Life cycle and management of Fletcher scale on Taxus and Arborvitae
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.
scale overwinters as young scales on the twigs and leaves. The small
scales (less than 1/8 inch long) are difficult to see until they begin
feeding in May, and in a matter of a few weeks double their size. Eggs
are produced under female scales in early June. All the live scale
insects found in early to mid-June will contain eggs as only females are
known. Eggs will begin to hatch and young penny-shaped, clear crawlers
emerge from under female scales in mid- to late June. The nearly
invisible crawlers soon settle on the needles or green stems and begin
feeding. They grow larger in July and early August. By late August and
early September, some females may again produce eggs. In Ottawa County,
Michigan, the proportion of scales that go through a second generation
in the fall varies from 10 to 100 percent. The second generation of
crawlers hatches in September. (view photos)
The best management strategy for Fletcher scale is natural control. This is what happens in the urban landscape after yews are planted. Predatory insects and parasites keep the scale population to such a low level that they are rarely seen. Some nurseries do not spray anything for Fletcher scale, and have been very successful in growing clean plants. However, when going from insecticide management to natural control, it is likely that you will see an initial outbreak of Fletcher scale that may last one to three years before predators and parasites provide adequate control. Also, natural control may not be as effective in large, clean fields as it is in small weedy fields because predators like ground beetle, rove beetles and ants tend to be more abundant where there is abundant ground cover and a diversity of plants.
If insecticides are necessary, foliar sprays are most effective when sprayed soon after most of the crawlers have emerged in mid- to late June. Crawler emergence can be monitored by finding 10-20 female scales each week, lifting them up and examining the eggs underneath. When most of the eggs have hatched into crawlers, it is a good time to spray then or within the next two weeks. In recent testing, the most effective foliar sprays have been Dursban, Sevin, Supracide, Orthene, Flagship, Tristar, Safari and Discus. The goal of a good foliar spray is to get a thorough insecticide coating on the foliage and twigs. This is best done with spray nozzles that produce fine droplets. Avoid spraying fine droplets when the wind is above 5 mph. Large volume sprays of 100 gal per acre or more put most of the spray on the ground. Remember, fine droplets have a much higher concentration of pesticide (often 100 times more concentrated than large droplets) when you are applying the same amount per acre. If the pesticide label gives the rate in amount per 100 gallons, figure out the amount per acre rate based on 200 gallons per acre, then adjust the per acre rate to your spray volume. Optimum timing in an average year is a single spray in late June for the first generation, and another spray in late August or early September for the second generation.
Several products that are absorbed by plant roots and move systemically through the plant can also be used as a soil-directed spray. Marathon or Discus can be sprayed in late May, to give the plants time to take-up the insecticide and be in the leaves by late June. Safari and Flagship are taken up more rapidly, and should be applied in early to mid-June. All of the systemic products must have an inch of rain or irrigation to wet the soil enough to allow root absorption. They will not work on the soil surface. Another alternative is to lightly disc following application to cover the insecticide and protect it from photodecomposition and volatility.
Successful management depends are good scouting. Every field should be scouted for Fletcher scale in early June and mid-August to determine which fields need to be sprayed.