THE SILK MOTHS

Silk moths (family Saturniidae) are called such because of the silk they use to spin their cocoons. Around 1,200 species of silk moths are found worldwide; 70 can be found in North America (Tuskes, Tuttle & Collins, 1996). Most of the silk moths are quite large, some with a wingspan of six inches. Their size and beauty have won the admiration of many, including this author; the ease with which they can be bred and reared makes them good subjects for studies.

As adults, luna moths do not eat, and they live only a matter of days. The females release pheromones to attract males. After mating, the female lays her eggs on suitable forest host plants, including hickories, walnuts, white birch, sumacs, persimmon, and sweet gum. After 1-2 weeks, the eggs hatch and the caterpillars begin feeding. The caterpillars go through five developmental stages called "instars," and molting occurs between each instar. While higher temperatures increase the development rate of the caterpillar, an average of 50-60 days is required. The caterpillar then spins a cocoon around itself and becomes a pupa; the adult luna moth emerges a few weeks later. In Virginia, luna moths have two generations each year. The second generation spend the winter as pupae, and the adults emerge the next spring.

Since the late 1950s, wild silk moth populations have been declining in the northeastern United States (Schweitzer, 1988). For example, the imperial moth is listed as threatened in Massachusetts (Massachusetts List, 2001). Proposed hypotheses for the declines include habitat loss, disruption of mating and other effects of outdoor lighting, spraying of DDT and other pesticides, and biological control measures against the gypsy moth. Habitat loss has not proven to be a sufficient explanation; in New England, forest habitat is expanding and host plants are increasing. Even if outdoor lighting were more prevalent in New England than in the rest of the United States, the difference would probably not be large enough to account for the severity of the decline of the silk moth populations. Spraying of DDT was banned 40 years ago; the silk moths should have been able to recover by now if this were the sole cause. Other pesticides have been used very little in New England forest management over the last 20 years. Even when heavy spraying occurred, a small fraction of the total forest area was sprayed. Biological control, or the use of one organism to control the population of another, remains a likely cause.

PARASITOIDS AND C. CONCINNATA

The terms parasite and parasitoid are used somewhat interchangeably. A true parasite does not kill its host, while a parasitoid kills its host in the process of its development. I will use parasitism as a general term to refer to the act of attacking and using the host in some way, but I will use parasitoid to refer only to insects whose larvae kill their hosts.

Parasitoids are one cause of the death of silk moths. Many parasitoids, most of which are flies and wasps, attack silk moths. The life cycle of silk moth parasitoids begins with the female parasitoid placing her egg or larva on or inside the caterpillar host. As it grows and develops, the parasitoid larva lives inside the host. The parasitoid eventually kills the host and exits its body to pupate nearby. The adult parasitoid emerges a short while later. Variations of this pattern exist. For example, a parasitoid larva living inside a host late in the summer might not exit until after the host has spun its cocoon, which will then serve as protection for the parasitoid pupa during the winter. Parasitoids attack their hosts in many different ways. Most flies lay their eggs on the external cuticle of the caterpillar. In contrast, most wasps lay their eggs inside the caterpillar by piercing the cuticle with a specialized ovipositor, which then deposits the eggs inside. Certain flies and wasps lay their eggs on foliage; caterpillars eating the foliage ingest the eggs. To ensure success, these parasitoids seek out groups of actively feeding caterpillars and lay the eggs in front of them.

C. concinnata employs a less-used strategy of larvipositing. The eggs are prehatched in the female's oviduct very shortly before she deposits them inside the caterpillar. Up to 14 larvae per host have been recorded (Boettner, personal communication, 2001). The larva(e) will kill the caterpillar in about 10 days. The fly larva emerges from the caterpillar as a white maggot about 5-7 mm long, which then surrounds itself with a smooth, reddish-brown, elliptical case called a puparium. Inside the puparium, the larva molts into a pupa. C. concinnata is very flexible and can mold its life cycle to that of its host, a reason for its success on so many different hosts. For example, if a smaller caterpillar is attacked, C. concinnata will delay its development slightly and exit the host when the host is larger.

METHODS

The protocol I used was modeled after that used by Boettner, et al. (2000) in their Massachusetts study. Eggs were collected from a wild female moth, and the caterpillars were raised parasitoid-free in the lab. They were placed outside for a period of time during which they were susceptible to parasitoids and then brought back into the lab and reared to adulthood. Those caterpillars that had not been parasitized matured into adult moths. Caterpillars that contained parasitoid larva(e) would eventually die; the larva(e) would then come out of the caterpillar, pupate, and emerge as flies or wasps a short while later. All of my parasitoids are being identified by experts in fly or wasp systematics.

