Lewis mites! 2024 Updates in Poinsettia
Lewis mite is one of those pests that seems to go in cycles – some years we don’t see it at all, and some years I get reports as early as August.
This year there seems to be scattered reports. Given that Lewis mite populations can ramp up in a warm fall and rear their ugly heads in October, it seemed like a good time for an update on what’s working, and what’s not.
Lewis Mite Symptoms and Scouting:
Lewis mite is closely related to spider mites (Tetranychus urticae) but are an entirely different species (Eotetranychus lewisi). For side to side comparisons of the two species, see this article by UCANR.
Lewis mites are exclusively a pest of poinsettia crops in the greenhouse (unless you’re also growing strawberries.) Early detection is difficult, since the symptoms are rather subtle at first: faint speckling and chlorosis (see picture below), which can resemble nitrogen deficiency. Additionally, the mite is almost impossible to see without a microscope.
Often, Lewis mite populations go undetected until mid-October, when populations build up enough to start causing noticeable damage: the upper foliage will turn brown and the mites will form unsightly webbing.
If you haven’t treated preventatively for Lewis mite (more on this later), walk your crop a little more often and look for the characteristic stippling symptoms. If you find a spot on the bench with Lewis mites, consider throwing out infested plants in the “Circle of Love”; i.e. throw out the infested plant and all those immediately surrounding it. Or, do a spot spray of miticides.
Lewis Mite Treatment:
Propagation:
Given that they are difficult to detect, but end up causing a lot of damage, the easiest solution is often a single, preventative application of a miticide at the cutting stage. (Something to keep in your back pocket for next year).
Unfortunately, products traditionally used for Lewis mite don’t seem to be working as well since the last time I published this post,e.g. Floramite (bifenzate). So what other options are there?
Avid (abamectin; IRAC 6), although hard on biocontrol agents, can be used effectively without derailing your bio program if you use it early in propagation as a preventative spray, before natural enemies are released. The residues generally dissipate before parasitic wasps are put on at potting. (This strategy obviously will not work if you release bios in propagation).
Production:
Alternatively, Avid can work if done carefully as a spot spray. Because Lewis mite are a relatively slow-moving pest, you don’t need to blanket the whole bench or compartment. This keeps the effects on natural enemies to a minimum.
Although Floramite by itself no longer appears to be very effective for Canadian growers, adding in a product with ovicidal properties can help boost its efficacy. According to technical specialists, some growers have found Floramite, followed immediately by Shuttle (acequinocyl), to be effective , while remaining fairly soft on parasitoids.
Just be aware that Floramite and Shuttle are in a similar chemical class (IRAC 20B vs 20D), and therefore should be used in the SAME pest generation “window” (2 week period), as opposed to alternating sprays over the season, as this could promote resistance.
If you find yourself needing to do a second spray 2 weeks after the first spray, then Vendex (fenbutatin oxide; IRAC 12) may be a good option. It has worked in the past, although it’s unclear if resistance has been built up to this product or not.
Spear T (IRAC 32) is also a relatively new product on the market, derived from spider mite venom, and is reportedly effective on both two-spotted spider mite and whitefly, and therefore may be useful in Poinsettia production. However, close observation would be needed to determine it’s efficacy on both Lewis mite and Bemisia whitefly, as well as it’s effects on parasitic wasps.
As the season progresses, control with contact insecticides like Floramite and Shuttle will be much more difficult since the canopy is denser. Many translaminar/systemic miticides will be effective for Lewis mite (e.g. Avid, Kontos), but will likely interfere with whitefly biocontrol programs. However, if you’re on a chemical program for Bemisia whitefly (or need to switch to one if whitefly pressure is high), late September or early October this is the perfect time to apply Kontos, as it will also help control whitefly nymphs (see diagram below).
What about Biocontrol for Lewis Mite?
If you are a facility that is focused mainly getting through to sale with bicontrol only in Poinsettia, then your best options for Lewis mite are Amblyseius fallacis and Neoseiulus californicus. Both of these predators can eat other sources of food besides prey mites to survive (e.g. pollen) and are tolerant to the high temperatures Poinsettia are produced under.
If put in the crop early (e.g. in propagation or immediately after potting), they can help keep Lewis mite in check – but spot sprays may still be needed in a warm fall, such as 2024. According to a hot tip from Ecohabitat at last year’s Poinsettia Open House at Sawaya Gardens, A. fallacis also seem more tolerant to Avid residues that other bios, and could be released 3-7 days after spot sprays of Avid as extra insurance.
Special thanks to the consultants and IPM staff that keep me (and you!) in the loop for the content of this post!
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