UNIVERSITY OF CALIFORNIA ■ COLLEGE OF AGRICULTURE

AGRICULTURAL EXPERIMENT STATION

BERKELEY 4, CALIFORNIA

CONTROL OF THE SHOT-HOLE BORER1

LESLIE M. SMITH2

The shot-hole borer is a small, brown beetle
that attacks orchards and may kill small twigs,
entire limbs, or even whole trees. These
beetles can be found throughout the summer,
crawling rapidly over the bark of weakened trees,

brown to black; and the legs are lighter brown
than the body.

The females cut circular holes in the bark,
generally* in the centers of lenticels (fig. 1).
The lenticels are raised and roughened areas of

Fig. 1. --Entrance holes made by females constructing egg
tunnels. These holes represent sufficient brood to give rise
to the condition in figure 2, when the next generation of
beetles emerge. This view shows the bark enlarged two times.
(From Ext. Cir. 64- )

particularly through the warmer part of the day.
They can be recognized by the following charac-
ters: the body is roughly cylindrical in shape;
the length is about l/lO inch; the color is dark

xThis leaflet supersedes Extension Circular
64, The Shot Hole Borer, by Leslie M. Smith
(1932), which is out of print.

2Associate Entomologist in the Experiment
Station.

the bark, which contain soft, spongy tissue,
adapted to the exchange of gases. Such areas
afford the beetles better footing and are less
resistant than smooth bark. From these holes,
tunnels are constructed through the bark to the
beginning of the wood . Here the females gen-
erally work up or down the length of the limb
and continue to make their tunnels through the
inner bark and cambium. Generally these tunnels

[1]

are cut slightly into the. wood, so that when the
bark and the frass, or undigested food deposited
by the larvae, are removed from a heavily in-
fested limb, the adult burrows are indicated by
shallow grooves in the wood. As soon as the fe-
males have constructed a short length of tunnel,
they begin to lay eggs along the sides. The
white, spherical eggs are enclosed only in a
thin membranous shell. To protect them the fe-
males shred bark and wood, cementing it along
the sides of the burrow until the eggs are com-
pletely imbedded. A completed tunnel is 1 inch
to 1-1/4 inches long. In the work of excavation
each female is generally accompanied by a male,
often seen standing on the bark near the mouth
of the burrow. When the burrow is completed and
the eggs are all laid, the female backs to the
opening of the burrow and dies there with the
posterior end of her body projecting. She thus
blocks the opening and prevents the entrance of
parasities and predators, which might kill the
eggs or young larvae .

The eggs hatch within a few days after being
laid. The young, or larvae, are at first almost
too small to be seen with the unaided eye. Feed-
ing on the cambium and inner bark, they advance
through it, generally at right angles to the egg
tunnel. They grow rapidly; and the burrow con-
structed by their feeding becomes larger as they
advance. Unlike the egg tunnels, the larval
burrows are packed full of frass. They are small
at the beginning and widen rapidly, whereas the
egg tunnels are of uniform bore. In outline the
burrows or tunnels of a single, isolated female
and her progeny somewhat resemble a centipede;
the primary burrow represents the body, and the
secondary burrows represent the legs. Such a
formation is practically never seen isolated;
when the beetles are numerous enough to assure
economic importance, the primary burrows are
close together, and the larval burrows are com-
pletely intertwined. In heavy infestations the
entire inner bark is reduced to powder. Young
feeding larvae are pinkish because the ingested
bark is visible through their skins. The full-
grown larvae are white, with small brown heads
sunken into the enlarged anterior part of their
bodies. They are legless, and their bodies are
generally curved. The fully matured larvae gen-
erally burrow toward the center of the limb and
penetrate about 1/16 inch into the wood. They
then turn up or down the limb and construct a
cell in which to pupate (transform to the adult
stage). The short burrow leading to the pupal
cavity is tightly packed with chewed wood. Some
larvae of the summer broods pupate in the bark,
but all of the overwintering generation pupate
in the wood. The length of the larval period is
imperfectly known; in summer it is probably a
little more than one month.

In the pupal cells, the larvae molt and trans-
form to pupae--white, motionless bodies that in-
distinctly show the form of the adult. After
some time the pupae molt and change to adult
beetles, which are white at first, but which .

soon turn brown, and then black. The adults re-
main in the pupal cells for a few days, then cut
their way through the bark (fig. 2) and begin
the cycle again.

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Fig. 2. --Adult beetles and exit holes in bark
of French prune, shown enlarged two times. (From
Ext. Cir. 64. )

Seasonal History

The shot-hole borers pass the winter as larvae
living and feeding, to some extent, in the inner
bark. Some of these overwintering larvae pupate
in the wood during the winter. The remainder
pupate very early in the spring. The first
beetles emerge from orchard wood about March 15.
The last adults from overwintering larvae emerge
before the middle of May. By this date, most of
the adults from the overwintering brood have
laid eggs, and the eggs have hatched, so that
the first spring generation is represented, on
May 15, mainly by large larvae. No pupae are
found at this time. By the middle of June, in
the observations here reported, the development
of the first generation was estimated as follows:
larvae 40 per cent , pupae 50 per cent , and new
adults 10 per cent. Since April 15 represents
the peak of emergence for the overwintering
brood, the complete cycle, from overwintering
adults to emergence of the greatest number of
adults of the first spring generation, is rough-
ly two months. By the middle of July the broods
overlap: eggs, larvae, pupae, and adults are all
present in numbers. The beetles continue to
breed through the summer and fall. A few adults
still construct egg tunnels and deposit eggs
early in November. Eggs and young larvae are
very common at this time; only a few larvae have
constructed pupal chambers in the wood, ahd none
have pupated. With a generation completed every
two months and a breeding season extending from
the latter part of March to the latter part of
October, three complete generations and a partial
fourth are possible.

