THE GYPSY MOTH AND ITS NATURAL ENEMIES AGRICULTURE INFORMATION BULLETIN NO. 381 U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE
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-//' ■*iS3l^ THE AUTHOR
ROBERT W. CAMPBELL is principal ecologist at the North- eastern Forest Experiment Station's research unit maintained at Syracuse, N. Y., in cooperation with the State University of New York College of Environmental Science and Forestry at Syracuse University. He received his bachelor's degree in forestry from the State University of New York College of Forestry in 1953 and his master's and Ph.D. degrees in forestry from the University of Michigan in 1959 and 1961. He joined the USDA Forest Service's Northeastern Forest Experiment Station in 1961.
ACKNOWLEDGMENTS
My thanks to both Wayne Trimm and Robert W. Brown, whose beautiful illustrations reflect careful study of their sub- jects. I also thank the many gypsy moth watchers who have shared their observations and experiences with me.
Issued February 1975
11 THE GYPSY MOTH AND ITS NATURAL ENEMIES
by Robert W. Campbell
CONTENTS
BEHAVIOR 2 Hatch and dispersal 2 Young larvae 2 Older larvae 4 Pre-pupae and pupae 4 Adults 6 Eggs 6 MORTALITY 8 Young larvae 8 Older larvae 11 Pre-pupae 18 Pupae 18 Adults 21 Eggs 21 AGENTS THAT KILL THE SEXES DIFFERENTIALLY 22 CHANGES IN GYPSY MOTH POPULATION DENSITY 23 A FEW LAST WORDS 27
111 CAMPBELL, ROBERT W. 1974. The Gypsy Moth and its Natural Enemies. Agr. Inf. Bull. No. 381,27 p., illus. Patterns of gypsy moth behavior are described, especially those related to population density. Natural mortality-causing factors that operate against this insect are also described. Several agents kill subadult male and female gypsy moths at different rates. Major determinants of year-to-year changes in gypsy moth numbers are described.
Library of Congress Catalog Card Number 74-600201
IV jYÍOST OF US have grown more or less than just sitting around and watching our immune to the sort of long-range, remote trees be gobbled up. threats that we read or hear about so fre- For example, millions of forested acres in quently in this age of instant communication. the northeastern United States were treated But those who have seen a gypsy moth out- with DDT in the 1950's, even though we had break can testify that the threat this insect far too little factual information about either poses seems immediate and sometimes almost the natural life system of the insect itself or terrifying. the possible interactions between the insect's life system and this one-dimensional method Millions of dollars have been spent in ef- of treatment. forts to control this pest or to eliminate it My introduction to this insect was based on from this continent; yet it continues to spread the notion that somebody should do nothing and to defoliate our woodland, park, and or- at all except sit on a stump and watch the namental trees. insect. The late Dr. Donald L. Collins, the Gypsy moths, Porthetria dispar (L.), were state entomologist of New York, offered me a imported to the United States from Europe summer job as "gypsy moth watcher" in 1957. in 1868 by a scientist who thought he could I took that job, and found my niche, I guess, use them to make silk. They escaped, bred, because IVe been a gypsy moth watcher ever thrived, and built up populations that soon since. began to damage trees in Massachusetts. Insect-pest management has been identified so closely with chemical pesticides in recent The gypsy moth is a defoliator. It devours years that we tend to ignore the fact that the foliage of hardwood trees— especially these pesticides are only a part of pest-popu- oaks— which can kill the trees. From the first Iktion management. We should know our infestations in Massachusetts, the gypsy moth enemy beforehand in enough detail so that has spread until it is now the major insect we can use pesticides only in bona fide emer- threat to the hardwood forests of the north- gencies; and other methods—including doing eastern United States. nothing—^would become much more promi- Some of our failures to control this pest nent in the management scheme. have had their roots in the long-standing The following notes about the gypsy moth, notion that we must at all times do something. its behavior, and its ecology have been col- This notion, in turn, seems to have been based lected during 17 years of observing this in- on the premise that doing anything is better sect pest. BEHAVIOR
Hatch and Dispersal on a silken thread, which is easily broken by Gypsy moth larvae usually begin to hatch wind movement, so they may become wind- around the first of May. Hatch time is deter- borne. The insects are buoyant at this stage, mined primarily by temperature, so you will and the wind may carry them for long dis- probably find the earliest hatch from egg tances. masses on the exposed bark of dark-colored This windborne dispersal period, which may trees, like red oak. Egg masses that were de- continue for about 2 weeks, often results in posited in sheltered places—under rocks for rather massive re-distribution of the popula- example—are likely to hatch a few days later. tion. Re-distribution serves as a potent sur- The tiny caterpillars are much lighter in vival mechanism for the species and frequently color when they hatch than they will be a few tends to prolong gypsy moth outbreaks. Ul- hours later (fig. 1). Because they nearly al- timately, of course, the caterpillars must ways spend a few hours on the egg mass, you either establish themselves on edible foliage or may see caterpillars of various hues on a single perish. egg mass. Early-stage caterpillars orient toward light; Young Larvae so when they move away from the egg mass, Long-range dispersal of larvae probably they nearly always move upward. Only on a ceases shortly after the insects establish them- heavily overcast day may you find a few cater- selves on suitable foliage. The larvae grow pillars moving downward from the egg mass. rapidly. The male molts its skin four times, Most of the larvae ascend to the top of the the female five times. The caterpillar stage tree, which by this time has produced foliage between molts is called an instar; so male about the size of rabbit ears. Many of these gypsy moths usually have five caterpillar in- larvae immediately spin down from the foliage stars, and females have six. Extra-instar cater- pillars are sometimes present in the popula- tion. Young caterpillars usually remain on or near the foliage until they have molted twice. Once, though, I worked in a place containing an enormously dense population, where mil- lions of the second- and third-instar insects had spun silken threads from the canopy to the forest floor (fig. 2). At this time, a walk through these woods was like walking through an endless spider web—a horrible experience. Fortunately, this situation is rare.
