United States

Department of

Agriculture

Combined Forest Pest

Research and

Development Program

Agriculture Handbook

No. 539

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Gypsy Moth t^ c VW

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Diseases of the

Gypsy Moth:

How They Help

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Populations ^ .

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Diseases of the Gypsy Moth:

How They Help to Regulate

Populations

by J. D. Podgwaite*

In 1974 the US. Department of

Agriculture initiated the Combined

Forest Pest Research and

Development Program, an

interagency efifort that

concentrated on the Douglas-fir

tussock moth in the West, on the

southern pine beetle in the South,

and on the gypsy moth in the

Northeast. The work reported in

this publication was funded in

whole or in part by the program.

This manual is one in a series on

the gypsy moth.

In the Northeastern United

States, the gypsy moth is subject to

a variety of naturally occurring

diseases of two types—infectious

and noninfectious. Infectious

diseases are the most easily

recognized and are caused by

several kinds of bacteria and fungi

as well as a virus specific to the

gypsy moth. In dense gypsy moth

populations, infectious diseases are

spread from one insect to the next

by direct contact or through the air.

The noninfectious diseases are not

as easily recognized. They are not

contagious but result from adverse

physiological changes in the insect,

generally in response to weather

conditions, food quality, or

hereditary factors.

Natural diseases do not appear to

prevent gypsy moth populations

from exploding to outbreak levels;

parasites and predators are probably

more important in this regard.

However, after gypsy moth

populations have reached outbreak

levels and have caused significant

defoliation and tree damage, it is

disease, particularly infectious

disease, that intervenes to reduce

these populations to harmless levels.

In this booklet, some of the natural

diseases of the gypsy moth are

described as well as some of the

disease agents that have been

developed for controlling this pest.

Microbiologist, Forest Insect & Disease

Laboratory, Forest Service, Hamden, Conn.
Infectious Diseases

By far the most devastating disease

of the gypsy moth is that caused by

the specific nucleopolyhedrosis

virus (NPV) that affects the larval

and pupal stages of the insect. The

disease results in the formation of

virus-containing, polyhedral

inclusion bodies (PIB's) in the nuclei

of cells in the infected insect. The

virus is so small that it is best seen

with the aid of an electron

microscope (figs. 1 and 2).

Figure 1.—Polyhedral inclusion

bodies (PIB's) of the gypsy moth

nucleopolyhedrosis virus.

Figure 2.—Cross section through

PIB's showing the infectious virus

rods (virions) within.
An insect becomes infected by

eating foliage that has been

contaminated with PIB's. In the gut

of the insect, the PIB's dissolve and

release the virus rods, which first

cross the gut wall and then infect

the hemocytes (blood cells). The

disease progresses to the fat body

and finally to cells of the

integument. An infected larva will

show signs of the disease by loss of

appetite, listlessness, a darkening in

color, a moist-appearing integument,

and often a tendency to climb

upward. Infected larvae usually

die within ^11 days and

characteristically hang from foliage

or bark in an inverted "V" position

(fig- 3).

The dead insect is a sac of PIB's,

and when this fragile sac is

ruptured, its contents are spilled

onto foliage and bark (fig. 4),

providing a source of infection for

healthy larvae.

In many dense gypsy moth

populations, this virus kills up to 70

percent of the larvae and reduces

populations to levels where they

cause only minimal defoliation and

tree damage in the following year.

Because this virus is such an

effective natural pathogen, it has

been developed for use in gypsy

moth management programs. The

product is called Gypchek (fig. 5), a

dry powder containing PIB's of the

virus. It has been shown to be

effective in reducing gypsy moth

populations and is safe for man and

wildlife. In April 1978, Gypchek was

registered as a microbiological
control agent with the

Environmental Protection Agency

(EPA), for use under the supervision

of the Forest Service.

Figure 3.—A typical NPV-killed

gypsy moth larva.

Figure 4.—Ruptured NPV-killed

gypsy moth larvae on leaves.

Figure 5.—Gypchek, an NPV

product recently registered with

EPA for gypsy moth control.
Figure 6.—Streptococcusfaecalis,

a bacterial pathogen often causing

small-scale epidemics in gypsy

moth populations.

Figure 7.—Senatia marcescens,

overall probably the most effective

naturally occurring bacterial

pathogen.

Figure 8.—Serratia liquefaciens, a

bacterial pathogen but of lesser

importance than 5. faeccilis or S.

Marcescens.

Figure 9.—Pseudomonas

aeruainosa, a common bacterial

pathogen of man and the gypsy

moth.

