The shot hole borer

CALIFORNIA
AGRICULTURAL EXTENSION SERVICE

CIRCULAR 64

April, 1932

THE SHOT HOLE BORER

LESLIE M. SMITH

Cooperative Extension work in Agriculture and Home Economics, College of Agriculture,
University of California, and United States Department of Agriculture cooperating. Distributed
in furtherance of the Acts of Congress of May 8 and June 30, 1914. B. H. Crocheron, Director,
California Agricultural Extension Service.

THE COLLEGE OF AGRICULTURE

UNIVERSITY OF CALIFORNIA

BERKELEY, CALIFORNIA

Digitized by the Internet Archive

in 2011 with funding from

University of California, Davis Libraries

http://www.archive.org/details/shotholeborer64smit

THE SHOT HOLE BORER

LESLIE M. SMITHi

The shot hole borer (Scolytus rugulosus Ratz.), sometimes called
the fruit tree barkbeetle, is a pest of European origin. Within the
last few years articles have appeared reporting the shot hole borer as
injurious to fruit trees in Argentina, Austria, Crimea, England, Jugo-
slavia, and Bulgaria. It is believed to have been accidentally intro-
duced into the United States some time prior to 1877. In 1916,
Brooks2 stated that the beetle occurred throughout practically all of
the United States east of the Mississippi River, and in many localities
to the west and in Canada. The first authentic record of the occur-
rence of this pest in California is found in the files of E. 0. Essig,
who discovered it in Ventura County in 1910. At the present time
this pest is known to occur in the majority of counties in California.
Its known distribution and importance in California are shown in
figure 1. During the past two or three years this insect has been
responsible for severe injury in some sections of the Santa Clara, San
Joaquin, and Sacramento valleys and in parts of southern California.

DESCRIPTION AND LIFE HISTORY

The body of the adult beetle is roughly cylindrical in shape. There
is considerable variation in the size of the adults, due to differences
in the amount and quality of food obtained by the larva, or immature
stage. The majority of adults, however, measure from 0.094 to 0.112
inch in length and from 0.039 to 0.048 inch in diameter. The color is
dark brown or black; the legs are generally lighter brown than the
body.

The females cut circular holes in the bark, generally in the cen-
ters of lenticels as shown in figure 2. The lenticels are raised and
roughened areas of the bark, which contain soft spongy tissue, adapted
to the exchange of gases. Such situations 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

i Besearch Assistant in Entomology and Parasitology.

2 Brooks, F. E. Orchard barkbeetles and pinhole borers, and how to control
them. U. S. Dept. Agr. Farmers ' Bui. 763:1-15. 1916.

4 California Agricultural Extension Service tClR- 64

wood. Here the females generally work up or down the length of the
limb and continue to construct their tunnels through the inner bark
and cambium. Generally these tunnels are cut slightly into the wood,
so that when the bark and frass, or undigested food deposited by the

□ Counties in which th
to be present.

e borer is not known

Present but not of economic importance.

Economic importance.

...mm.

;an o,E^5M: ■•■ ■■■'■ :■-, •-. J

Fig. 1. — Present distribution and importance of the shot hole borer
in California.

larvae, are removed from a heavily infested limb the adult burrows
are indicated by shallow grooves in the wood. As soon as the females
have constructed a short length of tunnel they begin to lay eggs along
the sides. The eggs are white and spherical, and are enclosed only in
a thin membranous shell. In order to protect the eggs, the females

1932] The Shot Hole Borer 5

shred some of the bark and wood, and cement it along" the sides of the
burrow until the eggs are completely imbedded. These protecting
walls can be seen in figure 3, where the whiter wood contrasts with
the darker bark. The females continue the tunnels until they are from
1 inch to \x/\ inches in length. Eggs are laid along the sides of the
tunnels as they are excavated. These tunnels are called the primary,
or egg, tunnels. In the work of excavation each female is generally
accompanied by a male, which is frequently seen standing on the bark
near the mouth of the burrow. When the burrow is completed and all
of the eggs are laid the female backs to the opening of the burrow and
dies there with the posterior end of the body projecting. The body
of the female thus blocks the opening and prevents the entrance of
parasites, or predators which might kill the eggs or young larvae.