Figure 1.—Hatching gypsy moth larvae. Light in color when first hatched, they turn dark within a few hours. These insects will soon begin crawling up the tree. Figure 2.—This young larva has spun a sill After about 2 weeks as pupae, the winged in the fall of 1958, 296 in 1959, only 80 in adult moths emerge. 1960, and 588 in 1961. This variability in egg-mass size is deter- Adults mined primarily by the size of the adult fe- Adult female gypsy moths cannot fly, and males that deposited the eggs. Their size, in they usually remain close by the pupal shell turn, is related closely to the density of the from which they emerged for all of their brief larval population from which they were pro- adult lifetime. Virgin females emit a sex at- duced. Thus, if you find a gypsy moth egg- tractant chemical that serves as a potent sig- mass population that is composed of tiny egg nal to the adult males, which are strong fliers. masses, you can be sure that these masses Mating usually takes place as soon as the male were produced by the tiny adults that sur- finds the female, after which the female insect vived the extreme competition characteristic deposits her eggs. She lays all her eggs in a of an extremely dense larval population during single mass, unless she is disturbed, and covers the preceding summer (fig. 5). Conversely, them with hairs from her abdomen as she pro- large egg masses indicate that competition ceeds. (usually, I think, for food) was not severe. Embryonation begins soon after the eggs are deposited and is completed within about Eggs 6 weeks. Usually these eggs do not hatch until The average number of eggs per egg mass spring, but hatching larvae have been noticed varies greatly, both from one place to another in the fall. These latter insects have never sur- and from one year to the next. Egg masses in vived in the temperate Northeast, but the one population I studied in northeastern New possibility that they could survive in a more York State contained an average of 474 eggs southerly climate cannot be discounted. 6 Figure 5.—The trees have been stripped of foliage, but the gypsy moth population remains high. Each of the lighter colored fe- male moths on this tree is deposit- ing an egg mass. The buff-colored egg masses may range from the size of a dime to the size of a quarter, depending on the density of the population. MORTALITY Young Larvae moths. Occasionally you can find enormous Caterpillars may die even before they reach numbers of dead caterpillars floating on the the surface of their egg mass. Dead caterpil- surface of pools. For example, about 75 per- lars may occasionally be found imbedded in cent of the ground surface of one site I was the mass of egg shells, hairs, and non-viable studying was under water in the spring of eggs; apparently they were unable to get out 1958. (This site, characterized by swamp of their egg mass. white oak and red maple, contained a 6-inch When enormous numbers of tiny caterpil- layer of organic muck lying directly over an lars are present, a few become so entangled in impermeable hardpan.) I counted an average their own webbing that they are unable to of 96 dead caterpillars per square yard of open escape. This phenomenon has been observed water in this site on May 14. most frequently on the underside of large, Late frosts can be lethal to the gypsy moth, heavily infested branches of favored-food but temperature regimes cold enough to kill trees. the insects will probably raise havoc with the Spiders take a small toll of the young cater- trees as well. Even when late snow covers the pillars, but not so much because they actively ground, and caterpillars are immobilized by prey on them; rather, the caterpillars simply it, the insects are likely to revive as soon as become entangled in the spider webbing and the temperature rises. die. Several species of birds prey on young Most ants show no interest in gypsy moth caterpillars. Both nuthatches and downy wood- caterpillars, but you may occasionally see ants peckers pick the insects off the tree bole, while prey on newly hatched ones. scarlet tanagers tend to pick them from the Pools of water trap newly hatched gypsy leaves and twigs in the tree crown (fig. 6). Figure 6.—Scarlet tanagers probably don't eat enough of the small larvae to alter the course of a gypsy moth outbreak. They do use this insect for food, however. 8 In this country only one parasite, a wasp It is easy to collect Apanteles cocoons and named Apúnteles melanoscelus, is known to rear them. We have used small lip vials for parasitize the young caterpillars. Although rearing containers, placing one cocoon in each you are unlikely to actually observe it, the container. A bit of nylon stocking, held in adult female wasp attacks the gypsy moth place by a rubber band, will seal the container. larva, and after inserting her ovipositer (a Either the gypsy moth parasite, or its para- hollow, needle-like apparatus) in it, forces an sites, are likely to emerge within a few days. egg into it (fig. 7). Usually only a few of the young gypsy moth Laboratory studies have shown that this caterpillars succumb to disease, but occasional stinging activity may itself be fatal to small reports suggest that pathogens have played caterpillars, but even those hosts that survive an important role in destroying first-, second-, the stinging are doomed because the growing and third-instar caterpillars. I have seen few parasite maggot destroys vital organs of the diseased gypsy moths during these stages; so caterpillar. This parasite spins a silken cocoon my notes on disease among gypsy moths will when it emerges, and, incidentally, tends to be deferred to a discussion of mortality among bind its erstwhile host to this cocoon; so you older caterpillars. may see both the parasite cocoon and the In summary, our evidence indicates that dying host together (fig. 8). These are com- most of the gypsy moths that establish them- monly found on foliage and in bark crevices. selves on suitable foliage during the dispersal Apúnteles melunoscelus cocoons are often period will survive until they are at least half- themselves heavily parasitized, especially the grown (fourth-instar) caterpillars. One ex- overwintering brood. Curiously, the females ception to this general rule, which occurred of one of its parasites, Gelis upuntelis, are al- in northeastern Connecticut, may have been ways wingless. You may mistake one of these induced by predaceous flocking birds. females for an ant, at first glance. Figure 7.