THE GYPSY MOTH AND ITS NATURAL ENEMIES AGRICULTURE INFORMATION BULLETIN NO. 381 U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE i^Q^^áh nú'3^1 '/■*X. -//' ■*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<en thread from the tree crown. This thread will probably break as he drops, and the wind may carry him to another loca- tion . possibly as much as 20 miles away. The many long hairs on his body increase his air-borne buoyancy. Older Larvae in the woods. They can be easily induced to use artifiicial resting locations, such as burlap Beginning about half-way through the third bands placed on the trees; and this method instar, you may find increasing numbers of has been used with some success to control caterpillars on the boles of trees. The insects the insect. are then undergoing a profound behavioral At dusk these caterpillars leave their rest- change. They are beginning to use dry, dark ing places and climb back up into the tree places for resting during daylight hours, and crown. You may need a flashlight to see this, they will continue to do this for the balance too; but you can almost set your watch by of their caterpillar lifetime. the time this re-ascent of the trees begins. Since the population on any given date will Caterpillar behavior changes as population probably contain caterpillars in three or four density increases. At intermediate densities different instars, this behavioral change may you are likely to find the caterpillars resting seem to be in a state of ñux for about 3 weeks. under bark flakes on the living boles of species The behavior of the older caterpillars is re- like white oak, which they would not use un- lated closely to population density. der lower density regimes. By the time most of the caterpillars in At extremely high densities, the caterpillars sparse populations are midway through their apparently do not use resting places at all un- fourth instar, most of them descend from the til they are ready to pupate. Rather, they crown of their host tree at the first detectable remain on the foliage, feeding continuously sign of morning light, to find a daytime rest- both day and night. ing place. You have to get out of bed early to At the end of this last behavioral phase, see this, and you'll need a flashlight. which is characteristic of outbreak conditions, Typically, the caterpillars descend to the the insects that are about to pupate usually tree base and spend the daylight hours under descend from the foliage, fill every available dead leaves on the forest floor (fig.