Articles

Five-Linked Food Chain of insects

By Frank L. Marsh 1986

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Abstract

Within southwestern Chicago the food relations of a five-linked food chain of insects (a case of hyperparasitism) was studied. The larvae of the large Saturniid moth Hyalophora (formerly Samia) cecropia (Linnaeus), while feeding upon black willow, box elder, and wild black cherry, served as the key industry for four successive links of hymenopterous parasites (more accurately named parasitoids). The primary parasite was the ichneumonid Spilocryptus extrematis (Cresson); the secondary parasite was Aenoplex smithii (Packard); and the tertiary and quaternary parasitic positions were held, respectively, by the chalcids Dibrachys boucheanus (Ratzeburg) and Pleurotropis tarsalis( Ashmead).

Contributing to the delicate dynamic balance of this food chain were the tachinid fly Winthemia cecropia (Riley) (formerly W. datanae Tns.), two additional ichneumonids Ephialtes aequalus(Provancher), and Hemiteles tene/lus (Say) and the chalcids Dimmockia incongruus (Ashmead) and Cirrospilus inimicus (Gahan).

Reference is made to an assumed controversy throughout all the natural world between the Creator and Satan. A brief discussion is also included suggesting how, from a creationist viewpoint, a change in food in some animals from plant sources to animal sources, may have occurred.

Introduction

Within the Chicago area near Summit, Illinois, there is a level prairie community supporting scattered clumps of black willow, box elder, and wild cherry. These trees were found to be heavily infested with cocoons of Hyalophora (formerly Samia) cecropia (Linnaeus), the infestation being reasonably constant from year to year. The writer became engaged in a study of the feeding interrelationships existing between these trees, the Cecropia larvae, and the involved chain of hymenopterous parasites and hyperparasites-see Marsh, 1934: PQ. I-IV, 1-98). This opportunity is taken to discuss briefly certain general aspects of the problem, e.g. the biotic balance between moth, predators, and parasites.

Methods

In collecting the material for this study, the cocoons were kept in three separate groups: (a) those found on the ground-chiefly beneath brittle-stemmed willow trees, (b) those spun from the ground to a height of 15 ft., and (c) those spun from 15 to 35 ft.-the upper limit of cocoons in this region due to the absence of high trees. Age or condition of cocoon made no difference in the uniform sampling of the area. Thus the regulatory factors of several years were determined. The separation into groups according to the stratum occupied was made in order to learn the vertical spread of the factors involved.

The life-history details of the host and parasites were learned from much observation and collection in the field and from laboratory study. In the latter the ichneumonids and chalcids were reared in test tubes lightly stopped with cotton. Honey diluted with an equal part of water proved the most successful diet for the adults.

Ecological Observations

A quantitative analysis of the biological influences acting upon the Cecropia cocoons is illustrated in Table I. The table represents the results obtained from a dissection of 2741 specimens. The feeding interrelations discerned in this study are given in Figure 1. From these two groups of data certain interesting facts may be noted. Thus the ichneumonid Spilocryptus extrematis (Cresson) appears to be one of the most important influences in regulating the emergence of the moth, destroying 22.8 percent of all cocoons. Again, the birds (Hairy Woodpecker and Downy Woodpecker) were second with a destruction of 6.8 percent, the mice (Meadow Mouse and Whitefooted Mouse) were third with a destruction of 3.8 percent, and finally the tachinid was fourth, destroying 3.1 percent of the Cecropia cocoons.

In the Chicago area Cecropia begins to spin about the middle of July. From the time the first mature larva surrounds itself with as much as' a thin shell of silk, up to the time when the last pupa case has hardened, about the end of August, the moth is open to attack by its chief enemy S. extrematis. The abundance of Cecropia in the area studied can be imagined from the finding of 19 cocoons in a single cluster on a young black willow tree, while as many as 253 old and new cocoons were found on single adult willows. See Figure 2. Still, a casual visitor in the region would not suspect their presence due to the rapid leaf replacement by the willow and box elder, and to the habit of the larvae while feeding, of scattering over the entire food plant. !able II lists the food plants of Cecropia in this region III graduated series from most stimulating to least stimulating as judged from the abundance of cocoons found on these plants.