—This braconid wasp, Apanteles melanoscelus, is laying an egg in a small gypsy motli caterpillar. Tine wasp liere is shown larger than real life size in relation to the caterpillar. Figure 8.—The Apanteles egg hatches and the wasp larva grows to maturity inside the caterpillar, chews its way out, then almost at once begins to spin its cocoon. The co- coon is firmly anchored to a piece of bark, but has also bound the now-dead cater- pillar to its cocoon. 10 Older Larvae We have seldom found a Calosoma beetle Predaceous ground beetles kill gypsy moth in an area when the resident gypsy moth pop- larvae (fig. 9). Most important is a large iri- ulation was sparse, even though both gypsy descent green importation from Europe called moths and Calosoma beetles had been numer- Calosoma sycophanta (fig. 10). The adult ous in that area in the preceding year. Al- beetles begin to appear while the gypsy moth though beetle densities increase from year to population is still in its third instar, and most year if the density level of their prey also of these adult beetles will have disappeared increases, the gypsy moth populations I by the time the gypsy moth population has studied have usually crashed before the beetle pupated (although their larvae will have be- population had time to increase to a level at come predaceous on the gypsy moth pupae which the beetles could consume a substantial by that time). part of the gypsy moth population; and the These Calosoma beetles may be difficult to beetle population subsequently disappeared. find unless many are present, because they One exception was observed in 1967 in Old seem to be more active at night than during Lyme, Connecticut. Gypsy moth numbers had the daylight. Each adult beetle, which had been high for several years running, and the spent the winter months in an underground beetles finally caught up numerically and chamber, apparently tends to establish a caused heavy mortality among their prey. The rather restricted home range. About two- gypsy moth, in this case, behaved rather thirds of the beetles tagged in some sites in strangely, possibly for reasons suggested be- northeastern New York in 1960 were later low. recovered on the trees where they had been Calosoma beetles have a rather unpleasant originally. (Each beetle found during that odor, so much so that an experienced observer year was marked by a series of dots inscribed can detect one by odor alone. Pick up the next into its wing covers with a portable drill.) one you find in one hand, and let it go again. Figure 9.—One of the native ground beetles, Calosoma frigidum, attacl 11 Figure 10.—This ground beetle, Calosoma syco- Now place this hand in the path of a wander- phanta, was imported into tliis country from ing gypsy moth caterpillar, and notice that he Europe. These beetles often hang head down from the tree bole as they prey on wandering will crawl around it. Yet this same caterpillar larvae they have captured. will crawl across your other hand. In the Old Lyme situation, the fully grown gypsy moth caterpillars, which had remained on the foliage during their later instars, tended to pupate in the tree crowns rather than des- cend to the boles. I suspect that the gypsy moths did this to avoid the odor of Calosoma beetles, which had thoroughly saturated the lower tree boles. Although Calosoma adults are agile in climbing tree boles, they seem to have great difficulty making their way along small twigs and foliage. Consequently, we have seen them much more frequently on the bole than on the small branches. Skunks often find sites where Calosoma beetles are active, and may then turn to an al- most total diet of Calosoma beetles. Since the skunk cannot digest the exoskeleton of the beetle, you may find what appear at first glance to be iridescent green skunk droppings (scats) in these areas. A closer look will con- vince you that skunks, indeed, eat large num- bers of Calosoma beetles when they can get them (fig. 11). 'i«*4^«» Figure 11.—The sl 12 Mice, too, eat Calosoma beetles. Typically, the prey population is sparse. My observations they discard the exoskeletons of their prey in on the food-getting behavior of such birds in rather neat piles. These remains are usually woodland situations are sketchy, for two rea- found on level spots near tree bases. sons. First, flocks of these birds are difficult to You may find both the nymphs and the approach closely under woodland conditions. adults of a stink bug (Pentatomid) with their Second, such flocks may range across several elongated beak inserted in gypsy moth cater- square miles of woodland, presenting the ap- pillars or pupae. Although I have usually inter- pearance of constant movement from place preted this activity as scavenging, rarely pré- to place. dation, on the caterpillars, these bugs clearly Our evidence from a generally sparse gypsy do prey on both gypsy moth caterpillars and moth population in northeastern Connecticut pupae from time to time, although this parti- and adjacent Massachusetts indicated that cular predator/prey relationship is probably these birds tended to find relatively high- of little biological significance. density gypsy moth caterpillar populations Other potential invertebrate predators of and that they then concentrated on these the older gypsy moth caterpillars, such as ants high-density spots as food sources. and spiders (fig. 12), have been watched (The expression "relatively high density" closely; but in 17 years of observation I have cannot be closely defined at this time, except seldom witnessed prédation on the older cater- by example. In this case, gypsy moth egg-mass pillar stages by these organisms. density had probably averaged only about 3 