Scouting and Control of Common Pests of Fraser Fir
The balsam twig aphid (Mindarus abietinus), (BTA), is found in
virtually every Fraser fir field in western North Carolina, as well as
in the natural stands of Fraser fir. Almost three-quarters of the insecticides
and miticides used in Fraser fir in this area are to control this pest.
Control is not difficult, but the treatment window is narrow. The severity
of twig aphid damage is determined by the weather in the spring, which
affects both the aphid survival and reproduction, as well as the ability
of the tree to recover from damage.
Symptoms. The BTA feeding on new growth at
bud break causes the needles to curl. Some of the damaged needles will
straighten before they mature; other needles will be damaged permanently.
The BTA does not damage already mature needles even though they may feed
on them. When numbers of twig aphids are high, sooty mold can also be found
on needles.
Pest description. The BTA is a small, green
aphid. Newly hatched aphids are very small, and require a hand lens to
see well. The small, individual aphids can be more easily spotted by looking
for the beads of honeydew, which they exude from their abdomen. Adult aphids
are much larger than most of the pests of Fraser fir Christmas trees and
are easily seen with the naked eye. Each individual aphid molts four times
before reaching adult size. Because of this, in heavily infested trees,
the white caste skins of the aphids can be readily seen.
Early in the spring, the aphids are wingless. By
mid- May, individuals with wings can also be found.
Twig aphid eggs are small, black, and tear-drop
shaped. White, waxy rods are scattered across the surface of the egg.
Similar appearing problems. Damage from the
herbicide Stinger can cause needles to twist around the stem. However,
the tips of the needles do not bend toward the base as they do with twig
aphid damage. Immature shoots of Fraser fir can also be twisted or distorted
by wind or other damage, making the underside of the needles show. This
can appear to be twig aphid damage, but on closer inspection it is observed
that the needles themselves are straight.
When to scout. The BTA is only active from
March through June. The rest of the year is spent as an egg. When trees
are scouted during this period depends on the goals of scouting.
There are two problems in scouting for the BTA One
is that twig aphids are mobile and will move back into the canopy of the
tree during wet or cold weather where they are harder to find. The other
is that the stem mother, the fist stage which hatches from the egg in the
spring, produces many live young. In April, a scout may find hardly any
aphids one week and return the next week to find many.
Because of this and because the BTA can cause so
much damage, it is currently recommended that trees be treated for BTA
each spring before bud break when they are 2 years from harvest. This is
the only pest of Fraser fir for which scouting is not necessary to justify
a pesticide application. To determine if trees need to be treated earlier
in the rotation, assess BTA damage in young trees in late May or early
June before needles have had a chance to straighten.
Scouting method. To assess twig aphid damage
on young trees, use the random walk pattern and look for trees with curled
needles. Estimate both the percentage of trees with some twig aphid damage
and the percentage of foliage damaged on the majority of these trees.
Treatment threshold. No treatment threshold
has currently been developed for BTA damage. However, if twig aphids are
readily found, some treatment is necessary on older trees. On younger trees,
if more than 1 out of 5 trees has some twig aphid damage, and that damage
is more than just a few curled needles, it may be necessary to treat for
twig aphids the following year.
Effects of weather. Twig aphid survival and
reproduction are favored before bud break during early, warm, dry springs.
Wet, cold weather reduces the survival of the stem mothers, and slows their
development, delaying reproduction. Sometimes late frosts can also kill
many aphids.
Weather also affects whether or not young damaged
fir needles will straighten. If the weather is dry in May and June, it
will be more likely that damage will be permanent.
Control. The BTA is easily killed with a
wide variety of pesticides. The key to control is timing. Once the trees
have broken bud and damage has already started to occur, pesticide applications
are ineffective. Pesticides need to be applied after the twig aphids have
started hatching or are completely hatched (depending on how long the pesticide
persists in the trees), but before bud break when they start to cause their
damage. Twig aphid hatch is currently being monitored every year in North
Carolina, and county extension agents can advise if hatch has occurred.
If Di-Syston 15 G is used to control twig aphids,
weather at the time of application is also very important. This material
must be applied when winds are calm and when there is enough moisture to
activate the chemical. Be sure to have in your possession the current North
Carolina Special Local Needs label, and follow all directions.
Scouting after control. It is necessary to
come back into trees soon after treatment so that if aphids are still alive,
the trees can be retreated before many trees have broken bud. If the majority
of trees have broken bud and there is already a lot of damage, retreatment
will not be successful, and will probably kill natural predators already
working to control the twig aphid.
Scout in trees 1 or 2 days (longer if Provado was
used for control) after treatment to determine if twig aphids have been
killed. Use the random walk pattern to move through trees, sampling 10
to 15 trees per acre. Move toward any trees with cones to examine these.
Twig aphids are easiest to find by beating foliage
on a tray, white board, or paper. Use a hand lens to clearly see the smallest
aphids. Also, be sure to note any predators found, including syriphyid
fly larvae, lady beetles and larvae, and lacewing larvae, which are the
most important predators of twig aphids. If predators are frequently found,
retreatment may not be necessary.
Twig aphids can also be found in immature cones,
which usually break about 2 weeks before the vegetative shoots. Once the
shoot buds have broken, twig aphids can be found inside immature shoots
with twisted needles.