Thinking ahead: What you can do THIS WEEK to prevent Lewis mite damage in your poinsettias.

2016 turned out to be a bad year for Lewis mite (Eotetranychus lewisi) in poinsettia.  Although it’s too early to say how 2017 is going to go, you should be considering possible preventative measures THIS WEEK for Lewis mite, especially if you have a history of Lewis mite with your cuttings. Treatment of this pest is more difficult later in the crop (though not impossible).  Keep reading for biological and chemical control options for this pest.

Lewis mite, a species of spider mite, are exclusively a pest of poinsettia crops in the greenhouse (unless you’re also growing papaya or citrus in there.)  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 October, when populations build up enough to start causing noticeable damage: the upper foliage will turn brown and the mites will form unsightly webbing. (Although last year we saw damage as early as Aug/Sept).

Given that they are difficult to detect, but end up causing a lot of damage, a preventive strategy is the easiest solution.  However, WHICH preventative strategy you pick is up to you.  As with many pests, there are many possible solutions, each with their pros and cons.

Option 1: is early applications of the predatory mite Amblyseius (Neoseiulus) fallacis .  These can be applied directly onto cuttings at a rate of 2/m2 as soon as they are off mist.  OR, they can be applied at a rate of 20/m2, after the plants are transplanted.  Only one application of this natural enemy is likely needed to control Lewis mite.  I should say that I have not actually seen this strategy work in person, but it’s been shown to be highly effective for our colleagues in B.C.

Option 2: is a single,  preventive application of a miticide at the cutting stage.  These include products like Forbid (spiromesifen), Floramite (bifenzate), FujiMite (fenpyroximate), and Vendex (fenbutatin oxide).  Note that control with contact insecticides like these will be much more difficult later in the crop, since the canopy will be denser.   If you’re using parasitoids for whitefly control, Floramite and Fujimite are likely your best option from these, since they’re relatively “soft” on whitefly parasitoids.

However, preventative treatment of Lewis mite with pesticides, though effective, comes with a caveat (of course).

It’s possible that early season applications of ANY pesticides could potentially drive your Bemisia population from the B-biotype to the Q-biotype, making your whitefly harder to control later (see my last post on this).  So if you haven’t had problems with Lewis mite from your supplier in the past, you may want to weight this option carefully, lest you inadvertently mess up your Bemisia control.  (Isn’t IPM fun???).

Option 3 (there’s always an Option 3!):  is the wait and see approach.   This requires regularly walking your crop and looking for the characteristic stippling symptoms of Lewis mite damage.  If you find a spot on the bench, immediately throw out infested plants in the “Circle of Love”; (i.e. throw out the centre infested plant and all those immediately surrounding it).  You should also consider a spot spray of miticides for nearby plants (a spot spray being less likely to interfere with bios for Bemisia control).  If the canopy is dense, systemic miticides might be more effective (e.g. Avid, Kontos).  

As always, when applying chemicals, make sure to READ THE LABEL, as some of these miticides are known to cause phytoxicity to poinsettia if applied after bract formation or on certain varieties.  Not sure which?  Download my pesticide summary table here: registered-insecticides-2016_omafra_2-0