Birds, especially grackles—and perhaps also or 4 egg masses per acre. Thus even sites that starlings and red-winged blackbirds—can be contained as few as 20 egg masses per acre important predators on fourth-, fifth-, and were of relatively high density. ) sixth-instar caterpillars, but usually only when In 1971, as outbreak conditions developed to the south of this study area, I noted that nocks of grackles were easy to find in the northern edge of the outbreak, but almost never a few miles farther north under condi- tions of lower density. Unfortunately, these birds seldom have much effect on the gypsy moth under outbreak conditions, because un- der these conditions there are too many gypsy moth caterpillars per acre for the birds to remove a significant proportion. One other bird should be mentioned in rela- tion to the gypsy moth: the cuckoo, or wood crow (fig. 13). Both black-billed and yellow- billed cuckoos are noted as predators of hairy caterpillars, and they are always present in small numbers around outbreaks of the gypsy moth. I always associate the unique call of these birds with gypsy moth outbreaks, prob- ably because I have rarely heard them else- where. These are relatively shy birds, but a patient observer with field glasses can assure himself that they are eating gypsy moth cater- pillars. Northeastern hardwood forests contain a Figure 12.—This Theridion spider has attacked the un- more numerous and more varied community fortunate gypsy moth larva and wrapped It in silk. The of small mammals than most people, including photo was taken in Branford, Connecticut by R. D. Neely. many foresters, realize. The white-footed 13 Figure 13.—Both the black-billed cuckoo, shown here, and the yellow-billed cuckoo are attracted to outbreak areas. They seem to specialize in eating hairy caterpillars such as the gypsy moth. mouse, Peromyscus leucopus, is both a promi- are most likely to find the caterpillar remains nent member of this community and an im- around tree bases, but if you happen to have portant predator of the gypsy moth (at least a tree house in that old apple tree in your while the prey population is sparse). Usually backyard, look for them there. We have found these mice prey much more on pupae than on these remains at least 35 feet above the older caterpillars, for reasons that will be de- ground, in tree crotches, which goes to prove scribed. On the other hand, I have observed a that the mouse is an excellent climber. few cases where mouse prédation on the cater- These mice are much more active at night pillars was extremely high. than they are during daylight hours, and at When a mouse finds a gypsy moth cater- night the older caterpillars are on the foliage pillar, he sometimes grasps the insect in his and inaccessible to the mice. Thus most of forepaws and pulls off its head capsule with the prédation by mice is restricted to dawn his teeth. He discards the head capsule, to- and dusk. gether with the bright green upper digestive Other small mammals, such as shrews, voles, tract, then rolls the caterpillar skin back on and chipmunks, have not been significant itself—just as you might peel a banana (fig. predators of any gypsy moth stage. In cage 14). experiments, both voles and chipmunks have Only a small portion of the prey insect is usually refused to eat the insect. Although the consumed. Under woodland conditions you short-tailed shrew, Blarina breuicauda, ate 14 Figure 14.— White-footed mice, such as the one pictured, can be effective predators of the gypsy moth as long as the insect popu- lation remains sparse. They are more likely to eat the pupae, but they will also attack larvae. This mouse has already killed several caterpillars and is about to pull the head off another. He will then probably roll the skin of the caterpillar back on itself and will eat a small portion of the insect, discarding the remainder. 15 large numbers of both caterpillars and pupae know nearly enough about these parasites, in cages, the available evidence indicates that either individually or collectively, to forecast the shrew seldom comes in contact with this their effectiveness in reducing gypsy moth insect in the natural state. populations. One other mammal, the gray squirrel, oc- One common parasite of the larger cater- casionally preys on the gypsy moth. On two pillars is a tachinid fly, Compsillura concin- occasions, I have seen a gray squirrel remov- nata. You are most likely to notice this fly ing bark flakes from infested white oak trees after it has emerged from its host and has it- and eating a few gypsy moth caterpillars. self pupated—although the chestnut brown More frequently, I have noticed white oaks puparium you see is actually the expanded with a litter of bark flakes around their bases, integument of the mature fly maggot (the and have suspected that this litter represented living fly pupa is inside this outer shell) (fig. a byproduct of the hunting behavior of gray 15 and fig. 16). Most of the fly maggots drop to squirrels. the ground after emerging from their gypsy Several insect species parasitize older gypsy moth host and pupate in the litter and duff, moth caterpillars. The broad range of habitats but you may also find puparia under bark flaps used by the host insect and the uniqueness of and even some that appear to be clutched by each parasite species suggest that we can ex- the legs of dead host insects. pect parasite effectiveness to be highly vari- C. concinnata pupae that have emerged able, both from one place to another and from from gypsy moth caterpillars transform one year to the next. Unfortunately, we do not rapidly into adult flies. They must find and ''" "Nu, Figure 15.—This taciiinid fly, Compsillura concinnata, cuts a slit in the skin of the caterpillar and lays an egg inside. The fly at- tacks so rapidly that the caterpillar probably doesn't know it has been parasitized. The fly has been drawn slightly larger than life size. 16 Figure 16.