Page Intro: One of the toughest pests to deal with. A physical toughness: scales are either armored or covered in a protective waxy coat. It’s as if they don’t want to be eaten! On this page you’ll first get some background, then you’ll discover how they become a problem (and continued), learn that scale and mealybugs are a serious pest, followed by some advice, something for the scout, and then some solutions.
Some Background
Scale insects and mealybugs are all members of the same order: Homoptera — just like aphids. Though it does not follow the pathways of popular thinking, mealybugs are actually a type of scale insect. To most growers, they are either dealing with “scales” or “mealies.” The title of this chapter is so named so people will not tell me I forgot to include mealybugs on this website — which would have been likely if I named it Scale Insects. Scales are broken down into several families, of which I will discuss three major ones. These three distinct families are: Coccidae, which are soft scales; Diaspididae, which are armored scales; and Pseudococcidae, which, of course, are mealybugs. Whew? now that that’s out in the open I can get to the details. [Intro]
How They Become a Problem
Collectively, there are thousands of scale insects. Of the families mentioned, hundreds have a destructive behavior which impacts crops worldwide. So wide is the distribution and concern of these pests, two are mentioned on the site (History of Biocontrol and IPM) in which I mentioned two scale insects: the cottony cushion scale, Icerya purchasi (of the family Magarodidae — which will not be discussed on this page) and the cassava mealybug (again, of the family Pseudococcidae).
Scales for the most part are sedentary creatures which are difficult to control — especially with sprays — as most have a natural talent for self-preservation. First of all most of these pests, at least as adults, permanently attach themselves to the plant or tree which plays host to them. Secondly, most are equipped with a protective covering, usually composed of waxy secretions forming either a shell of sorts or dense mat of waxy filaments or fuzz. These characteristics contribute to them being very troublesome and difficult to control.
Scale insects affect many different plants, shrubs and trees. Most herbaceous greenhouse crops — especially those whose production cycle is short, i.e. bedding plants-may never become afflicted with scale insects. More enduring crops such as tropical foliage, trees, etc., are far more likely to see these pests — especially if they are under duress. Most often I find that tropical interiorscape plantings fall victim to these pests. This, I surmise, is due to these plantings, as the name interiorscape suggests, being indoors and subjected to stresses which are not of the natural kind they are usually subjected to. In other words, if a tropical palm tree grown in the bright, warm and humid outdoor climate of Florida gets relocated to a dimly lit, stagnating, warm and dry climate in a Milwaukee, Wisconsin mall, it will be stressed and very likely to develop a scale insect infestation. It may, perhaps, come with some scale insects of its own which begin to take over when the plant becomes stressed as it tries to acclimate to its new surroundings. Very often, though, scales naturally present in Wisconsin will be drawn to these plants during the summer months as, to them, they probably look like easy-pickin’s and will therefore try to get to these plants. Outdoors, even in Wisconsin, natural enemies would normally keep scales in check-at least on native plants and trees. Where this often doesn’t work is in a situation where the scales or plants are not indigenous to the region, in which case may lack natural enemies and vigor.
Another situation where scale insects can become problematic may be in mass agriculture. Take, for instance, the citrus groves of Florida and California. These groves are often under a constant assault from pesticides, thus reducing the native population of biocontrol agents (thank goodness oranges have a peel else they’d have no appeal at all). If the sprays were not applied, assuming we’re talking about an indigenous pest, the natural enemies would, more often than not, be capable of restoring balance. Unfortunately, many growers will not allow this natural transition to take place as the lag time required for the natural enemies to play catch up could come at too high a cost. However, since many of the pests are from out-of-town, we’d have to also introduce out-of-town biocontrols. Just like that which was done to combat the cottony cushion scale.
Scales take up residence on fruits and stems and leaves — everywhere and anywhere. And as I wrote, they are well protected. In other words heavy spraying must be done in order to penetrate their defenses. Most of the spraying, it seems, does little good unless applied heavily enough to actually smother these insects, but which, of course, is often repelled by the waxy secretions. (Horticultural oil is a good remedy against scales if smothering is the cure de jour. Unfortunately it is not conducive to plant health as it smothers the plants as well.) This is why biocontrols are such a logical choice when it comes to battling these tenacious pests indoors. The tricky part is finding the right parasitoid or predator to do the job and allowing it enough time to work. If time is given and sprays not applied, natural enemies do work, especially, again, if the pest is indigenous.
For the past nine years I have been following the progress of a stressed-out beech tree in southern New Hampshire. This tree has been moderately infested with beech aphids and beech scale. The scale has been kept in check by its natural enemy, the twice stabbed lady beetle, Chilocorus stigma of which there is a persistent perennial population. The aphids, however, have prevented the tree from really meeting its full potential. I released a small amount of multicolored Asian lady beetles — which eat scales and aphids — on this tree as an experiment. I had reservations as I didn’t want these beetles to eat the scales and thus displace the twice stabbed lady beetles which have done so well at maintaining balance. I did, however, want them to eat the aphids. I applied a small number of beetles to the tree’s leaves where the aphids were hanging out. They rapidly consumed the aphids. I then opened the surrounding area a little to allow the tree more sun in hopes it would reduce the tree’s stress. It seems to have worked. The scales and corresponding beetles are still there, but the tree has been free of aphids for three seasons now and is starting to make a comeback. Continued… [Intro]
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