Figure 10.—Bacillus cereus, a

common bacterium that is weakly

pathogenic for gypsy moth larvae.
Infectious diseases caused by the

bacterial pathogens Streptococcus

faecalis, Serratia marcescens,

Serratia liquefaciens, Pseudomonas

aeruginosa, and Bacillus cereus are

also important in gypsy moth

regulation (figs. 6-10). Although the

collective mortality caused by these

bacteria in dense populations

usually does not exceed 15 percent,

there are at times pockets of

bacterial disease that may account

for up to 60 percent of the total

mortality within a given area. These

bacteria find their way onto foliage,

either carried there by the wind or

deposited in the droppings of birds

and small animals, where they are

consumed by larvae. Once in the

larva's gut, the bacteria produce

enzymes that allow penetration of

the gut wall and the invasion of the

hemocoel (body cavity). Unlike

NPV which infects blood cells,

these bacteria simply multiply in the

fluid portion of the hemolymph

(blood) and kill larvae either through

the production of toxic substances

or by depleting the insects of

nutrients. Of these naturally

occurring bacteria, S. faecalis and

5. marcescens are probably the

most effective in killing gypsy moth

larvae (figs. 11 and 12). In fact, S.

faecalis has been reported to cause

small-scale epidemics in gypsy moth

populations.

related to bacteria that cause disease

in man and other animals. In fact,

Pseudomonas aeruginosa is a

common pathogen of plants and

man, and there would be some risk

to nontarget animals if such bacteria

were introduced into the

environment.

Figure 11.—A healthy (large) gypsy

moth larva and one killed by 5.

faecalis.

Figure 12.—A larva killed by 5.

marcescens. Note the rose-colored

underside.

Unfortunately, most of the bacteria

mentioned do not lend themselves

to development as microbial control

agents, because they are all very

susceptible to deactivation by

sunlight when exposed on leaf

surfaces and some of them are

Interestingly, one bacterial species

that is relatively unimportant in

natural regulation of the pest has

been found to be effective as a

microbial insecticide. This

microorganism, Bacillus

thuringiensis (Bt) (fig. 13), has been

commercially available for several

years and is effective against many

insect pests, including the gypsy

moth. Unlike some of the other

bacteria, Bt is not harmful to man

or wildlife. Hovv'ever, it is not as

specific as the nucleopolyhedrosis

virus and does affect certain other

insects.

Bt kills only the larval stage of the

gypsy moth, in a manner quite

different from the other bacteria

mentioned. Bt normally lives in the

soil, and at a certain stage in its life

cycle it produces a spore and a

crystal (fig. 14). The spore is a

protective stage that allows the

bacterium to survive under harsh

environmental conditions, while the

crystal is believed to be a waste

product of the cell. It is the crystal

that is very toxic for gypsy moth

larvae. Within a few days of

ingesting a mixture of spores and

crystals that have been sprayed on

foliage, gypsy moth larvae cease

feeding and die. The crystal, which

is a large protein complex, breaks

down into smaller toxic substances

that cause paralysis of the larva's

gut. The larva then cannot feed and

soon dies of starvation (fig. 15).

Figure 13.—A stained smear of a

larva killed by Bacittus

thurinfiiensis, showing the large,

rodlike cells of this bacterium.

Figure 14.—A scanning electron

micrograph of spores and the

toxic diamond-shaped crystals of

Bacillus thuringiensis.

Figure 15.—A gypsy moth larva

killed by Bacillus thuringiensis.
The gypsy moth is also susceptible

to at least three species of

pathogenic fungi: Beauvaria

bassiana, Paecilomycesfarinosus,

and Aspergillus ßavus (figs. 16-18).

The combined mortality caused by

these three microorganisms is

usually less than 1 percent in natural

gypsy moth populations, but like the

bacteria, they can at times cause

significant mortality within a small

area.

Spores of fungi are usually the

infection units. They are airborn

and often are trapped in the hairs on

the surface of a gypsy moth larva. If

temperature and humidity

are right, the spore will germinate

and with the help of enzymes

penetrate the integument of the

larva. Once in the hemocoel, the

fungus grows to produce threadlike

structures called mycelia. During

growth, the fungus may produce

toxins that cause death or it may

simply multiply until it virtually fills

the larva with mycelia. After death

of the larva, the fungus may grow

back through the integument to

produce spores on the outside of the

insect (fig. 19).

Figure 16.—Colonies of the fungus

Beauvaria bassiana.

Figure 17.—^The fungus

Paecilomycesfarinosus.

Figure 18.—A giant colony of

A sperf>illus ßavus.

Figure 19.—A gypsy moth larva

killed by Beauvaria bassiana.

Unfortunately, environmental

conditions that cannot be controlled

(particularly humidity) are so critical

to the initiation of fungal disease as

to make these microorganisms

difficult to use as microbial control

agents. More importantly, many of

the fungi produce toxins that affect

man and other life forms, so their

use in pest management programs

could present some risk that

probably would not be offset by

their effectiveness.