—A nearly mature fly completely defoliated in 1959, while the bor- larva as it would appear if you could see Inside the gypsy moth dering area, which supported a mixed growth caterpillar. of red maple and American hornbeam, had lost only about 5 percent of its foliage. ^i«/*"^ The dense 1959 gypsy moth population in this site had been preceded by a relatively high-level moth population in 1958. The tachinids that lived upon this preceding gypsy moth generation may have exhausted their supply of alternate hosts within the site. In addition, severe defoliation by the gypsy moth caterpillars inevitably reduced the food avail- able to subsequent potential alternate hosts, which would cause a reduction in their den- sities. Under these conditions, it seems likely that the effectiveness of parasites like C. con- cinnata may be determined primarily by a limited supply of alternate hosts. Sweeping disease epizootics among the larger larvae sometimes cause the abrupt termination of outbreaks. Although these epizootics are usually associated with high- density gypsy moth populations, coupled with at least 50-percent defoliation of the overstory trees, epizootics also develop occasionally among moderately dense gypsy moth popu- lations and under conditions of less than 10-percent defoliation. Larvae that are about to succumb to dis- ease typically descend to the lower portion parasitize alternate hosts during that same of the tree bole and die there. Their remains summer to maintain their species. can often be found dangling from the bole, This parasite is probably most susceptible since the dead insects do not usually drop to attack by its own natural enemies while to the ground. Experienced observers can it is within its puparium. Some of these pupae detect a disease epizootic in this insect from are parasitized, some are eaten by small afar, because the stench produced is both un- mammals or by the larvae of Calosoma beetles, pleasant and distinctive. and some appear to succumb to pathogens, Although a polyhedral virus probably is particularly fungi. In total, however, this the primary cause of mortality in the disease mortality seemed much too low to account complex, many apparently diseased insects for the reduced rate of C. concinnata effec- examined have revealed no infectious agent tiveness in an area we were studying between or overt cause of disease. This indicates that 1958 and 1960, but we find a clue to this noninfectious diseases have a major role in reduction from another source. natural populations. Other pathogens include Parasitism of the gypsy moth by C. con- an array of aerobic bacteria and some fungi. cinnata was less than 2 percent in 1959 toward Epizootics, which usually develop in late the center of one site we were studying in June and early July, are often related to northeastern New York State, but it averaged heavy rainfall in late May and early June, about 20 percent along the border of this but several other factors are clearly involved. same area. The site proper, which supported For example, a disease epizootic is likely to a stand composed primarily of oaks, was be more severe among a gypsy moth popu- 17 lation in an aspen stand than it is among Mortality from desiccation did not become the insects in a typical oak-hickory forest. significant until 2 or 3 days after the larval Conversely, epizootics may be less severe, population had nearly stripped the trees of other things being equal, among the insects their foUage. The amount of material remain- living in the oak-pitch pine forests that are ing in the gut, however, indicated that these typical of Cape Cod than among the insects insects did not die of starvation per se. in an oak-hickory type. In any case, much Possibly a drastic modification in the water of the natural history of disease in this species balance of the doomed insects occurred, remains to be unraveled by further research. resulting directly or indirectly in death. Unfortunately, we cannot presuppose that I have seen no other significant causes of high incidence of disease among the large death among pre-pupae, although nearly all larvae will be an invariable consequence of the causes of mortality among pupae have heavy defoliation. All too often, the supply also been found among pre-pupae from time of foHage lasts just long enough for the cater- to time. pillars to complete their larval development and pupate. Pupae Conversely, I remember a few situations If you see adult Calosoma beetles during where the gyspy moth exhausted its food the larval stage of the gypsy moth, you will supply before it was able to pupate; and certainly be able to find the larvae of these although a typical disease epizootic did not beetles as they prey on gypsy moth pupae. ensue, mass starvation did. These beetle larvae, which hatched from I witnessed several examples of what eggs that were deposited on the ground, appeared to be mass starvation in 1972 ascend tree boles and prey on the gypsy moth among some of the populations we studied on pupae they encounter (fig. 17). They will also Cape Cod and in southeastern New Jersey. nip your hand if you hold them in it, but they Most of the larvae moved down from their cannot exert enough force to break the skin. host trees soon after the foUage was ex- When they are mature, the Calosoma larvae hausted, and then continued to crawl about return to the forest floor and construct pupal on the forest floor. For a few days, the ground chambers in the soil. They emerge from these under these defoliated trees resembled a chambers as young adults the following spring. moving carpet—a (carpet composed of starving Adult Calosoma beetles can also return to the caterpillars. Although most of these insects soil and overwinter. Thus the adult beetles eventually died, some were able to find enough you see in the early summer may be 2 or pitch pine foliage to complete their larval even 3 years old. development, and to transform into adult In the Old Lyme situation described earlier, moths. the gypsy moth larvae tended to pupate in Pre-Pupae the foliage rather than descend to the tree Dead pre-pupae are difficult to find within a boles to pupate; and this unusual behavior sparse population, except for occasional indi- may have been an avoidance response to the viduals that were killed by tachinid flies. odor of Calosoma beetles on the tree boles. However, I have often found a large number In any case, many of the beetle larvae either of dead pre-pupae in dense populations within starved or cannibalized one another, since sites where the food supply had been ex- they could not reach a food supply that was hausted. Most of these individuals appeared only a few feet away. to have dried up; they were shriveled and Birds have shown little or no interest in stiff. Such individuals nearly always had a gypsy moth pupae as a food source, but solid mass of dried food material within their white-footed mice frequently found and gut. For lack of a better term, all the above preyed on more than half the pupae in the symptoms have been grouped together as sparse stable gypsy moth populations we desiccation. studied in northeastern Connecticut. In this 18 Figure 17.