The Five-Linked Chain

In Figure 1 it will be noted that S. extrematis in turn served as the host of five parasites. However, attention will now be directed to the central chain leading through S. extrematis which, in this study, gives the best case of hyperparasitism, ending with the accidental quaternary parasite (Smith, 1916) Pleurotropis tarsalis (Ashmead). In this case Cecropia is the primary host. It is possible that S. extrematis is attracted to its host by the odor of freshly spun silk. As soon as cocoon spinning has progressed to a thin-shell stage, females of the icbneumonid have been observed coming up the wind to it as Canthon beetles follow up wind to fresh horse droppings. The ovipositor is thrust through the cocoon, and eggs are deposited on the inside of the cocoon or on the surface of the larva. Over 1000 eggs have been counted in one early-spun cocoon resulting from the oviposition of several females, while the greatest number of cocoons of S. extrematis in a single Cecropia cocoon was 172. As no starved larvae have been found, cannibalism is indicated. The average infestation of Cecropia cocoons with S. extrematis was found to be 33. During oviposition, the host larva is thrust with the ovipositor and invariably dies within a few hours. The larvae of S. extrematis move about freely over the dead host larva at first feeding on cuticle, later burrowing down and drinking the body fluids. In cases of heavy parasitism all the host body is eaten except the few chitinized parts. In the Chicago area S. extrematis is double brooded, completing a cycle in about 18 days.

Aenoplex smithii (Packard), the secondary ichneumonid parasite of this chain, appeared in about 13 percent of the Cecropia cocoons which were infected with S. extrematis. Because its host larvae are available throughout the year, the number of broods of A. smithii appearing in a season is governed by the duration of
the warm weather. Five successive groups of adults commonly appeared in a season are in the area studied. The host larvae are located by careful palpation of the infected Cecropia cocoon with the antennae of the female. Due to the rather short ovipositor only those larvae cocooned in the periphery of the cavity inside the Cecropia cocoon can be reached. A single egg (rarely two) is laid inside the cocoon of each host larva. This larva is then thrust with the ovipositer and dies within a few hours. The larva of A. smithii is very active, moving about freely over the the dead host drinking at numerous punctures made by itd mandibles. As in the case of S. extrematis the cycle of A smithii required about 18 days.

The omnivorous, cosmopolitan chalcid, Dibrachys boucheanus (Ratzeburg) appeared most frequently as a secondary parasite on S. extrematis, but because of the biological interest in its very frequent appearance as an accidental tertiary parasIte of A. sm2thn, It IS so listed here. Entrance to infected Cecropian cocoons is effected through holes previously made by woodpeckers, mice, or escaping ichneumonids, or i~ no holes are present, by crowding through the loose sIlk of the valve. Once inside, by palpation of the cocoons of S. extrematis with its antennae it determines the presence of a host larva and inserts its ovipositor, placing eggs on the surface of the larva. If a cocoon of A. smithii chances to be inside, its thin wall is also punctured and eggs deposited. An average of seven eggs were placed on A. smithii and 12 on S. extrematis. As many as 53 pupae have been found on the remains of one larva of S. extrematis. D. boucheanus is a very effective controlling factor. Each female may lay from 300 to 400 eggs and six broods were found in the field in a season. In the laboratory the writer reared 19 generations of D. boucheanus in 12 months. Contrary to the report of Muesebeck and Dohanian (1927), the writer found that in the Chicago area D. boucheanus always hibernated as a straw-colored pupa.

The last place in this five-linked chain of insects was occupied by the small chalcid Pleurotropis tarsalis (Ashmead). According to the easy shifting of its host from the role of an obligatory secondary to that of an accidental tertiary, P. tarsalis naturally frequently occurred as either an obligatory tertiary or an accidental quaternary parasite. The larva of this chalcid is an internal feeder. The female, after gaining access to the Cecropia cocoon in the same manner as D. boucheanus, places a single egg (rarely two) just beneath the cuticle of the mature larva or freshly formed pupa. In this case the adult parasite does not kill the host, but this is accomplished later in the pupa case by the development of the parasite larva. From laboratory rearings and field observations in this area, P. tarsalis appears to complete at least three cycles during a summer. It invariably hibernates in the larval stage.

Associated Parasites

The two smaller secondary ichneumonid parasites Ephialtes aequalus (Provancher) and Hemiteles tenellus (Say) were with the chain but much less effective than A. smithii as parasites of the primary parasite S. exrematis. Their handicap in this relation was the shortness of their ovipositors (2 mm. as compared with 4 am. in A. smithii). Where A. smithii could easily thrust :s ovipositor through the tightly spun wall of Cecropia ocoons and reach those of S. extrematis inside, these '10 smaller ichnemonids could only occasionally accomplish the task. Their effect in this food chain is comparatively minor.

The only dipteran observed in this willow-Cecropia community of parasites was the tachinid fly which J. M. Aldrich of the National Museum at first identified as Winthemia datanae (Tns.). See Marsh (1937). Since at time the National Museum told me the insect more properly is not W. datanae but W. cecropia seems likely under the conditions. I will accept the latter species name. At their request the specimen) shown in Figure 3 now rests in the National Museum,
Washington, D.C.

