Scouting and Control of Common Pests of Fraser Fir
Most Fraser fir scouting is focused on the spruce spider mite (Oligonychus
ununguis), SSM, which can be active in western North Carolina from
March through November. Populations of the SSM large enough to damage trees
can build up within a few weeks. Mite populations are affected by the temperature
and rainfall, the number of predators and past pesticide use.
Symptoms. The SSM cause tiny, yellow spots
to appear on an otherwise green needle. When the damage is light, the spots
will be confined to the base of the needle. Use a hand lens and spread
apart the needles to see well. When damage is heavy, the entire needle
becomes spot- ted and is easily seen. The yellow spots merge and turn brown.
If SSM damage is allowed to continue, the general color of the tree foliage
can turn yellow and rusty orange. Mite-damaged needles are more likely
to drop off in the fall or after the tree is harvested.
Pest description. The SSM in Fraser fir is
a small, red to almost black-colored mite. Young mites are smaller than
the adults and have six legs. When immatures molt, they may appear to be
dead as they go into a resting stage before they shed their old skin. Adults
are larger and have eight legs. The eggs are small and round with a single
hair coming up through the center. All life stages are present through
the growing season. The SSM overwinters as an egg though during warm periods
in the winter, live mites can be active.
Scouting for spruce spider mites using a hand lens
Similar appearing problems. Spider mite damage
can be mistaken for poor color, yellowing needles or other symptoms of
nutrient deficiency. Look at the needles through a hand lens to find the
yellow spots and the spider mites themselves.
When to scout. The first few years of a rotation,
Fraser fir can tolerate a low level of SSM damage. Scouting need not be
as frequent as it will need to be during the latter half of the rotation.
For trees 3 feet or larger during normal spring
temperatures, scouting may begin in late April and continue until late
October. If the year has an unusually warm winter and an early spring,
begin scouting in late March. When a balsam twig aphid treatment is made
that also has a miticide effect, the first scouting can be held off until
late May.
Scouting method. Use the rigid block scouting
method for the SSM. Look for SSM damage while walking through the trees.
Damage often begins on the most current growth that is shaded in the interior
and the lower half of the tree. While walking through the trees, focus
on these interior shoots from waist high to the bottom of the tree.
Take samples from trees showing SSM damage. If there
is no apparent SSM damage, choose a tree at random every 10 to 15 trees.
To sample for SSM, pull a single shoot of the most
current growth from the suspect or random tree. Take the shoot from the
interior and lower half of the tree. In the spring when trees have just
broken bud, also examine last year's growth. Occasionally, vary the sample
to examine shoots near the top of the tree.
Examine the shoot front and back with a hand lens,
looking for mites or mite eggs. There is no need to count the number of
mites or eggs, just note their presence or absence. If the shoot has any
mites or mite eggs, count it as a SSM infested tree. Keep count of the
total number of shoots examined as well as the number that are infested
with spider mites. Most scouts use hand counters or a note pad to keep
track of the counts.
To keep moving and spread the survey out more uniformly,
walk past at least six to seven trees after finding a positive SSM shoot.
Though the survey should encompass the entire field, be aware that SSM
are more likely to be found on trees grown on windy ridges and on dust-covered
trees near roads.
Examine anywhere from 15 to 30 shoots per acre.
The exact number of shoots examined and the exact acreage scouted doesn't
matter since the treatment threshold is based on the percentage of shoots
with mites or mite eggs.
Treatment threshold. After finishing scouting
a block, convert the number of shoots infested and sampled to a percentage.
For instance, if 5 shoots were found with mites or eggs out of 21 shoots
examined, divide 5 by 21 and multiply by 100 to calculate the percentage
of infested shoots. In this case the number is about 24 percent.
The treatment threshold changes as the trees grow
older and more valuable. The following thresholds are suggestions and can
be modified depending on your cost of control and markets.
Size of the Tree
Treatment threshold*
Less than waist high
40%
Waist high to year before sale
20%
Year of sale
10%
*Percentage of infested shoots to sampled shoots. Indicates pesticide treatment or some other action is necessary
Scouting frequency. If the SSM percentages
have not reached threshold levels, schedule the next SSM survey based on
the percentage of infested shoots and the weather, field location, and
past pesticide use.
• If no mites or eggs are found and no damage is seen, return
in 4 to 8 weeks.
• If less than 10 percent of the shoots have mites or eggs, or
if new SSM damage has occurred since the last scouting trip or pesticide
treatment, return in 3 to 5 weeks.
• If more than 10 percent of the shoots have mites or eggs, but it
is less than the treatment threshold, return in 2 weeks.
• If trees are go-to-market trees, scout at least once a month.
It may be necessary to scout sooner, if there are more than 10 days of hot, dry weather. Mite reproduction and life span increase under these conditions. Also, return sooner if pesticides that kill natural predators have been used in trees within the last 12 months, as SSM numbers are more likely to build up. In counties having greater rainfall, or in field sites that are sheltered by woods and have little air movement, scouting may be delayed.
Beating foliage to dislodge mites and predators. Examining mites to determine if they are pests.
Effects of weather. Spider mites become a
worse problem anytime during the growing season that unseasonably warm
or dry weather occurs. Hot spot scout identified problem areas during hot,
dry periods to determine how the weather is affecting mite activity. The
'hot spot' becomes a representation of what is going on in the rest of
the block. Identify the hot spot through other scouting methods and mark
it with flagging and on field maps. If mite activity is increasing in hot
spots, go back and use the rigid block scouting method over the entire
block to determine if treatment is necessary.
Spider mites can also be found by beating foliage
against a beater tray, white board, or white sheet of paper placed into
the mid or lower canopy. Examine the moving mites with a hand lens to determine
if they are SSM. This method misses finding spider mite eggs since these
will not shake out of the plant. However, this method is also best for
finding spider mite predators, which may be helping to control the spider
mites.
Control. No miticide should be used unless
mites are present. Many growers have treated mite-damaged trees where mites
had already been killed by natural causes, thereby wasting time and pesticide.
Miticides vary in how they kill spider mites, and by the stage of the mite
they attack. This affects control, because throughout most of the growing
season, all stages of the mite, from egg to adult, are present. Some miticides,
like Savey, only kill the eggs and immature mites. Others, like Joust,
kill all stages including the eggs. Avid and Di-Syston are systemic, and
may last long enough in the plant to kill the immature mites after they
hatch from the egg. There are many other materials such as Lorsban, Talstar,
dimethoate, Metasystox-R and Mavrik that have no activity against the egg,
and usually have to be reapplied after 2 weeks to kill the hatching eggs.
Contact your county extension agent to determine the miticides currently
labeled in Christmas trees and their mode of action.
Chemical control of the SSM is difficult because
good coverage is essential to good control. When applying a foliar miticide,
make sure the spray wets all of the current growth. The foliage at the
base of the tree should be especially targeted as this is where mites are
more numerous.
Scouting after control. Scout 2 to 3 weeks
after chemical control to detennine if mites were successfully killed or
if more mites are hatching out of eggs. Continue scouting for mites following
the regular scouting schedule through the rest of the growing season.