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With all the mite control products available, why are two-spotted spider mites still a problem?
By Dr. Richard K. Lindquist, Senior Technical Manager, OHP, Inc.

First, the good news: there now are many products registered for spider mite control on greenhouse and nursery ornamentals. If I counted correctly, there are at least 15 active ingredients in nine different mode of action groups registered for spider mite control on greenhouse and nursery ornamental plants. I am not counting some of the “fringe” products such as pyrethroid insecticides in this total. In addition, several species of predatory mites are also used for spider mite control – often in combination with one or more of the registered miticides. Given all this, there still are problems with mite control.The question is: why? There are several possibilities. The following summary is not a complete listing, but does include some of the most important reasons.

Pesticide resistance

This usually is the first thought entering the mind when mite control is less than satisfactory, and, in fact, resistance is a good possibility but not necessarily the reason for lack of control. Resistant mite populations are fairly common on long term perennial crops (e.g. roses, gerbera), or any situation in which the resident mites are exposed to products having the same – or similar – modes of action. Pesticide resistance in mites is helped along by genetics. Female spider mites can reproduce with or without mating. If a female mite mates with a male, the offspring will usually be mostly females with a complete set of chromosomes, reducing the chances of resistance being dominant. If she does not mate, she will still have offspring, but they will all be males with a single set of chromosomes. If the genes for resistance are present in the male mites, resistance will be dominant and more mites will survive. In addition, when the resistant male mites do mate, the genes for resistance will transfer to the offspring. Basically what happens is that genes that confer resistance tend to become more concentrated in this way.

Biology and life cycle

OK,we know that resistance is a possibility. Spider mites also have a very rapid egg to adult cycle when temperatures are high. Spider mites will develop at temperatures around 52° F (11° C), but the life cycle is long. When temperatures go up, the life cycle is reduced to 7-14 days, so numbers can increase rapidly if not controlled. This increases the number of generations in a given amount of time, probably requiring more pesticide applications and increasing the chances of resistant individuals being selected.

Application problems

Spider mites are tiny and generally live on undersides of leaves.This certainly makes spray coverage an issue. High-volume sprays seem to provide better control than low and ultra-low volume sprays. Several products have translaminar activity (move within the treated leaves but not up or down the plants), including Avid®, Judo™, Pylon® and TetraSan®. OHP’s new product, Kontos™, has upward and downward systemic activity. Both translaminar and systemic activity can help overcome spray coverage problems, but will not substitute for thorough coverage of spider mite-infested areas.

Expecting the miticides to do it all

No matter how effective, no miticide, or miticide rotation program will be able to do a complete job of spider mite control. Assistance in the form of a good scouting and monitoring program, minimizing the movement of workers from infested to non-infested areas,weed control, avoiding over fertilization, and integrating biological controls will help increase the effectiveness of a spider mite control program.

*Avid is a registered trademark of Syngenta. Judo is a trademark of OHP, Inc. Pylon is registered trademark of BASF Corp. TetraSan is a registered trademark of Valent USA Corp.