If, for some nefarious reason, you wanted to create the perfect pest, you should model it on an armored scale insect. They are hard to detect because they are small and camouflaged. They are hard to kill because they live under a waterproof cover. And, even if you do kill some, the survivors reproduce quickly and often continuously. These characteristics make armored scales among the most difficult pests to manage on nursery crops. The scale insect that has all these traits in spades is Japanese maple scale, Lopholeucaspis japonica. If you don’t have Japanese maple scale in your area or in your nursery yet — and just because you haven’t seen it doesn’t mean you don’t have it — you probably will soon.
Japanese maple scale was probably introduced to the U.S. from Asia and has been found in most eastern U.S. states from Connecticut to Georgia and west to the Mississippi. Adult Japanese maple scales are less than 2 mm long and oyster-shell shaped. Their armored covers, called tests, look white due to a wax coating but this can wear away leaving an even more cryptic brown test. Beneath the test, eggs, immatures, and adults are generally purple.
They are small and hard to see, but part of what makes Japanese maple scales so difficult to manage, even once you know you have them, is their lifecycle. In mid-Atlantic and mid-South states like Maryland and Tennessee, where most of our information is from, Japanese maple scales have at least two generations per year. In these cool regions, immature Japanese maple scales overwinter on the trunks and branches of host plants. In early spring, these nymphs mature and female scales begin laying eggs.
When scale eggs hatch, the tiny nymphs that come out are called crawlers. These are the only mobile life stage of armored scales. Crawlers are also the most vulnerable life stage of armored scales because they are tiny and soft and do not have a protective waxy test. Japanese maple scale eggs start hatching and crawlers begin emerging in mid-May for the first generation and in early August for the second generation in the mid-Atlantic and mid-South. However, in warmer areas like where I am in North Carolina there are probably more generations. I have found all life stages, including eggs and crawlers, in the dead of winter, which here can be 70°F.
Not all adults produce eggs at the same time and not all eggs hatch at the same time. Thus, crawler emergence goes on for many weeks. This is one reason Japanese maple scale management is especially difficult. Two generations of crawlers emerge over 8-10 weeks each, so there are always overlapping generations and life stages. The crawlers find a spot to feed and begin forming a waxy cover just three days after hatching, which protects them from insecticides. Thus, you never have all or even most of the population present in the vulnerable crawlers stage.
What else makes Japanese maple scales so tough? Well, don’t let the name fool you, they will feed on most trees and shrubs in your nursery, not just maples. Japanese maple scales can feed on dozens of plant species including red maple, Japanese maple, dogwood, redbud, elm, Itea, and broadleaf evergreens like cherry laurel, holly, Japanese holly, and boxwood. And, they accumulate on the interior branches because these are the oldest and so have been exposed to more scale generations. This makes scouting very hard because you can’t just inspect a few outer leaves — you really have to get there and inspect the main trunk. On something like a dense, tightly pruned holly this is not only difficult, it hurts.