10
11
Noninfectíous Diseases

Figure 20.—Healthy (left) and

starved fourth-stage gypsy moth

larvae.

12
After gypsy moth NPV, the

noninfectious diseases account for

the bulk of the gypsy moth

mortality (35 percent on the

average) in dense populations of the

insect. Unfortunately, very little is

known about these diseases,

principally because little is known

about the genetics and normal

physiology of the gypsy moth.

These two areas of study have not

been developed to the point where

baseline information on healthy

insects is available for comparison

to insects that are diseased.

Therefore abnormal physiological

conditions are very difficult to

diagnose and identify accurately.

Most noninfectious diseases of

the gypsy moth are undoubtedly

related to weather factors,

nutritional factors, or

heredity.

Unseasonably cold or warm

temperatures in the winter or spring

may result in gypsy moth eggs

hatching either at a time when the

weather is not favorable for the

development and survival of the

larvae or at a time that puts larvae

out of phase with the development

of their food source. This often

results in larvae dying on the egg

mass.

or, if they survive, become subpar

pupae and adults.

Genetics undoubtedly plays a role in

noninfectious disease occurrence. It

is known that larvae develop tumors

(fig. 21) and teratologies, although

these are rare. There are probably

many genetic-related diseases of this

insect that have yet to be

discovered.

Figure 21.—Gypsy moth larva

showing a tumor in the thoracic

region behind the head.

Poor nutrition is probably

responsible for most of the

noninfectious disease seen in natural

gypsy moth populations. Insects

that are severely limited in food

supply simply starve (fig. 20).

Larvae that feed on low-quality

foliage or on nonpreferred hosts

may die during the molting period

13
How to Recognize Gypsy

Moth Diseases

Careful observation of disease in

gypsy moth populations may

indicate how much of a problem the

insect will be in future years. This

knowledge can help in the planning

of control measures to use against

this pest.

Probably the gypsy moth disease

that is most easily recognized in the

field is nucleopolyhedrosis. As

mentioned earlier, and seen in figure

3, dead insects characteristically

hang in an inverted ''V" position

from bark and leaves. Dead larvae

are moist looking and dark in color

and may ooze a brownish illsmelling fluid that stains leaves and

bark. The widespread occurrence of

virus-killed larvae indicates that the

population is in decline and that in

the next generation there will be

fewer insects, and less defoliation,

to contend with.

Bacterial disease symptoms are

variable; discoloration of the

integument, cessation of feeding,

paralysis, vomiting, diarrhea, and

larval shortening are common

symptoms. Larvae infected with

Serratia marcescens show a

characteristic rose coloration on the

underside of their bodies (fig. 12).

Bacterially killed larvae are found

more often on the ground than on

trees.

Larvae aflflicited with fungal disease

often display restless behavior and

may become spastic in their

movements. They may choose to

settle on the undersides of leaves.

Living or dead larvae may be

covered with mycelia (fig. 19).

Noninfectious diseases are the most

difficult to recognize, because their

symptoms at times resemble those

of infectious disease, particularly

bacterial diseases. For instance,

larval shortening is a common

symptom of both starvation and the

disease caused by Streptococcus

faecalis. Without some laboratory

tests, it is difficult to know what is

being dealt with. However, in a

situation where a number of dead,

shortened larvae are found in the

midst of complete defoliation,

starvation can be suspected. On the

other hand, if there is obviously

enough food available for the

insects, suspect bacterial disease. If

a number of dead, first-stage larvae

are present on the egg mass,

physiological disease related to

weather or hereditary factors is

probably the cause. If this condition

is widespread, it could very well be

an indication that the gypsy moth

population is not healthy and may

be in a decline phase.

14
Acknowledgments

In closing this discussion, it must be

remembered that infectious and

noninfectious diseases are not the

only gypsy moth killers. There are a

number of parasitic insects as well

as some small mammals that kill

gypsy moth larvae and pupae. The

collective action of all of these

mortality agents—disease, parasites,

and predators—is what controls the

gypsy moth life system. When it is

better understood how these factors

operate in natural gypsy moth

populations, it will be easier to deal

with this pest and minimize its

harmful effects.

Special thanks to Roger Zerillo for

the preparation of much of the

photographic material in this

booklet and to Kathleen S. Shields

for kindly providing electron

micographs ofBacillus thuringiensis

and gypsy moth NPV. Thanks also

to Raymond Bruen for the

preparation of bacterial and fungal

specimens.

Issued January 1979

Available from the

Superintendent of Documents

U.S. Government Printing Office

Washington, D.C. 20402

Stock No. 001-000-03-889-3

^ U.S. GOVERNMENT PRINTING OFFICE : 1978 O-273-082