The eggs hatch within a few days after being laid. The young, or
larvae, are very small at first, and can scarcely be seen by the unaided
eye. They immediately begin feeding on the cambium and inner bark
and advance through it generally at right angles to the egg tunnel.
They grow rapidly and the burrow, which they construct by feeding,
becomes larger as they advance. These burrows, constructed by the
immature beetles, are called secondary, or larval, burrows, They
differ from the primary burrows in that they are packed full of frass
or undigested material, whereas the egg tunnels are open ; also, they
are very small at the beginning and widen rapidly, whereas the pri-
mary tunnels are of uniform bore. The tunnels of a single, isolated
female and her progeny somewhat resemble a centipede, wherein the
primary burrow represents the body and the secondary burrows
represent the legs. Such a formation is practically never seen, be-
cause of the fact that when the beetles are present in sufficient
numbers to be of economic importance, the primary burrows are close
together and the larval burrows are completely intertwined. In heavy
infestation the entire inner bark is reduced to powder. Young feeding
larvae are pinkish in color because of the ingested bark which is
visible through their bodies. 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 generally burrow toward the center of the limb
and penetrate the wood for a. distance of about y16 inch. They then
turn up or down the limb and construct a cell in which to pupate, or
transform to the adult. The short burrow leading to the pupal cavity
is tightly packed with chewed wood. Some of the larvae of the sum-
mer broods pupate in the bark, but the overwintering generation all
pupate in the wood (fig. 4). The length of the larval period is imper-

California Agricultural Extension Service [Cir. 64

Fig. 2. — Entrance holes made by females constructing egg tunnels. These
holes represent sufficient brood to give rise to the condition in figure 4, when the
next generation of beetles emerge. (Enlarged two times.)

1932]

The Shot Hole Borer

Fig. 3. — The inside of the same piece of bark shown in figure 2. This shows
how the inner bark may be reduced to powder while very few holes appear on the
outside of the bark. The more distinct burrows are egg tunnels. The white spots
are masses of chewed wood which form the outer portions of the plugs protecting
the pupal chambers. (Enlarged two times.)

California Agricultural Extension Service [d*. 64

Fig. 4. — Wood with bark removed, showing holes through which adults have
emerged. The round white spots are plugs protecting pupal cells from which
adults have not yet emerged. (Enlarged two times.)

Fig. 5. — Adult beetles and exit holes in bark of French prune. (Enlarged

two times.)

1932] The Shot Hole Borer 9

fectly known but during the summer months it is probably a little
longer than one month.

In the pupal cells, the larvae molt and transform to pupae. These
are white, motionless bodies which indistinctly show the form of the
adult. After some time the pupae molt and change to adult beetles.
These are white at first but soon turn brown, and then black. The
adults remain in the pupal cells for a few days, then cut their way
through the bark (fig. 5) and begin the cycle again.

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. In 1931 the first beetles emerged from cut wood
in cages on March 27, and some may have emerged from woodpiles
even earlier. The last adults from overwintering larvae emerged
before the middle of May. At this time the majority of adults from
the overwintering brood had laid eggs, which had hatched some time
earlier than the middle of May, so that the first spring generation was
represented mainly by large larvae on May 15. No pupae were found
at this time. By the middle of June 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 of the overwintering brood, the complete cycle from
overwintering adults to the time of emergence of the greatest number
of adults of the first spring generation, was roughly two months. By
the middle of July the broods overlapped, that is, eggs, larvae, pupae,
and adults were all present in such numbers that no conclusions could
be drawn as to the generations represented. The beetles continued to
breed through the summer and fall. A few adults still constructed
egg tunnels and deposited eggs in the early part of November. Eggs
and young larvae were very common at this time; only a very small
percentage of larvae had constructed pupal chambers in the wood,
and none had 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, it would seem that three complete
generations and a partial fourth are possible.

