Cyclamen Mites on Strawberries
Posted: April 5, 2011
Distortion of new leaves on plants infested with cyclamen mites – note that the leaf color of cyclamen-mite infested plants is often “off”, having a light green and bronze tinge. Photo Kathy Demchack
Molly Shaw, Cornell Cooperative Extension
Last spring a question came up during a phone call with berry extension specialists around the state—How prevalent are cyclamen mites in our strawberry fields? Summer 2010 presented a perfect chance to find this out in the southern tier. Since we were out taking soil and leaf tests for another project, I simply took another set of leaf samples on strawberry farms to examine for cyclamen mites. Cyclamen mites are microscopic arthropods (technically not insects, just as spiders are not insects) that hide out in plants and make their living by sucking on plant cells. Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA) has a good fact sheet with pictures, http://www.omafra.gov.on.ca/english/crops/facts/cyclam.htm. In the past cyclamen mites have been considered a minor pest of old strawberry fields that ought to have been removed anyway. But in 2010, we found them with surprising frequency in young strawberry fields. Cyclamen mites live in the crown of the strawberry plant, so you can usually only find them on the newest not-yet-unfolded leaves. Pick a leaf, gently spread it out, and look for almost-microscopic white graininess down by the leaf base. On heavily infested leaves I could see these white grains without a hand lens, but none of the farmers could. To reliably diagnose them you need a good hand lens, and I found a dissecting microscope came in very handy when finding small populations. (Note from KD – even if you have good enough eyes to see the grains Molly refers to, they are difficult to tell apart from sand or clay particles. I can only zero in on the mites or eggs with a dissecting scope.) My typical practice was to pick 25 baby leaves from each strawberry variety and examine them under the scope back at the office. Strawberry plants heavily infested with cyclamen mites will be stunted with deformed leaves. Interestingly, we found those symptoms on only a handful of plants on a couple farms, while nearly every farm had cyclamen mites on symptomless plants. In fact, of the 8 strawberry farms we sampled, only one was free of the cyclamen mites. What was even more surprising was that plants just planted in spring 2010 had cyclamen mites, sometimes as high as 40% of the leaves had mites, but typically they were at a somewhat lower level (10- 20%). This suggests that the mites were coming with the plants from the nursery—and most of the growers were using quite reputable nurseries!
So what? You can’t see them, customers can’t see them, and I just said that it’s hard to tell if you even have them by visual symptoms! The threshold for when their sucking activity takes a toll on the plant isn’t completely agreed upon. In California, 1 mite in 10 new leaves is considered a potential problem, while Manitoba uses 1 infested leaf in 15 as their threshold for treatment, with the added clarification that when you get to 45-65 mites per leaf it can cause a 1/3 yield reduction. These mites reproduce quickly, from egg hatch to adult in 2 weeks when conditions are right, and females don’t need males to lay viable eggs. With this type of exponential growth, going from a couple mites to the levels that cause 33% yield reduction can happen really fast! Besides yield reduction, the mites can cause general reduced vigor and winter hardiness, compounding problems for the poor plant. Cabot is a variety that some growers love and others can’t quite get to perform well after the first year, and coincidentally Cabot had some of the highest mite levels. Could the challenge with Cabot really be a cyclamen mite challenge at its root?
What can you do if you have cyclamen mites? That’s the problem, once you have them it’s really hard to get rid of them since they reside way down in the protected crown of the plant. Endosulfan, a strong insecticide, is the only in-field treatment labeled in NY, and the label will end in 2016. It’s supposed to be applied after renovation when the leaves have been mowed off, with high pressure and at least 200 gallons of spray/A. Anecdotally, growers haven’t found even this treatment to be very effective. Usually the best thing to do for a serious infestation is to start over with clean plants. But clean plants from where? This year we found disturbingly high levels of cyclamen mites on 2010 plants, which suggests that they might have come infested from the nurseries, and reputable nurseries at that. Hot water dips for dormant crowns used to be recommended (110 F for 30 minutes, with tight control on the exact temperature achieved), but varieties are different in their heat sensitivity and many new ones haven’t been tested. This is one of those areas where we don’t have enough information. Ideally nurseries would have techniques in place to assure that they’re shipping clean plants, but that’s easier said than done. More research is needed to establish where the infestations are coming from and to find environmentally sound controls. In the mean time, take a look at your plants this spring, bring leaf samples to your local extension office where you can use a microscope to examine them, and check out the fact sheet mentioned above for excellent pictures of what you’re looking for. The first part of the solution is identifying the problem.
Follow-up info for PA growers on cyclamen mites, K. Demchak, Penn State Horticulture
The above article is especially timely because in addition to at renovation, a second time that chemical sprays can be applied for cyclamen mite management is as soon as straw is pulled off in the spring when sprays can still get into the crown area, which is critical for effectiveness. Other chemical options in PA besides endosulfan are Portal, for which a 2(ee) label was issued for cyclamen mite control, and Temprano (same active ingredient as in Agri-Mek) which is labeled for cyclamen mite suppression. Finally, it should be pointed out that predatory mites can be quite effective in searching out cyclamen mites, and if the cyclamen mites are present in only certain areas of the field, predatory mites can be released in the problem areas, though sprays toxic to predatory mites should be avoided. (Ask the company from which you order predatory mites what specie(s) they recommend for your situation.) Typically we think of cyclamen mites as being a matted-row issue, rather than a problem in plasticulture. However, especially if dormant plants are used as the plant material, cyclamen mites could potentially be a problem in either production system.
For organic growers cultural controls including avoiding infested stock and susceptible varieties such as ‘Cabot’ are recommended. Azahar (Azadirachtin), Saf-T-Side (petroleum oil), Suff oil x (petroleum oil) and Trilogy (Neem oil) are labeled for cyclamen mites in strawberries. Limited efficacy data is available. See the Cornell Organic Guide for Strawberries for more information http://www.nysipm.cornell.edu/organic_guide/strawberry.pdf.