Bugs, bugs and more bugs. Do you ever think that the world is ruled by bugs (insects if we want to be correct)?
So far there have been 1.1 million different insects identified out of 1.47 million species cataloged of all animals. So yes, insects do rule! But most of these insects are either beneficial or neutral in their impacts to us. Today I am going to briefly discuss one insect that is a pest.
The oystershell scale is quite common in our area and can infest the bark of many different trees and shrubs. It gets its name because it looks like an oyster shell. The one tree we get the most calls on is the aspen tree.
This scale is a sucking insect that hatches out of eggs sometime in May or June, depending on temperatures. We call the newly hatched out scales crawlers because they have legs and are mobile. They soon molt (shed their skin), become immobile and hunker down to feed for the next few months.
Females produce eggs in late summer to early fall, under the shell of the scale, where they reside until they hatch the next year. The hard, waxy covering on this scale provides significant protection from weather effects and natural enemies. Dispersal to other trees can occur through winds during the crawler stage and by animals such as birds and squirrels.
While there is only one generation per year, during outbreak years this insect can cause serious stress and decline in the health of the infested tree or shrub and even kill it. We often don’t notice this pest until its numbers are quite high and cover the trunk or branches. Obviously keeping a tree as healthy as possible is the key to minimizing the impact of this pest.
But when all else fails, there are several controls available. Horticultural oils (dormant and summer oils) sprayed before leaves are on the tree can provide some control, but since eggs are protected under the waxy scale, control may not be sufficient. Insecticides timed to the crawler stage can be very effective. The various pyrethroid insecticides that contain active ingredients of bifenthrin, cyfluthrin, cyhalothrin, or permethrin can be used on trees and shrubs. Imidacloprid (Merit) as a spray may also be an effective crawler spray. Dinotefuran (Safari) as a systemic trunk treatment is probably the most effective but also the most expensive treatment.
Last but not least, hand removal can also work in some cases. I’m not talking about hand picking the individual scales off the bark but rather a gentle rubbing of the trunk with a soft plastic pad (such as used for dish washing) to crush, kill and remove an existing infestation. A DCI reader from Monument wrote to me and suggested using a pressure sprayer — very carefully. On thin barked trees like aspens, we do not want to damage their living bark. Until next time, enjoy your garden.
Jim Leser retired to Cedaredge after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a master gardener.