Last week I wrote about sooty mold (read the article here). Sooty mold is fungal growth on honeydew, or the excrement of certain insects, including aphids and some scale insects. We've seen a large number of tree and shrub samples with scale insects so far this year, due in part to the stressful spring last year.
Most mature female scale insects settle in one place and produce a waxy coating. Insecticide applications often must be timed for when the nymphs, or "crawlers" are present. The crawlers are immature scale insects which do not have the protective coating, making them vulnerable to the pesticide.
Magnolia scale on magnolia tree. The client submitted the sample due to the blackening of the bark (the aforementioned sooty mold, seen on the branch) and fewer, shriveled flower buds. The large, helmet-shaped structures are dead female scales, while the scores of much smaller, oval shapes are immature scale crawlers, or nymphs.
Oystershell scale on maple (first picture) and willow (second picture) trees. Oystershell scale is very common in Illinois. Because the insects are so small, it can be difficult to scout for them. These type of scale insects do not produce honeydew.
Scales are usually divided into two catagories: soft and armored. Soft scales, including magnolia scale, can be controlled using a systemic insecticide since they feed on vascular liquids. Armored scales, including oystershell scale, cannot be managed using the systemic chemicals; for these scales, treatment can only be accomplished at certain points of the year. Pesticide sprays must be timed to coincide with crawler activity, otherwise the insects will not be affected. For timing to treat specific types of scale, consult the Pest Management for the Home Landscape reference book (published by University of Illinois Extension), or your local Extension office. You can also search the Home, Yard, and Garden Pest Newsletter (here) for information on when to treat various types of scale.