[2]

- .

Injury to Trees

Injury to trees by the shot-hole borer results
from the feeding of both adults and larvae. In-
jury by the mature beetles falls into two fairly
distinct groups: twig injury and limb injury.
In producing twig injury the adults bore shallow
holes in the new wood at the bases of the buds.
They work in such places for only a short time,
and evidently seek them only if suitable larger
limbs are not available. In laboratory tests,
when terminal growth and suitable limbs were both
present, the beetles entered the limbs and rarely
attacked the twigs. Such punctures, made at the
bases of the buds and fruit spurs, exude con-
siderable gum (fig. 3). This gumming increases
throughout the summer; in severe cases, nearly
all the terminals on the tree may be gummed by
fall. Eggs are rarely laid in twig growth that
is less than 1/2 inch in diameter.

Limb injury by the adults is due to tunneling
through the inner bark and the cambium. Entrance
holes are frequently cut into limbs and trunks
where the sap flow is still strong. The beetles
abandon these holes, and gum exudes freely. Oc-
casional trees, particularly young trees that
lack water, become studded all over the trunk,
limbs, and twigs with masses of gum. In favor-
able situations — that is, in the limbs and trunk
of weak trees, where the sap flow is much re-

duced— the beetles continue their tunnels and
lay their eggs. They rarely girdle the limb,
however, since they nearly always turn up or
down it.

The larvae often girdle the limb; they tend
to travel at right angles from the vertical, pri-
mary burrow, and hence work around the limb.
Probably, however, a branch too weak to produce
gum to repel the adults, and therefore suitable
for egg deposition, could not survive even in the
absence of beetle injury.

In the Santa Clara Valley the shot -hole borer
attacks chiefly prunes and cherries. In the Sac-
ramento and San Joaquin valleys, peaches, prunes,
apricots, and almonds are often attacked. Essig3
gives the following host list: almond, apple,
apricot, cherry, chokecherry, elm, hawthorn, June-
berry, loquat, mountain ask, nectarine, peach,
pear, plum, prune, and quince.

Control

In control the most important consideration
is that the shot-hole borer cannot live in healthy,
vigorous trees. One factor largely responsible
for an epidemic is lack of soil moisture. Trees
may be suffering so slightly from drought that
no ill effect is visible, and yet be weakened and
attract the beetles. The pests single out such
a tree even if it shows no outward sign of sick-
ness. According to abundant field evidence, it
is on trees of low vitality that the beetles in-
jure the bases of the buds on the year-old wood.
The following year, when these trees have been
further weakened by bleeding through numerous
punctures, the beetles are able to attack the
larger limbs.

It is therefore extremely important to keep
the trees strong and vigorous. The chief aid to
the tree, in withstanding the borer, is water.
In the center of one of the most heavily infested
areas, several orchards have been almost com-
pletely protected by heavy irrigation, whereas
near-by trees suffering from lack of water were
seriously injured by this insect.

Other practices that increase the vigor of
the trees, such as reducing the tops, cutting
back, and fertilizing, should be followed when
the beetle is established in the vicinity.

Next in importance to keeping the trees vigor-
ous is orchard sanitation. Many growers have
already witnessed severe injury to trees growing
near woodpiles. Dead and dying wood removed from
the orchard in late fall or in winter contains
the overwintering larvae. If such wood is cut
up and piled for home use, the beetles will emerge
from it without difficulty the following spring.
In dispersing from such a center of infestation,
the adults stop and feed on the terminal growth
of trees in the immediate vicinity; they may, in

Fig. 3. --Twigs of French prunes showing gum-
ming as a result of punctures made by the shot-
hole borer at the bases of buds and spurs. (From
Ext. Cir. 64.)

^ssig, E,
America, p.
1926.

0. Insects of Western North
511. Macmillan Co., New York, N.Y.

[3]

a single season severely injure or even completely
kill these trees. Eventually, most of the beetles
fly onward to more distant trees, so that distri-
bution is fairlv uniform and the injury does not
show a definite connection with the woodpile.
The grower should therefore not underestimate the
danger from this source; he should burn all in-
fested wood before the end of February.

If there is a large woodpile, representing
considerable value as household fuel, the grower
may wish to treat it in some way to kill the
overwintering larvae. This treatment is usually
difficult: many of the beetles are in pupal
cells in the wood, with the entrance sealed. In
tests, the brood has been killed by fumigating
the wood for 24 hours with carbon disulfide,
used at the rate of 25 pounds per 1,000 cubic
feet. A thorough spraying from all sides with
stove distillate will likewise kill all over-

wintering beetles. Infested wood cut and piled
during the winter may harbor adult beetles un-
til, by the earJy part of June, all of the over-
wintering brood have emerged. Such wood is
never reinfested; . that is, no eggs are laid in
it. Being no longer a menace, it may be kept
indefinitely.

Wood from healthy, noninfested trees, which
may be removed for various reasons during the
winter, may retain enough sap to be acceptable
to the beetles the following spring and may
serve as food for the first spring brood.

Dead trees and limbs left in the orchard
also present a serious menace; they may carry
the beetles through the winter, precisely as
does the cut wood. Severe twig injury is com-
monly found on trees adjacent to a dead tree.
All such material, therefore, should be removed
and burned before the end of February.

[4]

5m-May, '45(4047)