eggs. In general, within a twospotted spider mite population, males only have one copy of a resistant gene (R) whereas females have two copies (RR). Genes affiliated with resistance are fully-expressed in haploid (single set of chromosomes) males. The primary mechanism of resistance exerted by most twospotted spider mite populations is increased metabolic detoxification by enzymes such as cytochrome P450 monooxygenases, esterases, and glutathione-S-transferases. Moreover, cross resistance may occur within a spider mite population, which is based on a single resistance mechanism conferring resistance to miticides in the same chemical class and/or having similar modes of action.
 

What you need to know

It is essential that producers know which miticides their suppliers use before shipping plants. For example, northern U.S. growers that receive plants from Florida or California may have difficulty suppressing twospotted spider mite populations due to the intensive selection pressure (frequency of spray applications) and environmental conditions (temperature) that favor the development of resistance in both states. In spring, producers that receive Mandevilla x amabilis plants from Florida often complain that they cannot suppress twospotted spider mites populations, which may be associated with resistance to similar miticides—especially those with the same modes of action—that were used to keep the crop “clean” in Florida. Therefore, it is important to contact suppliers to determine what miticides were used on the crop two to three weeks prior to shipping. This will help avoid using miticides with the same mode of action.

Resistant genes can be expressed rapidly in a twospotted spider mite population within a short period of time. However, the rate at which resistance develops, within a twospotted spider mite population, to a particular miticide will vary depending on the amount of selection pressure (frequency of applying miticides) placed on the spider mite population.

The primary ways to avoid resistance involve judicious selection and timely application of miticides, and integrating them into other plant protection strategies consistent with basic pest management philosophy. Since resistance is genetically based, it is the frequency of resistance in a spider mite population that a producer attempts to mitigate. The basis of resistance management is rotating miticides with different modes of action. In general, producers should only use a miticide once or twice within a generation (depending on the time of year) then switch to another miticide with a different mode of action. When rotating miticides with different modes of action it is assumed that the frequency of individuals resistant to one miticide will decline during the application of another miticide. Furthermore, producers should incorporate broad modes of activity such as insecticidal soap (potassium salts of fatty acids) and horticultural oils (petroleum-based) in rotation programs.

Finally, label directions must be adhered to. Many currently available miticides contain resistance management information on the labels, which may include the number of applications per crop cycle, number of sequential applications, and total amount of product that may be applied per crop cycle or a combination of these statements. These directions are intended to delay or reduce the onset of resistance.
 

Conclusion

Remember, to avoid the possibility of twospotted spider mite populations developing resistance, it is important to rotate miticides with different modes of action. This will extend the longevity and effectiveness of the currently available miticides.

In addition, disposing of heavily-infested plants and using forceful water sprays to dislodge all life stages (eggs, larvae, nymphs, and adults) will be helpful in dealing with twospotted spider mites.

Follow these steps, and you’ll win the battle against twospotted spider mite populations.

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