Monday, April 17, 2017

Clover mites: The other spider mites

A typical individual of the genus Bryobia, often called a "clover mite". Photo by Scott Justis.

Most gardeners and plant-keepers are familiar with spider mites, including well known pests like two-spotted spider mites (Tetranychus urticae), boxwood spider mites (Eurytetranychus buxi), and spruce spider mites (Oligonychus ununguis). However, some people each spring or fall become VERY familiar with another group of spider mites: so-called "clover mites" in the genus Bryobia.

Clover mites can become a nuisance when they invade homes in large numbers. This spring we have seen a  number of samples submitted to the clinic already. But first, a little bit about these mites.

What are they?

Clover mites are arachnids and true members of the mite family Tetranychidae (spider mites). All members of this family feed on plants throughout their life. Typical spider mites mentioned above are in the subfamily Tetranychinae - these are the most commonly encountered pest species. Clover mites belong to the other less common subfamily: Bryobiinae. Within this subfamily are several genera including a few that are also pests of plants. One of the more well known, for example, is the brown wheat mite (Petrobia latens (Müller)) a pest of wheat and barley that is also known to transmit barley yellow streak mosaic virus. Members of this subfamily do not spin silk as is common in Tetranychinae.

How do you identify these mites?

As is typical for many mites, clover mites are small. Adults are around 1 mm in length and the young are smaller.

A dried clover mite specimen on a US penny. Photo by Matt Bertone.

If you can observe them up close there are several characteristics that can help identify them as clover mites (or at least members of the genus Bryobia). They are typically green or brown (sometimes grayish) with red or orange legs. Their first pair of walking legs is elongate compared to the other legs. They also have a wrinkly body, appearing as if it was made by a fingerprint. One feature that seems fairly distinct for the genus is a four-pronged tip of the snout (above the mouthparts), each "tine" tipped with a flattened scale-like seta. Scale-like setae are also present on the body. Other that these traits, specimens need to be mounted on slides and viewed under a compound microscope to see the small structures.

High magnification photo of clover mite specimens showing traits of the group. Photo by Pia Scanlon DAFWA

What do they do?

Clover mites will feed on a variety of different plant types. In fact, the different feeding forms have long been considered different species in a species complex (often commonly referred to by the scientific name "Bryobia praetiosa"). A few of the variants mentioned by Jeppson et al. (1975) are as follows: (1) those that feed on fruit trees, are multivoltine (many generations per year) and overwinter as eggs; (2) those that feed on a wide range of herbaceous hosts, univoltine (one generation per year) or multivoltine, and overwinter at various stages; (3) those that are specific to English ivy, multivoltine, and overwinter at various stages; and (4) those that infest gooseberry, are univoltine, and overwinter as eggs.

A group of clover mites on grass. Photo by Whitney Cranshaw

Injury to plants is typical of other spider mites, showing a general linear or random stipple pattern:

Clover mite damage on impatiens. Photo by Rayanne Lehman

Clover mites are rarely an important pest of plants. They are most active in cooler weather - thus this post may already be a tad late. The following is a nice phenogram from Jeppson et al. (1975) showing when certain mite stages are present in Canada:

One of the main issues with clover mites - and the reason I mentioned for writing this post - is that high populations often hatch or become active in the vegetation around the foundations of human structures. This usually happens in the spring and fall. Despite only feeding on plants, they can travel into houses in huge numbers and worry homeowners. They are especially worrisome when the residents squish the mites, causing a red smear. Although it's the mite's blood, people mistake it for theirs and think they are dealing with a blood-feeding organism (which they are not). Although the stains can persist, the mites themselves very quickly dry out and die inside. Thus, no chemical control is recommended for inside the home; one can simply wait for them to die or vacuum up the mites. Insecticides can be used on the vegetation and foundation within 18" of the structure, but is rarely needed unless the mites have been an issue. Many general, over-the-counter insecticides used for various arthropod pests will work, but please heed local application laws and product labels.


Jeppson, L. R., Keifer, H. H., & Baker, E. W. (1975). Mites injurious to economic plants. Univ of California Press.


P.S. - People often mistake a few common mites that crawl around the foundations of homes for "clover mites" (enough for a future post, perhaps). Here are two common RED mites you might see.

velvet mites (Trombidiidae):

Velvet mites are like small strawberries, but large for mites (typically 4 mm long, but some over a centimeter!). They are parasitic on insects as larvae and predators as adults. They do not bite humans.

concrete mites (Erythraeidae: Balaustium sp.):

Concrete mites are smaller than velvet mites but larger than clover mites. They are often abundant on concrete (hence the name) where they scavenges food. In some situations they have been known to bite people, but do not suck our blood. These situations are rare.