Biology & Control Of Clover Mites

Prior to World War II, the clover mite, Bryobia praetiosa, was considered a minor pest which occasionally entered homes and other structures. Since that time, however, the clover mite has become a major recurring pest in structures, particularly homes, in many areas. This has been attributed by some entomologists to the increased expansion of suburban areas with large, well maintained lawns, the ideal environment for clover mites.

Identification. The clover mite feeds on the juices of plants such as grasses, trees and shrubs. It has been found on at least 200 species of host plants. The clover mite is very small in size, measuring less than 1 millimeter in length. It may be various colors such as dark red, rusty brown or olive green. The key identifying characteristic is the front legs which are about twice the length of the other legs. The young stages of this mite are bright red in color.

Clover mites reproduce parthenogenetically which means without fertilization. All adults are females. Males have not been found in the United states. The adults lay their smooth, spherical red eggs on trees and other vegetation throughout the summer and fall. The eggs laid in the fall hatch the following spring. Eggs may be laid in such great numbers in the crotches of trees that the tree appears the color of brick dust. The eggs also may be found in large numbers on the foundations of buildings and in exterior cracks in buildings.

Although the clover mite overwinters mainly in the egg stage, the other stages may also overwinter. The winter eggs begin to hatch very early in the spring. A single generation is completed in the spring. Several generations may develop during warm summer months and only one generation in the fall.

The clover mite is tolerant of cooler temperatures and has been observed moving about in temperatures just above freezing. During the fall, these mites begin to seek out places to overwinter and lay their eggs. Cracks in foundations and heavy vegetation and leaf litter next to foundations are attractive overwintering sites. The mites are sensitive to small changes in temperatures and will begin to move about during warm weather. Mites which have entered buildings through exterior cracks may move into the living areas of buildings and be seen during these warm days in winter.

When the temperature begins to warm as spring arrives, the eggs hatch and overwintering stages become active. The bright red larvae may be seen crawling into structures through windows and under doorways. A key area to look for mites is on windows and in window sills. The moving red dots are easily seen here. The exterior of the building may appear as a red moving mass, the number of mites present are so great. The south side of the building - the warmest side of a structure during the spring - is usually where the greatest amount of mite activity is found. Clover mite problems are most evident in the spring, but infestations inside buildings may also occur during summer and fall months.

Once inside, the mites are not a threat to people or pets since they do not bite. When crushed, however, these mites leave a bright red stain which is difficult to remove.

Control. Control of clover mites involves both chemical and non-chemical methods. The customer needs to keep the vegetation cut low around the foundation and remove leaf litter. Exterior cracks and holes in the foundation and around windows and doors need to be sealed. The maintenance of a vegetation-free barrier 2 feet wide along the foundation of the structure has been sown to reduce problems with the pest. The steps will not eliminate a present infestation but may reduce the size of chance of an infestation in succeeding years.

The mites which have already migrated inside should be removed using a vacuum cleaner to avoid crushing them and laving red stains. Spraying mites with liquid residual insecticides indoors should be avoided. Application using an aerosol insecticide such as Whitmire’s PT 270-Dursban into the cracks inside the moveable portion of the window will help prevent mites from moving into the house via the window.

The key to elimination of a current infestation is in treatment of the exterior where the mites are originating. All exterior cracks in the foundation and around windows and doors should be dusted with a residual dust such as Drione. This will kill the mites which have already crawled into the cracks. If the cracks are not treated, mites residing in the cracks will continue to enter the structure, resulting in callback services and an unhappy customer.

The voids inside hollow block foundations also may need to be dusted if cracks are present which might have allowed mites to enter. If the structure has a crawl space, the foundation inside the crawl space should be inspected and treated if mites are present.

When exterior cracks have been treated, the foundation and the lawn surrounding the structure must be treated using a residual insecticide. The foundation will need to be treated about 2 or 3 feet up the side. Application must be made to the lawn or ground surrounding the structure up to 15 feet from the structure. Dursban 2E, 4E, and 50W are all labeled for clover mite control.

Rainfall soon after an application outdoors may affect the level of control achieved and reapplications may be necessary. If a heavy infestation is present when the first treatment was done, then a follow-up service five to seven days afterward may need to be scheduled to be sure the problem has been solved.

__ Stoy Hedges

The author is corporate technical manager of WMI Urban Services, Inc., Oak Brook, Ill.