10

California Agricultural Extension Service [Cm. 64

INJURY TO TREES

Injury to the trees caused by the shot hole borer, is the result of
feeding of both adults and larvae. The injury by the adults falls
into two fairly distinct groups which may be designated as twig
injury and limb injury. In producing twig injury the adults bore
short holes in the new wood at the bases of the buds. The adults work

Fig. 6. — Twigs of French prunes showing gumming as a result of punctures made
by the shot hole borer at the bases of buds and spurs.

in such places only for a short time. This is evidently a method of
feeding when suitable larger limbs are not available. Laboratory
tests showed that when terminal growth and suitable limbs were both
present, the beetles entered the limbs and rarely attacked the twigs.
Such punctures, made by the adults at the bases of the buds and fruit
spurs, exude a considerable amount of gum, as shown in figure 6.

1932] The Shot Hole Borer 11

Some twig injury is done by the first brood since an appreciable
amount of gumming was seen by the middle of May. This type of
gumming increases throughout the summer and in severe cases, nearly
all the terminals on the tree may be gummed by fall. Eggs are rarely
laid in twig growth which is less than % inch in diameter.

Limb injury by the adults is due to tunneling through the inner
bark and 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 from them freely. Occasional trees, particu-
larly young trees which are suffering from lack of water, become
studded throughout the trunk, limbs, and twigs with masses of gum.
In favorable situations, that is, in the limbs and trunks of very weak
trees, where the sap flow is much reduced, the beetles continue their
tunnels and lay their eggs. However, since they nearly always turn
up or down the limb, they rarely girdle it.

The injury by the larvae frequently results in girdling the limb
since they tend to travel at right angles from the vertical, primary
burrow, and hence work around the limb. I It is possible, however,
that a branch which is so reduced in vitality that it cannot repel the
adults, and is 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 Sacramento and San Joaquin valleys,
peaches, prunes, apricots, and almonds are frequently attacked. Essig3
gives the following host list : almond, apple, apricot, loquat, mountain
ash, cherry, choke cherry, elm, hawthorn, Juneberry, nectarine, peach,
pear, plum, prune, and quince.

CONTROL

The most important fact bearing upon the control of this pest, is
that it cannot live in healthy, vigorous trees. The factor which is
almost solely responsible for th^ present epidemic of the shot hole
borer is drought. Trees may be suffering so slightly from lack of
water that no ill effects are visible but they may be weakened so that
they are attractive to the beetles. The beetles appear to be able to
select a weakened tree which frequently shows no outward sign of
sickness. It is on such trees of low vitality that the beetles inflict
injury at the bases of the buds on the year-old wood. The following

s Essig, E. O. Insects of western North America, p. 511. Mnemillan Co.,
New York, 1926.

12 California Agricultural Extension Service [^r- 64

year these trees are considerably weakened as a result of bleeding
through numerous beetle punctures, so that the beetles are able to
attack the larger limbs.

It is of the utmost importance therefore to keep the trees as strong
and vigorous as possible. The chief aid to the tree, to enable it to
withstand the attack of the shot hole borer, is water. In the center
of one of the most heavily infested areas, several orchards have been
almost completely protected from beetle attack by heavy irrigation,
whereas trees on all sides which were suffering from lack of water
were seriously injured by this insect.

Other practices which 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. Cody4 recommends 4
or 5 pounds of sulfate of ammonia or nitrate of soda to the tree to
stimulate new and vigorous growth.