—The larvae of Calo- soma sycophanta hatch at about / the same time the gypsy moth is beginning to pupate. This Calo- soma larva (right) has attacked the pupa and is now chewing into it. The pupa will be merely a hollow shell before the preda- ceous beetle larva moves on. area, the few gypsy moths that pupated in the host pupa as a mature maggot. The adult the litter on the forest floor were almost cer- females of this fly deposit large numbers of tain to be preyed upon, and even those insects their tiny eggs on the foliage in infested that pupated in relatively sheltered places, areas. These eggs, which may or may not be like the undersides of bark flaps, were often ingested by the caterpillars as they eat, then found and consumed by mice. Neither birds have to hatch and enter the body cavity of nor mammals have ever played a significant the host through the gut wall before they are role as predators of pupae in any of the dense excreted. This particular fly is sometimes gypsy moth populations that I have studied. important in reducing dense populations to Several species of insects sting and kill lower levels, but it was rare in most of the gypsy moth pupae, and one species—a large dense populations I studied in Glenville, New fly named Blepharipa scutellata—enters the York. gypsy moth larva as an egg, but emerges from Four species of wasps, all of which belong 19 Figure 18.—A double attack. The ichneumonid wasp, Itoplectis conquisitor, is attaci 20 to the family Ichneumonidae, have been seen plays a major role in destroying gypsy moth attacking gypsy moth pupae in this country. pupae, but that is about all we know about They stung and killed up to one-half of the this relationship. pupae in some of the dense populations we have studied, but seldom successfully para- sitized them (if we measure success by the Adults development of an ichneumonid offspring Although adult gypsy moths are preyed within the host). In fact, the ratio between upon by a variety of vertebrate and inverte- total hosts stung and those producing ichneu- brate animals, this mortality usually has no monid offspring ranged from 4 to 1 for the biological significance. Losses among the species Theronia atalantae, to more than adult female insects, between the time they 1,000 to 1 for another species, Itoplectis emerged as adults and the time they began conquisitor. In some cases, it seemed likely to lay their eggs, has averaged less than 5 that this stinging activity by the wasps was percent in most of the populations we have part of their food-getting behavior, since they studied. These losses have increased occa- often drank some of the body fluid as it began sionally to about 20 percent in a few situ- to ooze from the punctured gypsy moth host. ations where adult gypsy moth density was Since the sting by an ichneumonid is not very low. usually discernible after a day or so, and because the scavenging activities of a group of flies called Sarcophagids almost always Eggs followed those of ichneumonids, the number Few gypsy moth eggs have been preyed of pupae containing maturing sarcophagid upon among the dense populations we have larvae has been used to indicate the true studied, but a large proportion of the egg effects of ichneumonids on the host population masses have sometimes disappeared among (fig. 18). some of the sparse populations. Ants are Interactions between ichneumonids, sarco- one cause of this. In places where ants are phagids, and the gypsy moth illustrate both active, they remove all the eggs from the the sort of biological relationships about which mass, one at a time (fig. 19). We do not know we know far too little, and the need for more than simple collecting and rearing to deter- mine the real effectiveness of many of the apparently straightforward mortality factors that operate in nature. For example, this entire group of wasps had been dismissed as unimportant natural enemies of the gypsy moth because they were seldom recovered from field collections. Another wasp that stings gypsy moth pupae is Brachymeria intermedia. I seldom found this wasp between 1958 and 1971, but we found it in large numbers in a whole series of widely separated locations in 1972. I have no explanation for this apparently sudden change in the density of this species, but the insect obviously was very much involved in the gypsy moth life system in 1972. This species, like the ichneumonids, stung many more hosts than later rearing work showed Figure 19.