Every summer samples come into the plant clinic that exhibit the characteristic stippling and browning of spider mite feeding but mites are nowhere to be found. Nursery and landscape plants with these symptoms are often treated with pesticides even though the culprit is long gone. The culprit in these cases is frequently cool season mites. As their name implies, cool season mites are active in spring and fall when they suck fluid from cells on plant leaves and needles. In hot summer months these mites are dormant. However, it is summer when their damage becomes apparent as chlorophyll bearing cells die. Thus, by the time plants exhibit aesthetic damage the mites are gone and treatment is wasted.
In North Carolina, the most important cool season mites are the spruce spider mite (Oligonychus ununguis) and southern red mite (Oligonychus ilicis). We posted about southern red mite a couple weeks ago. Cool season mites are often not managed effectively if at all. Although we do not have all the tools we would like, we can improve management by scouting to locate infestations and treating them appropriately. Spruce spider mite feeds on coniferous evergreens such as spruce, juniper, hemlock, and arborvitae. The most efficient method of scouting for cool season mites (and other mites) is to hold a piece of white paper or a paper plate below a branch and strike it with a pencil or stick to dislodge arthropods. Spider mites will appear as tiny moving specks about the size of the period at the end of this sentence.
Conifers damaged by spruce spider mites will develop areas of brown, dead needles some of which will begin to drop from the plant. However, damaged needles are generally not replaced on conifers so aesthetic injury will be permanent. Thus, it is important to scout for and treat mite infestations before damage occurs.
Effective spider mite control relies on several factors that are common to pesticide applications in general. First, select a product that will target spider mites but without killing all the natural enemies that will help clean up mites and eggs that were missed (there will always be some that are missed). Second, make applications early in the season when populations are small, and mites are young and thus most susceptible to pesticides. Finally, thorough coverage is essential. Think about where these mites live: on the undersides of leaves and on old needles that are deep in conifer foliage. Be sure to cover these areas when spraying. Check out Nursery Management article by Adam Dale for more information.