Next in importance in keeping the trees vigorous, is the need for
careful orchard sanitation. Many growers have already suffered
severe injury to their trees in the immediate vicinity of woodpiles.
Dead and dying wood which is removed from the trees during the
late fall and winter contains the overwintering larvae. If such wood
is cut up and piled for home use, the beetles will emerge from it with-
out 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, and in the course of one season may severely
injure, or even completely kill these trees. The majority of the beetles
fly on to other more distant trees until they are fairly uniformly
distributed and their injury does not show a definite connection to the
woodpile. The grower should not let this mislead him regarding the
seriousness of holding over the beetles in stove wood. Such wood
should be burned before the end of February.

When a large amount of wood has been removed, representing a
considerable value as stove wood, it would seem desirable to treat it in
some way to kill the overwintering larvae. It must be borne in mind
that this treatment is normally very difficult, owing to the fact that
the beetles are, in part, in pupal cells in the wood, with the entrance
to the cell protected by a dense plug of chewed wood. Laboratory
tests on a small scale have demonstrated that the brood can be killed
by fumigating the wood for 24 hours with carbon bisulfide at the rate
of 25 pounds per 1,000 cubic feet. Spraying the wood thoroughly
from all sides with stove distillate likewise killed all of the overwinter-

4 Cody, L. E. Bark beetles' control studied. Sunsweet Standard 14(6) :6. 1930.

1932] The Shot Hole Borer 13

ing beetles, in a small-scale test. Whether or not these methods can be
satisfactorily developed to fit field conditions remains to be proved.
Any work of this sort undertaken by the growers had best be done in
close cooperation with the local farm advisor or agricultural commis-
sioner. Infested wood which has been cut and piled during the winter
may serve as a source of adult beetles until the early part of June, at
which time all of the overwintering brood will have emerged. Such
wood is never reinfested, that is, no eggs are laid in it, so that it is
then no longer a menace and 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 accept-
able 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 and serve as a means of carrying- 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 dead trees and
limbs should be removed and burned before the end of February.

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Feeding Beef Cattle in California.

Lettuce. (Series on California Crops
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Suggestions on Grapefruit Culture in
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Rabbit Raising.

The Home Preparation of Fruit Candy.

Cauliflower Production.

Wool Production in California.

The Manufacture of Monterey Cheese.

Selection and Care of Electrical Equip-
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Pork Production in California.

Irrigation of Orchards by Contour Fur-
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Liver Fluke and Stomach Worm of
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Bovine Tuberculosis.

Thinning Sugar Beets.

Strawberry Culture in California.

Bush Fruit Culture in California.

The Home Vegetable Garden.

Control of Pocket Gophers and Moles in
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Elements of Grape Growing in Cali-
fornia.

Powdery Mildew of the Grape and Its
Control in California.

What to do About Bovine Tuberculosis.

Rearing Dairy Heifers Free from Tuber-
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No.

34. Plum Growing in California.

35. Alfalfa Production.

36. Beekeeping for the Beginner in Cali-

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3 7. Home and Farm Preparation of Pickles.
38. Alfalfa Varieties and Seed Supply.

40. Frost Protection in California Orchards.

41. Prune Culture in California.

42. Peach Culture in California.

43. The California Avocado Industry.

44. Bang's Disease (Infectious Abortion).

45. Zinc Chloride Treatment for Pear Blight

Cankers.

46. Cherrv Culture in California.

47. Equipment for the Bulk Handling of

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48. The Manufacture of Cottage Cheese.

49. Sheep Production in California.
51. Apricot Growing in California.

53. Home Floriculture in California.

54. The Control of Weeds.

55. Growing and Handling Sweet Potatoes

in California.

56. Girdling Grape Vines.

57. Commercial Fertilizers and Soil Fertility

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58. Turkey Raising in California.

59. The 1932 Agricultural Outlook for Cali-

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115. Grafting Vinifera Vineyards.

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212. Salvaging Rain-Damaged Prunes.

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239. Harvesting and Handling Apricots and

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240. Harvesting and Handling California

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241. Harvesting and Handling California

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244. Central Wire Bracing for Fruit Trees.

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321. Treatment of Lime-induced Chlorosis

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322. An Infectious Brain Disease of Horses

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12m-5,'32