Actias luna eggs were collected from a wild female at a UV light in Nelson County. The female was placed in a paper bag, where she laid clusters of eggs that were cut out and put into containers. The eggs hatched in 10 days. Caterpillars in the lab were fed fresh hickory leaves every other day, and their containers were cleaned out daily to keep them as free from disease as possible. One hundred luna caterpillars were placed outside during each of the 2nd, 3rd, and 4th instars, and 76 were placed outside during the 5th. The 2nd, 3rd, and 4th instar caterpillars were deployed for the entire instar (2-8 days); when they began to molt, they were collected and brought inside. Fifth instar larvae were left out for 4-5 days and then brought back in, as they begin to wander before spinning their cocoons and pupating. All of the caterpillars were deployed during the period from June 5 to June 29, 2001.

A small area of Fern Woods on the Sweet Briar campus was used for the deployment of the caterpillars. Small understory hickory trees were used for ease in finding the caterpillars. Four caterpillars were placed on each tree and checked daily. The trees were marked by flags placed in the ground near the tree. Had markings been placed on the tree, they might have attracted parasites and skewed the data. Also, each tree was used only once. Since parasitoids use olfactory cues (including detection of caterpillar frass) to find their hosts, it is possible that reusing the trees would have artificially increased parasitism rates.

After collecting the caterpillars, I reared them in individual containers and recorded whether they had parasites emerge, died from disease, or pupated. All of those that pupated were allowed to harden for 2-3 days and then placed in an individual wire cage enclosed in a section of pantyhose (to keep in any emerged parasitoids). The pupae were sprayed with water every 2-3 days to prevent them from drying out. The emergence of the adult moth served as final evidence that the caterpillar had not been parasitized, as some parasitoids emerge from pupae. As of the time of submission of this article, not all data had been collected. Not all of the moths or parasitoids had emerged as adults.

RESULTS AND DISCUSSION

I recovered 35-46% of the deployed caterpillars (Table 1). Some of the disappearance is due to predation. A predatory stinkbug was observed attacking a caterpillar. Other likely predators include birds, small mammals, spiders, and other insects. Disappearances also resulted from caterpillars wandering off the tree and sometimes being dislodged by heavy rain; small caterpillars are easily knocked off their leaves and then are likely to die. This probably occurred with the 2nd instar caterpillars I placed outside; after a heavy rain one night, nearly half of the caterpillars were missing the next day.

Of those caterpillars recovered, parasitism rates were low except for those exposed in the 5th instar. The parasitism rate for the 5th instar is 78% of only those caterpillars that were recovered. If it is assumed that the unrecovered caterpillars were parasitized at the same rate, then this gives us an upper limit of 78%. If, however, those not recovered were not parasitized, then this means only a 21% parasitism rate. Unfortunately, this is a large range; it demonstrates how difficult it is to determine naturally occurring parasitism rates.

A total of four species of parasitoids were observed (Table 2). C. concinnata accounted for approximately 3/4 of the 5th instar group parasitism and all of the 4th instar group parasitism. A wasp accounted for the lone parasitized 2nd instar caterpillar, and two unidentified species of flies were responsible for the remaining 5th instar deaths.

Hyperparasitoids, or secondary parasitoids, were also observed. This type of parasitoid attacks the parasitoid originally present in the caterpillar. The hyperparasitoid remains inside the primary parasitoid until the latter pupates. Soon after, the hyperparasitoid kills the primary parasitoid. Hyperparasitoids were present only with C. concinnata. The C. concinnata pupae produced wasps rather than the expected flies.

When looking only at caterpillars whose C. concinnata larvae were attacked by a hyperparasitoid, half of the 4th instar and over 3/4 of the 5th instar caterpillar hosts of C. concinnata were attacked by hyperparasitoid wasps (Table 3). In three of the 5th instar caterpillars, two different species of hyperparasitoids were present. It is possible to have a tertiary parasitoid that attacks a hyperparasitoid, but it is also possible to have multiple parasitoids or hyperparasitoids on the same host. Until the hyperparasitoids are identified, it is not possible to say which occurred. Most likely is multiple parasitism, since the body sizes of the two species of wasps are nearly the same, while tertiary parasitoids are significantly smaller than the secondary parasitoids.