—This ant and his brethren will in some of the populations we studied. probably return to this egg mass again and again until they have removed every We know that disease in general often egg in the mass. 21 what becomes of these eggs, but it seems eggs only near the surface of the mass with reasonable to assume that the ants eventually its ovipositor. Thus the percentage of the eat them. total egg mass that it can reach is deter- Neither birds nor small mammals have mined by the size and shape of the mass itself. shown much interest in gypsy moth eggs as a Kill seldom exceeds 30 percent, but a much food source, possibly because the hairs from higher percentage kill of eggs has been the adult females' abdomen, which cover the reported from populations that were composed egg mass, are distasteful or even toxic to of the tiny egg masses that often follow vertebrates. outbreaks. Two species of egg parasites have been Gypsy moth eggs are killed by temperatures successfully introduced into the United States below about —20°F., and this probably limits against the gypsy moth. Ooencyrtus kuwanai, the further spread of this insect to the north. which was imported from Japan, is the more Winter-kill can also decimate a potential out- abundant of the two. This insect, which can break, at least among the egg masses that be found stinging eggs from the time they are not insulated from temperature extremes are deposited until late in the fall, can reach by snow. AGENTS THAT KILL THE SEXES DIFFERENTIALLY Some mortality factors are known to kill than 25 percent females among pupae after subadult male and female gypsy moths at an epizootic. different rates. These factors can cause phen- Second, white-footed mice are more likely omenal departures in the adult sex ratio from to prey on female pupae than males. Only the expected 50 percent females and 50 per- about 7 percent of the females that pupated cent males. in the litter in the areas we studied in north- eastern Connecticut survived, whereas 20 We have found as high as 83 percent percent of the males that pupated in this females and as low as 2 percent females among stratum survived and became adults. I also the adult insects. On the other hand, the sex suspect that mice are more likely to prey on ratio among embryos and newly hatched female larvae than on male larvae, although larvae has been constant at 50:50, and the sex this remains to be proved. ratio at the start of the fourth larval instar, Third, one of the insects discussed earlier, although it varied somewhat, was consistently Blepharipa scutellata, is more likely to kill close to 65 percent females and 35 percent female pupae than males, probably because males among the populations we have studied. the large female larvae consume more foliage Through what mechanisms was this tremen- than their male counterparts, and thus are dously variable adult sex ratio achieved? more likely to ingest the eggs of this species. First, disease among the older larvae was Finally, ichneumonids—apparently because strongly selective against the female insects— they prefer to sting host pupae of a certain mainly because disease incidence reached its size—usually killed more male pupae than peak among the larvae just as they were females, except when host size was reduced pupating, and male larvae tend to pupate be- by excessive larval density and competition. fore females. This differential mortahty in The net result of all this differential mortality Glenville, New York, caused a change in the was a series of adult sex ratios varying sex ratio among pupae from about 65 percent from more than 80 percent females to only females where no disease occurred to less 2 percent. 22 CHANGES IN GYPSY MOTH POPULATION DENSITY Some areas, such as the one we studied in this dispersal as an equally sparse population Glenville, New York, between 1958 and 1964, would receive in an innocuous situation. may support a system of extremely unstable This phenomenon is especially important gypsy moth populations, whose densities along the edge of an outbreak area, because ranged from 2 or 3 egg masses per acre to the virtual flood of windborne newly hatched more than 20,000. Other areas, such as the larvae, which may inundate an erstwhile in- Eastford area in northeastern Connecticut, nocuous situation with gypsy moths, tends which we studied between 1965 and 1971, may to create an even more massive outbreak in support a sparse and stable population system following years. Re-distribution also tends to whose densities seldom exceeded 20 egg maintain outbreaks within the outbreak area, masses per acre. because populations that have just receded Relationships between egg-mass numbers to sparse levels tend to be bolstered tremen- from year to year were strikingly different dously by the windborne larvae from nearby between the innocuous Eastford populations more densely populated sites. and the outbreak situation in Glenville. For Pressure on the gypsy moth from vertebrate example, the hypothetical density at which predators may be severe within sparse popu- a population should stabilize in the innocuous lations of the prey species and may increase situation in the absence of disturbance was as prey density increases, but only across a somewhere between 5 and 10 egg masses per limited range in prey densities (fig. 20). Per- acre. Steady-state density in the outbreak centage kill by vertebrates diminishes rapidly situation, on the other hand, was between to almost nothing as prey numbers continue to 2,500 and 4,000 egg masses per acre. increase beyond this limited range. Similarly, starting with 100 egg masses In generally innocuous situations, on the per acre, one would have expected only about one hand, these predators tend to concentrate 5 egg masses per acre the next year under on the few relatively high-density populations innocuous conditions, but might expect about present, and cut these down to lower densities. 1,000 egg masses per acre in the following Conversely, these predators are usually in- year under outbreak conditions. Thus the effective within the sparse populations in out- populations we have studied under innocuous break areas, since they tend to concentrate conditions have exhibited a high degree of their food-getting efforts in sites that are numerical stability at low population levels, supporting larger numbers of the prey species. but those studied under outbreak conditions Both the availability and the character of have shown no tendency toward low-level certain locations that the insect uses for stability. Rather, they tended to increase resting sites may be of critical importance dramatically to dangerously high densities. within generally innocuous areas. For ex- Why did these striking differences exist? ample, the probability that the gypsy moth First, the small caterpillars begin to dis- would survive to become an adult in Eastford perse as soon as they reach the expanding was several-fold higher in places where the foliage of their host trees. This re-distribu- insect rested and pupated in bark flaps than tion tends to have much more dramatic con- in places where the insect rested in the litter. sequences in outbreak situations than in Concerning parasitoids in innocuous situ- innocuous ones, because a sparse population ations, the available evidence suggests that within an outbreak situation is likely to they were capable of reducing the rate at receive several times as many insects through which gypsy moth densities would increase. 