This tachinid occurs in the Chicago area as a primary rasite, along with S. extremotis, of the key industry ralophora cecropia. Normally this tachinid fly erges the second week in July, just as its Cecropia tal host has molted the last time before spinning its cocoon. This is important in the life of this tachinid because use it lays its eggs on the larval surface and if placed earlier the eggs could be shed with the discarded skin. When considered over the whole area the parasitization of Cecropia by W. cecropia is rather t (3.1 percent, see Table I). Its distribution is in highly concentrated areas which frequently are quite ely scattered. In these spots (including several small trees) the destruction of Cecropia is commonly total. During the oviposition the female hovers over the larva’s back, clinging to the larva s tubeercules while the eggs are securely attached by their adhesive coating to the skin of the host along the dorsal line. An average of 21 eggs was found. In about 36 hrs the larvae (maggots) hatch and gnaw their way directly to the celomic fluid of their host. The host usually completes its cocoon before dying. The parasites eventually devour everything about the host except its bloated and blackened skin.

The mature tough-skinned maggot of W cecropia finds itself imprisoned in the Cecropla cocoon commonly 10 to 15 ft. above ground. The writer has photographs of the escape of the mature maggot through the cocoon valve, and of its burrowing into the ground after falling from the tree (Marsh, 1934). Pupation occurs in a small cavity constructed three to eight inches underground.

The only parasite of W, cecropia discovered in this study was the small chaicid Dimmockia incongruous (Ashmead). He often observed

Ichneumonid and chalcid parasites trying to pierce the cuticle of the maggots of W. cecropia, but to no avail because because of its toughness. However he did once find two puparia of W. cecropia which contained 52 pupae of D. incongruss, 12 of which were in turn hyperparasitized by larvae of the tertiary chalcid Pleurotropis tarsalis. The tough surface of the maggot and also of the puparium of W, cecropia rendered it generally safe from danger of parasitization.

Off the main chain but still a very definite deterrent to the numbers of the primary parasite S. extrematis was a small, pretty, black-and-yellow chalcid wasp (body length: male 1.5 mm., female 2 mm.; see Figure 4D) which had a life cycle of from 18 to 21 days, and appeared to run at least three broods a summer. As parasites their behavior was quite identical with D, boucheanus. In his identification of the writer's chalcids, A. B. Gahan thought this could be a new species of the genus Cirrospilus. A total of about 100 was sent to Gahan (the writer was raising them on his desk) which made the fact of a new species certain, So I invited him to assign the new name. This he did, calling it C. inimicus Gahan (1934). For some biology of this new chalcid species, see Marsh (1938). Those readers who desire more detail on the biology of the insects associated in this willow-Cecropia community are referred to the sources listed under Marsh in the References. An impressive point in the field study of this five-linked insect food chain is the fact that the fertile parasite arrives exactly at the most vulnerable period in its host's life cycle. This obviously involves perfect timing and points toward design rather than evolution,

This nicely balanced series along with other regulatory factors of Cecropia in the Chicago area seems to have produced an equilibrium in the abundance of the moth. The infestation of this moth is abnormally heavy, yet has apparently been of this degree for some time, See Figure 5. As expressed in the words of an interrogated "native" who had grown up in the area, the cocoons had "always been just that thick." A reasonable balance does appear to have been reached in the infestation by this moth so that, in recent years at least, Cecropia has neither increased nor decreased.

Discussion

True natural science consists of two parts, (1) demonstrable facts and (2) undemonstrable speculation about the facts. It is a must in true natural science that the scientist keep very clearly in view what is demonstrable fact and what is mere speculation. Up to this point in this article the author has reported demonstrable facts. These items should be the same whether reported by Bible-believer or atheist. Scientific facts are simply facts to all. The author of this article is a Bible-believer.

In my report I have called the parasitic wasps and a fly "parasites" in harmony with H. S. Smith's recommendations (1916, pp. 477-486). In 1966 R. L. Smith (p. 405) states that insects which attack their host indirectly by laying their eggs in or on the host, and later the eggs hatch and the larvae feed on the host until it dies, should be called parasitoids. Still more recently (1986) May and Seger (p. 260) continue to call parasitic wasps and flies parasitoids. Thus today when discussing predation I am careful as to distinguish between parasites and parasitoids. My present report concerns the latter.

From the writer's point of view (as a Bible-believer) he suggests that the only way to begin to comprehend our natural world is for the student to bear in mind that since a time soon following Creation Week, our world has been the battleground of a controversy between the great amI loving Creator (Christ the Son) and the jealous and crafty destroyer Satan (Ezekiel 28). God's wisdom and fairness is shown in His permission to Satan to live and demonstrate his manner of directorship were he the King of the Universe. Briefly, all that is fair, loving, and beautiful comes from the Creator, and all that is evil, hateful, repulsive and selfish comes from Satan. Both contestors in this controversy use natural laws as their implements.