When looking at the total number of C. concinnata pupae rather than parasitized caterpillars, it is even more obvious how often C. concinnata was attacked by a hyperparasitoid (Table 4). Although less than half of the emerged adults from C. concinnata pupae of the 4th instar were hyperparasitoids, it was a much different story for the 5th instar pupae. Of these emerged adults, more were hyperparasitoids than were C. concinnata. Most were wasp species "A."

CONCLUSIONS

Populations of C. concinnata exist in Virginia. The parasitism rates support the argument that C. concinnata may be having a significant effect on either silk moth populations or other parasitoid populations. If C. concinnata's presence has not altered native parasitoid populations, and its effects are supplemental to the effects of native parasitoids, the result should be an overall increase in parasitism rates with a detrimental effect on silk moth populations. When considering the UV light data and the lack of published reports of declines, this does not seem very likely. Since few native parasitoid flies were observed, it is reasonable to hypothesize that C. concinnata is displacing some native parasitoids. In this case, the net effect on silk moth populations might be minimal. A third possibility is that the hyperparasitoids are keeping C. concinnata's population levels in check. This could be happening along with displacement of native parasitoids.

The high rate of hyperparasitism observed was unexpected. In other studies done in New England, hyperparasitism of C. concinnata was usually around 5% (Boettner, personal communication, 2001). It has been hypothesized that C. concinnata is very successful because it escapes hyperparasitism by developing quickly (Piegler, 2001). However, this study suggests that C. concinnata in Virginia may not escape hyperparasitism very effectively.

Based on a small amount of available short-term data, although C. concinnata appears well-established in Virginia, silk moths have not experienced the population declines that are occurring in New England. Many factors may account for this. In Virginia, C. concinnata has not existed as long, DDT spraying for the gypsy moth never occurred, and hyperparasitism of C. concinnata may be much higher. The findings of this study warrant further investigation of the complex interactions between populations of C. concinnata, native silk moths, and native parasitoids.

Table 1: Fates of Caterpillars

Instar	Number deployed	Days deployed	Recovery rate	Number becoming adults*	Number killed by parasitoids*	Number killed by pathogen*
2	100	2-7	41%	36	1	4
3	100	3-7	46%	44	0	2
4	100	4-8	35%	28	4	3
5	76	4-5	36%	6	21	0

* of those recovered

Table 2: Caterpillars Killed by Each Parasitoid*

Instar	Number parasitized	Number killed by C.concinnata	Number killed by fly species A	Number killed by fly species B	Number killed by wasp species D
2	1	0	0	0	1
4	4	4	0	0	0
5	21	16	3	2	0

*of those recovered

Table 3: Hosts Containing Hyperparasitized C. concinnata*

Instar	# of hosts killed by C. concinnata	# of hosts producing no hyperparasitoids	# of hosts producing hyperparasitoid species A	# of hosts producing hyperparasitoid species B	Hosts producing both hyperparasitoid species A & B
4	4	1	2	0	0
5	16¥	2	10	0	3

*not all puparia had produced adults as of 9/1/01
¥the identity of one dead pupa is unknown

Table 4: Fates of C. concinnata Individuals

Instar	Total # of C. concinnata pupae	Emerged adult C. concinnata	Emerged hyperparasitoid species A*	Emerged hyperparasitoid species B*	# of puparia containing adults not yet emerged
4	9	5	2	0	2
5	62	23	20	4	15

*of the total hyperparasites

REFERENCES

Boettner, George H., Joseph S. Elkington, and Cynthia J. Boettner. 2000. Effects of a Biological Control Introduction on Three Nontarget Native Species of Saturniid
Moths. Conservation Biology 14: 1798-1806.

Fink, Linda and Lincoln Brower. 1999 & 2000. Unpublished data.

Massachusetts List of Endangered, Threatened, and Special Concern Species. (Online)
Available http://www.state.ma.us/dfwele/dfw/nhesp/nhrare.htm, August 27, 2001.

Piegler, Richard S. 2001. We Now Know What Happened to Our Biggest Moths. News of the Lepidopterists Society 43: 30-31.

Schweitzer, D.F. 1988. Status of Saturniidae in the Northeastern USA: a quick review. News of the Lepidopterists Society 1: 4-5.

Tuskes, P. M., J. P. Tuttle, and M. M. Collins. 1996. The Wild Silk Moths of North
America; a Natural History of the Saturniidae of the United States and Canada.
Cornell University Press, Ithaca, New York.