23 Figure 20.—Typically, the sparse, stable gypsy moth populations have been maintained at innocuous levels largely through the activities of vertebrate predators such as the white-footed mouse. Once populations increase numerically beyond this sparse density range, they may continue to increase to outbreak levels despite the fact that they will probably be subjected to higher parasitism rates. Commonly, outbreak populations are decimated by disease, as represented by the "wilted" larva on the right. Starvation, of course, may cause massive mortality when all else fails. sparse intermediate dense DENSITY but that they did not usually prevent or locations for the insect can upset this rela- reverse increasing trends completely. Their tively stable situation. activities did serve to maintain several of the Gypsy moth populations in outbreak areas gypsy moth populations we studied within a tend to collapse (numerically), but only after low enough range of densities that the verte- their numbers have increased into a range brates that eventually concentrated their where the population becomes increasingly activities on these populations were able to susceptible to an array of mortality-causing reduce them to lower levels. factors whose effects are themselves closely Collectively, vertebrate predators and para- related to density. Disease among the older sitoid insects can maintain an innocuous caterpillars is a good example of this. When system of gypsy moth populations within an all else fails, the insect population may ex- innocuous density range indefinitely, but haust its food supply and simply starve (fig. either the intrusion of an outbreak from out- 21 and fig. 22). side the area or alterations in the sites that In outbreak areas, individual sparse popu- result in a larger number of secure resting lations tend to increase rapidly to high den- 24 sities for two reasons: First, neighboring more because they have abundant food sources dense populations supply large numbers of available nearby. Population increases are newly hatched dispersing larvae. Second, ultimately reversed, but usually only after vertebrate predators do not tend to seek out the population has increased to dangerously and consume the insects in these populations, high levels (fig. 23). Figure 21.—Across a fairly broad range in insect density, each adult female gypsy moth will probably produce at least 500 eggs. Egg production begins to diminish above this density, as a con- sequence of a reduced food supply that has started to limit fecun- dity. Egg production per adult female drops sharply, and starva- tion begins to become a major mortality factor when food is exhausted, in extreme cases, nearly ail of the insects starve, and the few insects that do become adults will probably deposit fewer than 100 eggs per adult female. □ □ciDCiDaaoczDaaaaoaaaaaGiiEZDa critical minimum food level Û 0 0 IL DENSITY 25 Figure 22.—Occasionally, gypsy moth larvae simply exhaust their available food before many of them can pupate. In such cases, the ground may resemble a mov- ing carpet as the insects wander through the defoliated stand. N/Iany of these in- sects will eventually die in windrows as they pile up against the bases of their erstwhile host trees. Figure 23.—Development and progression of an outbreai<. Year 1 (top): Newly hatched larvae from the outbreak population on the left spread out into a sparsely populated stand. Year 2: Both populations may have reached temporary medium densities by the second year. Year 3: Now the situation is reversed: the former sparse population has reached an outbreak level, and is providing a flood of newly hatched larvae to the sparse population on its left. 26 A FEW LAST WORDS Pest outbreaks, like runaway fires, tend to effective ways to prevent either pest outbreaks be spectacular. Our methods for suppressing or fires. both insect outbreaks and fires tend to The apparently spectacular success of mod- resemble a sort of man-against-nature mor- ern pesticides gave many entomological re- ality play, both dramatic and exciting. searchers a feeling that a permanent pest- control solution, rather than a short-term Of course, any professional fireman will tell emergency control, had been found. Now, you that the best way to cope with a potential however, as we seek new ways to live in rea- fire is to prevent it from occurring in the first sonable harmony with our insect neighbors, place. Similarly, pest managers would much we have found that we need to know a great prefer to contain potential pests at innocuous deal more about their ecology in order to and manageable levels rather than have to design pest-management systems that are cope with outbreaks. The problem in both broad and flexible as well as ecologically cases is that we have not yet found totally sound. We need to know the enemy better. 27 THE FOREST SERVICE of the U. S. Depart- ment of Agriculture is dedicated to the principle of multiple use management of the Nation's forest re- sources for sustained yields of wood, water, forage, wildlife, and recreation. Through forestry research, cooperation with the States and private forest owners, and management of the National Forests and National Grasslands, it strives — as directed by Congress — to provide increasingly greater service to a growing Nation. For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 60 cents Stocl< Number 001-000-03387-5 . TÎr U.S. GOVERNMENT PRINTING OFFICE : 1975 O-568-010