A Mite too Much or Did That Tiny Thing Do That?

Doug Whitt

Master Rosarian, Charlotte Rose Society

Roses & You, July 2020

A rose garden of colorful blossoms clothed in lush foliage has no peer in the world of flowering shrubs. Managing such a unique setting is a wistful fancy of many aspiring rosarians while navigating the various recesses of their garden. It will be the garden of a dedicated individual, who best steers through the ofttimes annoying issues, that attains such a lofty ideal.

Regrettably there is no ‘perfect rose’; neither is there a flawless environment in which to grow them. Mother Nature is kind in providing the chief ingredients for success such as soil, sunlight, and rainfall, but she also supports the existence of other sorts of biota, including that which can be harmful to roses. These take many guises; from that of large forms easily recognizable, to minutiae that must be viewed under magnification.

A cryptic pest of roses that falls into the latter category has been the topic of countless articles through the years. To add another would seem a redundancy, and yet a serious discussion of the subject from time to time may be just what is needed to refresh images of a menace to be dealt with, and the consequences of passiveness. I am referring to the presence of mites.

What is a mite? It is not an insect, therefore methods of insect control have minimal effect on it. Instead, it is an arthropod in the order Acari; class Arachnida which makes it a relative of spiders, ticks, and scorpions. Fossil records of its ancestors go back to the Devonian age: it is a crafty survivor! Thousands of species of the order can be found on all continents; from the Arctic to Antarctica, from deserts to alpine regions, and even species that live in water. Their colors range from shades of gray, green, yellow, red, and almost black. Their feeding mouth parts are adapted for piercing, sucking, biting, and sawing; perfect for creating the ruin observed in a garden where they are present.

Identification of any pest is the first step in controlling it. Mites that are parasitic to roses are in the family of red spider mites, Tetranychidae, with several members of the family that victimize roses. They also prey on many garden vegetables, defoliate fruit trees, attack various annuals and perennials, and even grass and clover crops. I would describe the family as very inhospitable to all gardeners to say the least. The primary member found on area roses is the two-spotted mite, Tetranychus urticae.

This mite is less than half a millimeter in length requiring a keen eye or magnifying glass to observe. It could seem that such a tiny creature would be incapable of inflicting much damage to a plant, but consider that a lone female can live for more than a month, laying 15 to 25 eggs each day. Then with the transformation from egg, to larva, to nymph, to adult requiring as little as a week or so in hot weather, and nothing to interrupt the cycle, the mathematics become meteoric. The progeny of this female could soon number in the thousands, and multiplied by the number of ensuing egg-laying females the condition spells nothing but concern as their offspring drain away vital plant fluids while feeding.

Spider mites spend most of their life upside down on the underside of a leaf, rarely straying far from where they hatched, and only then when they have depleted the leaf’s moisture content. They cannot fly, thus must crawl or use other means to reach a new food source which is usually a branch higher up the plant. Many perish before they can relocate. They love congested foliage areas such as clutter in a plants center since food is always near at hand. Like spiders they can spin a silken thread to float in the wind, attach themselves to mobile insects, or be carried by a bird to a new destination.

Most winters tend to leave the appearance of cleansing the garden of any prior season’s pestilence. Fresh unblemished foliage to begin a new season can instill a smug sense that last year’s dilemma is history. Then one morning a change in the appearance of a few bottom leaves kindles an awakening to reality. Inspection of the undersides reveals the ‘salt and pepper’ calling card of mite presence, and in our area, next to the blackspot disease they represent the most ominous threat to plant survival.

The ‘salt and pepper’ appearance may contain all four stages of mites, webbing, molted skins, excrement, and dead adults. Yellowing foliage that eventually drops off is the final result of their presence. The silken webbing they spin is used for many purposes including securing eggs in place. I have observed a plant with the remaining uppermost foliage, including the space between stems, heavily enmeshed in webbing being used for relocation. An infestation of this magnitude leads to complete defoliation, and possibly contamination of neighboring plants in a short period of time.

The continuity of a colony from year to year is assured by adults that overwinter on green foliage of the roses, or on neighboring weeds and perennials. Then, as temperatures rise in the spring they will crawl back to the plant where the new bottom leaves provide a food source and reproductive site, and the cycle begins anew.

Now that the adversary and its capabilities are understood a course of action to protect the rose garden can be contemplated. Once detection of its presence is confirmed, and by knowing the rapidity of its expansion, a hasty response is necessary in preventing further contamination. The extent of the response is a decision of the individual rosarian based on the intended use of the roses, or other personal considerations.

Some plants exercise their own means of self-defense or are aided by natural agents in the cause. Certain insects and predatory mites feed on the parasitic mites feasting on the foliage; a positive reason for guarded use of all-purpose insecticides in a spray mixture. In my garden I note that some varieties are only sporadically besieged by mites while others seem to act as a magnet for them. Whether this is the result of the location nearest to an egg-laying female’s habitat; the leaves’ cell structure; fluid nutrient content; or some other physiological concern, it is a matter of conjecture. Plants weakened by disease are always preyed on in fulfilling nature’s rule of ‘survival of the fittest’, thus keeping a plant strong improves its ability to cope with pestilence.

If weather conditions remain favorable for mite proliferation, sheer numbers can still overwhelm the natural defenses resulting in plant desiccation and leaf drop. It is then time for the rosarian to be a step ahead of this outcome. A method in wide use is simply dislodging the mites with a strong jet of water directed upward from the base of the plant to the topmost branches. A follow-up treatment in three or four days can displace any that hatch or escaped the previous time. Repeating the process every few days, especially during the heat of summer will be good for the plants as well as control mite numbers. This act should be performed early in the day to allow for rapid evaporation from the foliage.

If a colony has become thoroughly entrenched it may be necessary to use a miticide (acaracide) to restrain their activity. Mites are adaptable creatures and it is advisable to alternate two different brands with different modes of control to prevent offspring from developing an immunity to a single mode. Some soap or oil-based products are touted as being effective against mites, but may be phytotoxic to foliage in hot weather when mites are most active. Any pesticide should be utilized in the cool part of the day, and mixed and applied as directed on the container label.

I find it beneficial to take a daily walk through the garden, if only briefly, with a wary eye out for any untoward mischief as a first line of defense. Another step I take during the growing season is to clip off the bottom foliage to six or eight inches above soil level where the mites first colonize. I am aware of the benefit of leaving as much foliage on a plant as possible, but this act may serve to remove the mite’s vanguard. Importantly, it improves access to the underside of lower foliage and to the plant’s interior when using a strong jet of water or applying a miticide.

When winter arrives, after a hard freeze or two and all growth has ceased, I clip off all foliage remaining on the plants. This not only removes any mites overwintering there but also any fungus spores that may be present. Sometime afterward I apply a dormant spray to the canes, mulch, and any adjacent weeds or plants that may be harboring mites.

Having cared for roses for almost 60 years, and dealt with most disorders that can affect a garden, I have observed that, put simply, the most effective control of any pest is early detection coupled with a timely corrective measure. This is especially important when mites are the pest. It is so much simpler to eradicate the few upon discovery than to cope with an entrenched colony of profuse numbers just a few days later.