Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont
Birch leafminer and borer, eastern tent caterpillars, and fall webworms are some of the common pests on trees and shrubs in landscapes. Knowing these, and their least toxic controls, will help you to have healthier plants with the least harm to the environment.
Birch leaf miner larvae feed inside birch leaves, causing
tan-colored blotches. In severe infestations, leaves may be
almost totally brown. Gray birch and paper birch are most
susceptible, with other birches much less so. It is much easier
to control adults in spring as the new leaves emerge, before adult
miners lay eggs The larvae that hatch, being inside the leaves,
are much harder to control. If this pest is severe for several
years, trees can be weakened and attacked by the bronze birch
The bronze birch borer is the larva of a beetle which tunnels
through the bark, eventually resulting in tree death. This
happens from the borer eating and cutting off the vascular tissue—
that part of the tree that conducts water and nutrients below the
bark surface. Once you see raised areas in the bark, control may
be too late. Most commonly injured are paper and white birches,
with other species more resistant. Preventing injury from birch
leaf miner helps, as does keeping trees healthy, and not pruning
trees during summer when adult beetles are flying about. Larvae,
once inside trees, are difficult to control.
Eastern tent caterpillars are recognized by many from their white
webbed “tents” in branch crotches, teaming with black
caterpillars. These insects hatch from eggs in early spring as
buds open on apples, crabapples, cherries, and their relatives.
They emerge from the nests on warm days to feed on leaves, at
which time they can be controlled with sprays such as insecticidal
soap. Nests can be pruned out in early summer and destroyed, as
can the shiny black egg masses (about one inch long) found on
twigs in winter.
Fall webworms are similar in that they make web nests, but these
are on branch tips and appear in mid to late summer. There often
are several nests per plant, compared to one for tent
caterpillars. And the larvae of fall webworm feed from within
their silky nest, expanding the nest down the plant as they feed.
Nests can be pruned out as for tent caterpillars. Don’t use fire
to destroy these or tent caterpillar nests, as the fire may damage
the host plants. Many different plants host this pest, including
birch, lilac, crabapple, and cherry. Since feeding is late in the
season, it is more an aesthetic problem than harm to plants.
Viburnums are great landscape shrubs but unfortunately, the
viburnum leaf beetle is particularly fond of some species. Both
larvae and adults can quickly defoliate plants, only leaf
skeletons remaining. Eggs hatch in late spring when the young
larvae begin feeding, followed by adults in mid-summer. If you
monitor plants almost daily, as a friend of mine does, you can
pick off insects as they appear and you see them. If you use
pesticides, use least toxic ones such as insecticidal soap,
horticultural oil, or neem products so that you don’t harm
beneficial insects, which feed on adult beetles. You can learn
more about this pest, and which species are most resistant to it,
from Cornell University (www.hort.cornell.edu/vlb/).
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