Scale Insects of Woody Plants - Continued

In the last (Spring) issue of The Cut Flower Quarterly, I wrote an article on soft and armored scale which attack woody plants. In this edition, I will concentrate on the nastiest scales currently rampaging through many cut flower operations. Of the many species of armored scale, the Japanese maple scale, Lopholeucaspis japonica, has in the last five years become one of the most frequently encountered pests in Maryland and most states on the East Coast. We now have reports of this scale cropping up in Minnesota, Indiana and Ohio.

This invasive insect species is a gift from the Orient and is quickly spreading across the U.S. on a wide range of woody plant material. A body covering that conceals the insect, along with its cryptic coloration and nature, make this scale very difficult to detect. Over several years, populations can build up to levels which cause dieback and even death of the infested plants. Once established in a cut flower operation  it will often spread throughout, infesting many different species of woody plants.

This scale is not a quarantine pest on the East Coast but several states in the Midwest have quarantines on Japanese maple scale. Infested plant material can be condemned when shipped to Midwest states. Unfortunately, this scale has made it through the quarantine and is now found throughout the Midwest.

Understanding This Pest

First off, Japanese scale does not look like your typical insect. The cover of the insect is what you are going to see first. The actual body of the insect is under the hardened cover. Even when you pop off a scale cover the body looks sac-like rather than a typical insect’s with six clear defined legs and antennae. It does have legs and antennae but they are greatly reduced in size. The insect is basically a couch potato; not moving most of its life but feeding on your plants like a useless teenager hanging out in your living room playing video games and draining your refrigerator.

The Japanese maple scale is in a grouping of insects called the armored scales, members of the family Diaspididae. Entomologists love to group everything in families so we know the common characteristics of the pest. Kind of like knowing what type of family characteristics are for the feuding  Hatfields and the McCoys. For example, the Diaspididae have a hardened body cover that is not attached to the body. This characteristic is useful to distinguish armored scales from other families that may feed on different part of the plant, or damage a plant in a different manner.

The cover of Japanese maple scale is elongate, basically long and narrow. As the insect grows it sheds its skin and goes through growth stages called instars. A newly-hatched scale is a 1st instar, moving around at first, then settling down and covering its body with a wax that becomes the hardened cover. As the insect grows it moves to the second instar stage which is light brown. This is overlain with a dirty whitish wax. The male and female covers look very similar, but the male covering is smaller.

Life Cycle

The life cycle of Japanese maple scale is rather poorly understood. It is pretty well documented that it has three female instars and five male instars including adults. The information on what stage of development varies between researchers. In Pennsylvania and Japan it is reported to overwinter as fertilized 3rd instar females. Other researchers report that it overwinters as 2nd instar male and females. Miller and Davidson (2005) report that adult females and males are not found until April, which would indicate that they overwinter in Maryland as 2nd instars.

We do know that eggs are laid over a relatively long period. At the Central Maryland Research and Education Center in Clarksville, we have observed females laying eggs in early May in warm springs, but in most years egg laying starts in mid to late May to early June. Miller and Davidson (2005) report that eggs continue to be laid through June and early July. We have observed crawlers in late May to early June (2002 through 2007) and have observed them until the first week of August. Miller and Davidson report 2nd instars present from the last of June through late August and adult females from July through October.

Eggs of the fall generation are present from late July and continue through October. Because the egg laying period is extensive, it is very difficult to distinguish between the two generations.

Monitoring and Control

Plants with slow dieback of branches should be ex-amined for the presence of male and female scale. The scales tend to settle at branch junctures and like to concentrate populations in the center of the plant in early infestation. Later you can find them through the branches of the plant.

Horticultural oil can be applied at 2-3% rates in the fall after leaves drop, or in spring before bud break. Be sure to apply only when temperatures are above 50-60F for 4-5 days. In summer, 0.5-1.0% oil can be applied during the growing season when crawlers and early instars are present. This is a good suppression method but don’t expect better than 50% – 60% control of Japanese maple scale using oil applications.

Do not apply horticultural oils to drought-stressed plants. When crawlers and early instars are present, which is over a relatively long time, one of the two insect growth regulators (IGRs), Distance or Talus, can be applied. The IGRs will prevent the crawlers or early instar stages from developing into the next instar stage, resulting in death of the insect. IGRs are slow acting so patience is helpful. The systemic dinotefuran (Safari) has been used to control several species of armored scale but there are no published data on efficacy of this material on Japanese maple scale.

We have worked closely with Maryland nursery owners to evaluate Talus applications over a three-year period. This IGR appears to give excellent control if timed when crawlers and 1st and 2nd instar stages are present.

And Finally

The best advice I can give you is do not plant Japanese maple scale-infested plants into your cut flower operation. If you have it, be very aggressive in controlling this pest or it will rapidly overwhelm you and your lovely cut woody plants. If you are not sure if you have the right scale send me a good clear digital picture at the address below.

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]