Satan cannot create new life (Psalms 36:9). But I believe that he can degenerate living forms by gene manipulation. Scientists today know that Genesis kinds of organisms cannot hybridize. But they also know that crossbreeding between varieties within a single basic type usually is successful, but not uncommonly the hybrid will produce some poisonous substance or be abnormal some other way. Examples here are known among hybrids between varieties of monkshood (Aconitum) and guayule (Parthenium), In animals such hybrids often manifest changes in their original instincts. See Marsh (1981, p. 166).

In my opinion Satan has employed this manipulation of genes very widely in his worldwide degeneration of organisms. Let us speculate a little. We know from Genesis 1:30 that the original ancestors of the ichneumonid Spilocryptus extrema tis were Herbivora (plant eaters) but in our study S. extrema tis showed a preference for the fluids and tissues of cecropia larvae. Is it not possible that Satan has used his gene technique here to change an original instinct? If we are correct here then he has been quite busy because according to Ross (p. 449) in North America alone there are probably about II,000 species of parasitic insects.

As in the case of Job, where the Creator set a limit to how far Satan could go in abusing Job, all through nature we can see where the Creator has checked some satanic activity. An example here may be the work of scientists in discovering the use of a Satan produced poison to kill Devil-generated organisms.

This five-linked insect food chain is in delicate dynamic balance. It is interesting to study what would be the effect on the moth if any certain link were to increase or to decrease in number of individuals*.

*"(Editor's note) Some creationists prefer to believe that the parasitic action of insects developed after the Fall. An alternate explanation would be imagining nature as a battleground (conservation \'5. degeneration processes). See Williams. Emmett L. 1976 A Creation model for natural processes. CRSQ 13:34-7 or in 1981 Thermodynamics and the development of order, Creation Research Society Books, Norcross, GA 30092. pp. 114·19. If one prefers to view parasitism as part of the balance in nature, it can be visualized as occurring because of the interplay of conservation and degeneration processes. However parasitism originated, it is the result of design rather than accidental natural events.

References

Throughout the course of this work I have had the advantage of counsel from Dr. C. L. Turner ( Northwestern University). It is also a pleasure to acknowledge the criticism of Dr. Orlando Park ( Northwestern University) and I am especially indebted to the following taxonomic experts, J. M. Aldrich, R. A. Cushman, A. B. Gahan, C. L. Metcalf, C. F. W. Muesebeck and C. W. Sabrosky for their care in determination of insect material.

Much of the material in this article first appeared in Ecology--see Marsh (1937). Permission for its use here was given May 5, 1986, by the Ecological Society of America.

Gahan, A. B. 1934. A new species of Cirrospilus Westwood (Chalcidoide). Proceedings of the Entomological Society of Washington. 36 (5).
Marsh, Frank L. 1934. A regional study of Samia cecropia and nine associated parasites and hyperrarasites. An unpublished master's thesis (containing 127 photos in the Northwestern Universitv Library, Evanston, IL. .
--- 1935. Cecropia-larva to pupa in pictures. Nature Magazine 25(6):312-3.
--- 1936. Egg placing by Dibrachys boucheanus Ratzeburg.
Canadian Entomologist 10:215-6
--- 1937. Ecological observations upon the enemies of cecropia, with particular reference to its hymenopterous parasites. Ecology 18:106-12.
_ _1937. Biology of the tachinid Winthemia datanae (cecropia).
Psyche XLIV(4):138-140.
--- 1937. Biology of the idllleurnolliJ Spilocryptus extrematis Cresson (Hymenoptera). Annals of the Entomolvgical Society of
America XXX(I):40-2. .
--- 1938. Biology of the new chalcid parasite Cirrospilus inimicus Gahan. J oumal of the New :York Entomowgical Society XLVI:27-9.
--- 1981. Have the Genesis kinds ever crossed? Creation Research Society Quarterly 18:164-7.
May, R. M. and Jon Seger. 1986. Ideas in ecology. American Scientist 74(3):200.
Muesebeck, C. F. W. and S. M. Dohanian. 1927. A study of hyperparasitism, with particular reference to the parasites of Apanteles mewnoscelus. United States Agriculture Bulletin 1487; April.
Ross, Herbert H. 1956. A textbook of entomology, second edition.
John Wiley & Sons, New York: p. 449.
Smith, H. S. 1916. An attempt to redefine the host relationship exhibited by entomophagus insects. Journal of Economic Entomology 9(5):477-86.
Smith, Robert L.1966. Ecology and field biology. Harper and Row, New York, pp. 33, 405-6.

Creation Research Society Quarterly Vol. 23, March 1987