“Northwest Missouri Horticulture”
by Tim Baker
Recently I was talking with a grower who was having problems with his watermelons. From his description, it sounded like it could be spider mites. With the hot and dry weather we have been experiencing lately, it seemed like a strong possibility. These conditions really favor spider mite growth and reproduction.
When I visited his watermelon field, he showed me melons with classic mite symptoms: the leaves had ill-defined yellow spots where the mites had been feeding. I later looked at the leaves under a microscope. They were full of spider mites.
Spider mites need to be addressed quickly. Under hot, dry conditions, spider mite populations can literally explode and take over fields extremely quickly. They have been known to destroy entire melon fields in a few days.
Mite damage may look like a disease problem. The leaves turn yellow and start to die. When I first started Extension in the Bootheel, many watermelon growers were unfamiliar with the damage that mites could cause. They sometimes would remark to me that when they saw these symptoms, they would step up their fungicide sprays, but it didn’t help. Of course in this case, a fungicide spray program won't help, since you're dealing with a mite, not a fungus. Miticides are what are needed.
Spider mites can attack almost any plant, including ornamentals in your front yard. I’ve seen them on ornamental bushes, apple trees, peach trees, cantaloupes, and watermelons. There are different species of mites which attack different plants. The mite usually seen in watermelons and cantaloupes is the two-spotted spider mite.
In Southeast Missouri, I frequently saw the worst cases of spider mite outbreaks in fields with heavy insecticide use. While some insecticides are labeled for mites, I have seen situations where the beneficial insects that usually control the mites are killed off by these insecticide/miticides, and then mite populations explode. In these situations, we especially recommend a regular miticide, which should not harm the beneficial insects.
To check for spider mites, a hand lens is useful. The female is oval shaped and about 1/50 inch in length. The male is smaller. They come in many colors: red, reddish yellow, yellow, greenish, or blackish. They have two large spots on each side of the body. Sometimes you can see small webs where eggs have been laid.
If you suspect spider mites, but cannot tell for sure, give me a call. Mites can be devastating and can destroy fields rapidly. It's best to catch them in the early stages when control is easier.
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