Think of leaves as food factories for trees. Within them, light energy is utilized to manufacture sugars that the plant will use for development. Chewing insects, by devouring foliage, can cause a great deal of distress in a landscape situation. In essence, trees with inadequate foliage will starve.
Chewing insects of concern to landscape contractors include certain butterfly and moth larvae, grasshoppers, beetles, webworms and bagworms. Perhaps the most notorious chewer to hit the U.S. is the gypsy moth, an invasive species causing large zones of forest defoliation in the East. In regards to his experiences with this chewer, Donald Zimar, a Manassas, Virginia arborist, says, ?I remember trees that were bare as January in the middle of June. These chewing caterpillars ate the leaves off entire hillsides ? defoliating them, except for a few species they didn?t like. It literally rained caterpillar feces. It was this way for three years in a row. Many trees died, but then they let up. Now they come and go.?
Of course, most chewers aren?t going to cause this much widespread destruction, but if left unchecked, many of the common ones can wreak havoc in a landscape setting.
Comparison to other pests
Generally speaking, and with a few exceptions such as the gypsy moth, chewing insects are rarely as great a threat to landscape tree health as are other categories of pests. According to Craig Wells, shrub and ornamental tree care manager for AgroScapes in Vienna, Virginia, chewers are easier to control largely because they?re so accessible. They?re right out in the open, fully exposed to any treatments that are practical.
Borers and root feeders, on the other hand, are protected by the soil or the tree itself. By the time borer damage becomes visible, it may be too late. With chewers, the landscape contractor can see the whole story, from small, beginning populations to full-blown epidemics, and it?s much easier to prescribe solutions early in the game.
Defoliation, while a serious matter, doesn?t automatically mean tree mortality. A majority of a tree?s leaves may be devoured one year, but by the following year, chewer populations may have fluctuated to the point that trees aren?t impacted at all.
But, it should be emphasized, chewers may instill a larger fear factor than the more covert borers and root feeders. For instance, Sugar Land, Texas consulting arborist Ken Six tells how, in Houston, tent caterpillar populations that are usually concentrated in water oaks and live oaks may move into smaller landscape shrubs. ?They?re cyclic in nature,? he explains. ?Some years they?re worse than other years. This year, we?re already seeing signs of them. They?ll gather up on the trunk, and they?ll look like a gray mat on the trunk of a tree.? In this phase, the larvae are preparing to move up into the canopy and feed on the leaves. ?I?ve seen tent caterpillars so bad here,? Six continues, ?especially in live oak trees, that if you stand underneath a tree, you can hear them chewing.?
In addition to invading plant life, caterpillars may gather on buildings, automobile tires or other spots where they?re unwanted. Of course, this is when the contractor?s phone will start ringing off the hook, and he?s liable to get more desperate cries for help than he can comfortably manage.
Keeping your eyes open
Kirkland, Washington arborist Brian Gilles says, ?I can tell you that the biggest problem with chewing insects revolves around the wrong tree planted in the wrong place, or trees planted in areas with insufficient soil and moisture. Once the trees get stressed, then the insects become a problem.? In other words, a tree that?s as healthy as possible will be much more likely to withstand a full defoliation than a tree that?s already stressed.
Knowing plants and chewers well will allow the most effective preventive treatments, because the contractors will be able to identify the most vulnerable stages of the pests? life cycle. Says Wells, ?I find that one of the easiest ways to control any of these insects ? primarily the chewers ? is to get them really early on in their development. We try to target specific pests at specific times of the year.?
For instance, Wells explains that now is a good time for the control of tent caterpillars, when they?re only in their first or second instars and their nests are limited. Treating the caterpillars now means pesticide applications in very small spots. It also allows the use of materials with less toxicity than will be possible later on. Finding evidence of caterpillars during this time of year also means that field personnel can simply pull them down and counter the need for future solutions.
Six, too, prefers to stay ahead of chewing insects? development, and if early signs of infestation are detected, he?ll often recommend bacillus thuringiensis (Bt) applications.
?That?s the hard part that you have with biological controls,? explains John Moran, president of Arbor Care in New Fairfield, Connecticut and past president of both the International Society of Arboriculture and the American Society of Consulting Arborists. ?You can put them out there, but they don?t always do what you want them to do.?
A plant health care program within your business will allow clients to pay for regular monitoring, and this will also lessen the likelihood of chewing insect ?crunch times.? In particular, market your plant health care program to those clients who have experienced epidemic proportions of chewers in the past, and let them know of your intentions to monitor the situation and solve the problems early on.
Even if few people will pay for plant health care per se, it will still be to your advantage to watch for warning signs in your day-to-day field operations. Also, if it?s determined through your early monitoring that outbreaks are likely, the use of systemic materials may be prudent.
Arbor Care is a distributor of Mauget products, and Moran estimates some 1500 people are certified to use these materials in New England. According to Moran, the strength of Mauget?s microinjections is that (1) the materials stay where you put them and (2) you don?t have drift onto neighbors? lawns as you would with spraying.
Abacide and Inject-A-Cide ?B? are two of Mauget?s systemic products that Moran has recommended for the control of certain chewers. Imicide for treatment of the lerp psyllid and the Asian long-horned beetle has proven itself over the past few years.
Advanced control options
Once chewers like tent caterpillars reach the epidemic stage, Six will then advocate the use of Tempo. Tempo is a contact pesticide, and even in heavy infestations, a small amount sprayed onto a tree with a high-pressure spray rig will solve the problem in a matter of seconds. He reports that the caterpillars will drop from the tree on a strand of web, as if trying to escape the Tempo, and will just dangle lifelessly there. (He jokes that the dead caterpillars on a web resemble Halloween ornaments.)
The chemical Orthene is a favorite of Wells, and he may recommend it for the control of Japanese beetle, oakworm, weevil and sawfly ? as well as tent caterpillar.
Wells expresses his preference for foliar-type products that are absorbed into leaf tissue but move very little within the plant?s vascular system. This ensures that the material stays within the leaves and reduces the need for reapplications.
In a lot of cases involving chewing insects, action on the part of the contractor may be unnecessary. For gypsy moth problems, Zimar says, ?Treatments included everything from banding trees to spraying biological controls from airplanes on very large forested areas. Nature herself helped by disseminating natural fungi and viruses that left their corpses hanging from the trees. The odor was sickening.?
Says Wells, ?Sometimes, we?ll opt to not even treat, if the numbers of pests are low enough that we don?t think it?s going to take the damage beyond a threshold point.?
While defoliation won?t kill a tree as quickly as will girdling by a borer or some other insect attack, leaf loss is nevertheless a matter not to be taken lightly. ?All the food for the tree is made in the leaves,? says Six. ?If the tree just expended a lot of energy making a lot of leaves and then caterpillars come and chew the leaves off, the tree has to use more energy to put more leaves on so they can photosynthesize and make food. So, it?s really not good for a tree to be defoliated, even though in most cases, the insects are not going to kill the tree.?
For the contractor, the challenge becomes deciding how much force to use, if any at all. Will the trees survive if the chewers are left to the forces of nature? Does the benefit of treatment outweigh the costs? Can the client?s fears be calmed without a full-fledged pesticide application? Again, though, as is the case with a great many insect and disease attacks, the best course of action is a preventive one.