Lecanium scale found in blueberry fields

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

Lecanium scale has been found in blueberry fields this summer, especially around the Holland/West Olive region. They have also been found in some vineyards in the northwest region. The waxy scales are small and brown (3-4 mm long) and typically on the new growth from last year. The high densities of scale will cause honeydew on the foliage and fruit beneath the colonies, and this can cause problems with growth of sooty mold. Additionally, there may be reduced growth of new foliage on shoots that are heavily-infested.

Our earlier article on this subject highlighted the need to wait until crawlers emerge from under the protective scale before applying treatments to protect bushes, and we have now observed crawler movement from the adult scales. In our weekly sampling for scale through June, the average number of scale crawlers on sticky tape traps were as follows: zero on June 2, 0.25 on June 10, 0.2 on June 14, and 49.1 on June 22. This rapid increase in scale crawler density indicates that this week would be the right time to protect fields where high scale density was found this spring. This will prevent scales from settling on this year’s growth and continuing their development.

Scouting to identify the start of crawler emergence can be done in your own fields by weekly checking of double-sided sticky tape placed near scale colonies, or by regular checking of colonies with a 10x hand lens. The scale crawlers will be visible around the adult scales, moving across the vine shoot surface. They are about 0.5 mm long and yellow.

We have already seen parasitic wasps emerging from scales collected in crop fields this spring, and fungal pathogens have also attacked colonies in some wooded areas near blueberry fields, so some natural biological control is underway. In recent sampling, scale populations in woods had less than 5 percent survival, but each surviving scale can produce hundreds of crawlers, so they have a high potential for reproduction.

Once scale crawler emergence is identified, protection of bushes is possible using effective insecticides. We have conducted a small demonstration trial in a West Olive farm this spring, comparing scale control in a field receiving these three programs for fruitworm control and measuring the control of scale. These programs included two applications applied 14 days apart after bloom 1) Guthion and then Imidan, 2) Assail applied twice, 3) Mustang Max applied twice. All of these programs provided complete control of scale, with no surviving adult scale a week after the final treatment. These results and the further details below indicate that many growers managing blueberry maggot and Japanese beetles at this time should be achieving control of Lecanium scale. For this pest, it will be important to get coverage through the canopy including the lower branches to make sure all the new young growth is protected.

The landscape entomologist at MSU, Dr. Dave Smitley, who has plenty of experience with Lecanium scale in shade trees recommends use of neonicotinoids or pyrethroids for control of Lecanium scale crawlers. Neonicotinoids include Provado, Assail, Actara and Scorpion and these products are systemic, being absorbed into the plant tissues after application and therefore resistant to washoff. Pyrethroids such as Danitol, Mustang Max, and Baythroid will provide quick knockdown of the scale crawlers. However, pyrethroids can also disrupt biological control for this and other pests, so they should be used with care. Dr. Smitley has also mentioned to us that oils have not worked very well against soft scales such as Lecanium scale, but that option may be something organic growers would be interested in trying next spring at delayed-dormant timing to try and reduce scale populations on bushes.

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