Fact Sheets And Publications

Factors that contribute to Spider Mite Outbreaks in Vegetables

Spider mites tend to be more severe and treatments required more often in vegetables for several reasons. On Delmarva, we grow many vegetables on black plastic to warm the soil early. Warmer conditions promote spider mite development. Our sandy soils hold less water causing weeds around field edges to frequently become water stressed. This stimulates mites to move to adjacent crops. Vegetable production also tends to use neonicotinoid and pyrethroid insecticides much more frequently than field crop production. Neonicotinoids can stimulate mite reproduction via a phenomenon known as hormoligosis2. Hormoligosis occurs when a mild stressor (such as a pesticide application) causes an organism to increase reproduction. Both pyrethroids and neonicotinoids kill beneficial arthropods that would otherwise eat spider mites. Frequent fungicides are required to maintain vigor and quality in vegetables, but they also can kill beneficial fungi that attack mites, such as Neozygites. Finally, high fertilizer use in vegetables creates plants that are more nutritious for the mites, increasing population growth.

Neoseiulus fallacis - A native Phytoseiid that is a light cream to mottled color. It is not as prey-specific as P. persimilis. It can sustain itself for some time on other mite groups, thrips, bark lice, and whiteflies, as well as some plant pollens^5,6. It can be an important natural enemy in more permanent environments such as orchards where several preferred prey items are available during the year. This species has been found in low numbers around the edges of some vegetable fields in Delaware. It does not control spider mite populations as quickly as P. persimilis but persists in the environment longer and is more tolerant of drier conditions. 

In our experience, it takes close to three weeks for commercially purchased predators to establish themselves and reproduce to a point where it may be possible to exert control on two spotted spider mites. Ratios between 1:10 and 1:20 predators: prey should be sufficient to cause an imminent spider mite population crash. If purchasing predatory mites, keep shipping costs in mind. Predators should be shipped overnight and deployed as soon as possible upon arrival. If immediate deployment is not possible, they can be stored in a cool dark room, or a ‘warm’ refrigerator for a brief period of time (no more than a day). Predators should be deployed either early in the morning or late in the afternoon to minimize abiotic stress during release and to prevent them from overheating in the bottle as they are being released. Phytoseiulus persimilis is the only predatory mite that can handle the trichomes of tomato, and even then, establishment is much slower than in other crops. 

Other non-mite, commercially available predators that are important biological control agents include predatory midges (Feltiella), pirate bugs (Orius), six-spotted thrips, and lady beetles (Stethorus). Predatory midges, pirate bugs, and Stethorus are commonly encountered in spider mite infested weeds and fields. However, their populations usually develop slower. We do not have any experience field releasing these, and they are much more expensive. In addition, they all have wings, allowing them to rapidly disperse out of the target area. As such, they would be of better use in a greenhouse and should be conserved by using IPM-friendly pesticides when they are present.  

Neozygites floridana is a species of fungus that can infect spider mites and multiply rapidly to kill a mite and mummify it and then infect more mites7. It is one of the most significant factors in spider mite population declines in field crops, but application of broad-spectrum fungicides can reduce its effectiveness^8,9. 

Chemical Control

TSSM can be difficult to control with chemicals. Their rapid reproduction and development increases the chances of resistance development within a single season. Spider mites can also form highly inbred colonies because an unmated female can lay unfertilized (male) eggs. Once offspring males mature, they can inbreed with the female and she can then lay fertilized (female) eggs. This allows resistance to spread rapidly¹⁰. The results of resistance include continued crop damage and the need to make a reapplication, both of which reduces revenue from the field. TSSM are resistant to most pyrethroids and in some cases certain chemicals can increase oviposition rates¹¹. Rotating miticides among mode of action group numbers will help prevent resistance. It is best to keep in mind the life cycle of the spider mite that a chemical targets because some chemicals affect the eggs while others focus on mobile growth stages.

OMRI approved miticides include horticultural oils, sulfur, insecticidal soaps, some biopesticides, and products containing plant essential oils. Oil and soap products work as contact agents that smother the mites. As such, they require both high use rates and high water volumes. Horticultural oils also run the risk of phytotoxicity if applied in warm to hot weather conditions. These products tend to have very little residual activity. 

The table below lists miticides labeled for various crops. There are generic formulations of some chemicals that may differ in use patterns, rates, and crops they may be applied to. Different Modes of Action (MOAs) target different parts of mite physiology. As always, consult labels for usage, application directions, and restrictions. Also pay attention to maximum use rates per acre; many miticides can also only be applied once or twice per season.

Green is compatible with predatory mites and beneficial insects unless otherwise noted; orange represents moderate toxicity, and red represents high toxicity. Sources include University of California IPM webpage, www.biobestgroup.com, and Bergeron and Schmidt-Jeffris 2020¹³