by Roger Bryan

One of the most comprehensive articles dealing with spider mites is one by Dr. Ray Cloyd of the University of Illinois . This article, with the title “Dealing with Twospotted Spider Mites on Roses,” appeared in the April 2006 issue of the American Rose magazine.  I wrote  this brief article for Basal Breaks to supplement the information provided in Dr. Cloyd’s article by identifying several new, and very effective miticides. Moreover, I also want to restate several of Dr. Cloyd’s observations pertaining to spider mites to serve as background for the introduction of the new miticides.

To begin, most rosarians already know that the two-spotted spider mite (tetranychus urticae) is a serious rose garden pest. This nearly invisible insect works on the bottoms of leaves sucking the chlorophyll, nitrogen and amino acids out of them causing the leaves to die and fall off the bush. Spider mites can take a healthy bush to a leafless stick in a matter of days unless thwarted by constant vigilance of plant health and condition, and by applying the intensive spray program described by Dr. Cloyd using Avid to kill the adult mites and Hexygon to kill the eggs and larva, or Floramite to kill all life stages.

The aforementioned chemicals all work as contact or, at best, translaminar (i.e., Avid) sprays so special care must be taken to spray the undersides of the leaves – Avid, being translaminar, does penetrate the leaf structure and will move from the top surface to the bottom, but it only kills the adult mites. The life cycle of the spider mite – from egg to adult – is five to 20 days depending on factors such as temperature and humidity. If you don’t get all life stages when you spray, the mites will probably return, and they may have become resistant to the spray(s).

You may be wondering why there are no systemic miticides. Systemic pesticides enter the bush through its roots (when applied as a drench to the soil around the bush) or through it leaves (when applied as a spray) and then move through the plant’s vascular system via the phloem and xylem. The phloem cells are laid out end-to-end throughout the entire plant, and serve to transport the sugars and other molecules created by the plant and systemic chemicals. The xylem of a plant is the system of tubes and transport cells that circulates water and dissolved minerals and systemic chemicals. Unfortunately, however, spider mites generally feed near the midrib and plant veins, and not in the phloem or xylem, so they never get into the plant’s vascular system and, in turn, would avoid any systemic chemicals.

Without access to systemic miticides, rosarians are left with contact sprays, which stay on the surface of the leaves only where they are sprayed, and translaminar sprays (sometimes referred to as locally systemic), which are absorbed by the plant’s leaves and move from one surface to the other. That is, if one sprays the tops of a plant’s leaves, a translaminar spray will migrate to the leaves’ bottoms – clearly, translaminar sprays are the best for mites which feed on the bottoms of leaves. As noted above, and in Dr. Cloyd’s article, Avid is a translaminar miticide, but it only kills the adult mites. Avid can be supplemented with Hexygon which kills both the mites’ eggs and larva, but Hexygon is a contact spray so great care must be taken to spray the bottoms of the leaves. Dr. Cloyd also mentions Floramite, and notes that it kills all the mites’ life stages but, like Hexygon, it is a contact spray and is not translaminar. Let’s examine some alternatives.

Bayer Environmental Science has just announced the registration of a new miticide, based on a proprietary new chemistry with a unique mode of activity. The name of this new miticide is Forbid 4F, and its active ingredient is spiromesifin, a tetronic acid, which blocks fat synthesis so the mites dry out and die. This mode of activity, inhibiting lipid biosynthesis thus causing dessication (i.e., drying-out), is virtually the same as that of soaps and oils. That is, the MOA for Forbid 4F can be considered, for the sake of simplicity, to be MOA 6 as defined in papers appearing on the Rose Society’s website (www.chattanoogarose.org), and in articles appearing in past issues of Basal Breaks. This is great because MOA 6 has little or no potential for the development of resistance – an analogy (a derivative of Dr. Cloyd’s “napalm” analogy) would be a flamethrower: you may miss some of your targets, but when they return they’ll be no less vulnerable to the flame. Another feature of Forbid 4F is that, like Avid, it is translaminar. Moreover, according to Bayer, Forbid 4F controls mites at all life stages and offers an excellent residual of four to eight weeks for mites.

The label for Forbid 4F indicates its toxicity rating is CAUTION (be aware that the Avid label carries a WARNING designation). The label further specifies a usage rate of 2 to 4 fluid ounces per 100 gallons of spray. This equates to 1/8 to 1/4 teaspoon per gallon of spray – Bayer recommends, however, using the lower rate. Also, Forbid 4F may be tank mixed with sprayable fertilizers, insecticides and fungicides, although Bayer recommends a jar compatibility test. This product is available through Bayer distributors, such as Southern Ag (www.southernag.com) in Boone , NC , and from Rosemania (www.rosemania.com). Now, here’s the downside of Forbid 4F: its cost. As an example, from Rosemania, 8 oz. (that’s one cup) of Forbid 4F costs $299 (gulp!). Ah, but here’s another alternative.

Most rosarians already have Avid in their pesticide arsenals, so what’s needed is a translaminar ovicide/larvacide – and there is one: TetraSan 5 WDG produced by Valent U.S.A. Corporation. The active ingredient in TetraSan is etoxazole, a juvenile growth inhibitor – this puts it in MOA 5 of the simplified MOA schema mentioned above. The label for TetraSan carries a CAUTION toxicity rating, and specifies the usage rate as 8 oz. to 16 oz. per 100 gallons – this equates to ˝ to 1 teaspoon per gallon. I tank mix Avid and TetraSan to produce a translaminar miticide that kills all spider mite life stages. And, I can buy a pound of TetraSan from Rosemania for $99 – it’s packaged in eight 2 oz. water soluble bags, and should create 100 to 200 gallons of spray. So, if you have some Avid, get some TetraSan, and you can produce a spray that’s as effective as Forbid 4F for one-third the cost. And, even if you don’t have Avid, you can get 8 oz. from Rosemania for $99.95, and still beat the price of Forbid 4F by $100.

Be sure to use an adjuvant like Indicate 5 so that your spray completely covers the leaves (at least their tops), and also be sure to Read the Labels of all the chemicals you use.