May Insect Problems
Periodically I pull out my crystal ball and predict
insect and disease problems. However, I don’t need the crystal
ball in May because every year a few insects routinely show up and
may cause damage to plants. Remember most of the insects in your
yard are beneficial but a few may cause problems.
Pine sawfly larvae are usually busy feeding in early
May. Their favorite foods are mugo and scotch pines, but they will
feed on other pines too. Although pine sawfly larvae look a lot
like caterpillars, the adult is a non-stinging wasp-like insect.
In early May the larvae are about 1/2 to 3/4 inch long with black
heads. Fully-grown larvae are 1/2 to 1 inch long with several light
and dark green stripes on their body. They feed for several weeks
on older pine needles, finishing about the time the new needles
elongate.
When disturbed, the larvae throw their heads back
in unison, a behavior that may scare away birds and other predators.
Since the larvae do not eat the new needles, their feeding doesn’t
kill trees and shrubs, but plants look sparse for a year or two
until the newer needles cover over the bare areas. Pine sawfly larvae
are easy to control, because they feed in large groups only on a
few branches. Hand remove by shaking the infested branch over a
pail of soapy water. Most of the larva will fall in the pail and
drown. Hand picking also works. If you choose to use an insecticide,
spray only the infested branches. Spray the larvae with carbaryl
(Sevin) as soon as they are seen. Products containing Bacillus
thuringiensis (Dipel, Thuricide) will not control pine sawfly.
Birch leafminers are also a type of sawfly. Shortly
after the birch leaves unfold, the female will lay her eggs in the
leaves. After about a week the eggs hatch and young larvae begin
to make mines in the leaves. Irregular brown blotches on the leaves
indicate leafminer activity. If you hold a mined leaf up in the
light you may see a tiny "worm" or dark pepper-looking excrement
in the mined area. The larva feeds or mines for two or three weeks
between the upper and lower leaf surfaces, then chews a hole in
the leaf and drops to the ground to pupate. A new adult emerges
several weeks later. In northern Illinois as many as four generations
per year may occur.
The best way to reduce birch leaf miner damage is
to watch for the first mines in May and then treat. Spray the foliage
with acephate (Acephate, Orthene) or dimethoate (Cygon, Ferti-lome
Ornamental & Evergreen Spray) when mines first appear and repeat
in three weeks. Read and follow label directions.
Ash plant bug and Honeylocust plant bug populations
fluctuate each year but we always have at least a few samples come
into the Extension office in late May and early June. Both types
are slender, flattened, quick moving insects that feed on the sap
of leaves and stems. Adult ash plant bugs are brownish and 3/16
inch long. Honeylocust plant bugs are green and 1/8 inch long. Immature
insects are similar in color, but smaller.
Ash plant bugs cause tiny light spots on leaves
and small dark black spots on the underside of the leaf. Honeylocust
plant bugs feed on expanding leaflets, causing distortion and twisting.
These leaflets usually stay on the tree, marring the appearance
for the growing season, but not harming the tree. Heavy infestations
will cause leaves to drop from the tree during June but damaged
trees will refoliate.
Treatment for plant bugs is rarely needed. Usually
when damage is noticed the plant bugs are too large to effectively
treat. The leaves and branches of small trees may be sprayed with
acephate, carbaryl, or malathion when the insects are young. The
possible environmental impact does not justify spraying of large
trees for these insects.
April - May 2000: Gardening
with Hebs - Part 1 | Discouraging Canada
Geese | Needle Evergreen Diseases |
May Insect Problems |
