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.
Integrated pest management programs for armored scales, like all IPM programs, starts with good scouting and monitoring. Scouting and monitoring starts with inspecting new stock. Not that anyone would intentionally unload scale infested liners but, again, these are very hard to see. At low densities, Japanese maple scales and other scales are common around branch collars and other rough areas where they can get a foothold. Inspect the trunks of existing stock to determine if they have scales. Pay special attention to plants with dense outer foliage like hollies and boxwoods and plants that have been in your nursery a year or more.
Natural enemies are important for helping regulate scale insects in nurseries. I am not talking about buying and releasing predators or parasitoids. Natural enemies of scale insects exist all around and work for free. The key is not destroying them with insecticides that leave long-lasting toxic residue. Contact insecticides kill natural enemies but not mature armored scales (waxy water proof cover, remember) so frequent applications of these products can increase the abundance of scales and other pests like soft scales and spider mites.
Armored scale control is most successful when insecticide applications coincide with crawler emergence. This is true for contact insecticides and for systemic insecticides. Especially when timed correctly, horticultural oil, can make a dent in Japanese maple scale populations but is not often enough if scale density is high. Insect growth regulators such as pyriproxyfen and buprofezin are often good choices for armored scales. Three neonicotinoids, dinotefuran, acetamiprid, and thiamethoxam, are available for armored scale management. Note that imidacloprid does not kill armored scales. Neonicotinoids are systemic so they move into plants making them toxic for pests to consume. This means that scales feeding on the plant may be killed even if you do not contact the insect directly. Even systemic insecticides are best applied when crawlers are active but can kill other stages that are feeding on treated plants. Check with your local extension personnel to see what is available and effective in your area.
Japanese maple scale has been around for a while and is spreading. The final insult from Japanese maple scales and other armored scales is that even if you kill them, the waxy tests (covers) stay on the plant. Therefore, plants look like they have pests even if they don’t, which makes them hard to sell. With this in mind, the key to Japanese maple scale management is to scout and stay ahead of them so you do not end up with dense populations that are uglier and harder to manage than small populations.