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Sawfly (Neodiprion)

by Mark Stewart

The adult conifer sawfly, or Neodiprion, is a serious pest of coniferous trees. A medium-sized insect, ranging from 18 to 25 mm in length when fully grown, is common throughout Northern Asia, Japan, and most of North America (except the Midwest). In Europe it occurs from southern Italy to northern Finland, in the European Alps up to elevations of 6,500 feet, and is characterized by a smaller body size. Sawfly larvae are notorious for defoliating various types of pine trees and other conifers.

At first glance this may not seem like a particularly noteworthy accomplishment. To many insects the pine tree is a virtual no-man’s land. Its resins and oils, which may smell sweet to us humans, are highly toxic — but not to the sawfly larva! This little critter not only can tolerate pine oil and resins, but it even “knows how” to chew them up without being poisoned.

A unique digestive system

The sawfly is aided in its curious dietary habits by a unique digestive system which allows it to regurgitate the poisonous oils and resins found in these leaves. While munching on its staple diet of pine needles, the larva separates the poisonous oils and resins from the digestible pulp, and stores them in two goiter-like sacs located at the sides of its oral cavity. If provoked, it instantly turns its head toward its aggressor and secretes a drop of its stored liquid. This foul-smelling brew is usually enough to discourage would-be predators such as spiders, ants, and birds.How does the sawfly accomplish this phenomenal feat? Exactly how the separation takes place — i.e., how the larva is able to let all resinous bits disappear into the sacks during the chewing process while the other components enter the rest of the digestive tract — is still a puzzle, but one which may eventually be understood. One sawfly expert believes the powerful muscles of the sacs somehow aid in this separation of ingredients. A chitinous membrane protects the storage sacs from the effects of the acidic resins.

An evolutionary enigma

The sawfly’s digestive system is more than just an oddity. It presents a number of thorny questions regarding its origin. The unique digestive ability of the Neodiprion could not have evolved step by step. Let me explain. First, try to imagine the difficulties some ancient variety of sawfly larva would have encountered had it tried to switch from a normal leafy diet to one of pine needles. Think about it. According to evolutionary theory, the changes necessary to enable it to digest the pine needles could only have occurred through small mutations in a step-by-step fashion. But in the case of the sawfly larva, such piecemeal transitions become totally impractical, and one is immediately confronted with one of evolution’s basic problems of logic. The sawfly larva’s diet could not consist of pine needles until the mechanism for separating the poisonous from nonpoisonous components was fully developed and working with perfect reliability. But such a mechanism, if it came gradually into existence by small mutations, would have developed only if the food already consisted of pine needles. Yet any intermediate transitional larva with an incompletely developed digestive system would clearly have been unable to adequately cope with the poisonous effects of the pine needles. On the other hand, if there were no pine needles around for the larva to feed on, there would be no reason for it to develop any mechanism for digestive separation and protection. In any event, there would have been no evolutionary progress.

No matter how intelligently the theory of evolution may present the reasons for the step-by-step development of already existing organs or parts of the body, the difficulties in trying to explain how certain independent anatomical-physiological systems could originate by evolution seem insurmountable.

References

(1) The German Weekly, Die Zeit, #33, Translated by Werner G. Gerlach (Aug. 9, 1974)
(2) Lyons, L. A., The European pine sawfly, Neodiprion sertifer (Geoffr.) (Hymenoptera: Diprionidae). A review with emphasis on studies in Ontario. Proc. Entomol. Soc. Ont. 94:5-37 (1964)
(3) Pschorn-Walcher, H., Unterord. Symphyta, Pflanzenwespen. In: Die Forstschadlinge Europas (ed: W. Schwenke). Parey, Hamburg & Berlin. Vol. 4:4-234 (1982)
(4) Borror, D. J. and R. E. White, A Field Guide to the Insects. Boston: Houghton Mifflin Co. p.316 (1970)
(5) Bland, R. G. and H. E. Jaques, How to Know the Insects, 3rd ed. Dubuque, Iowa: Wm. C. Brown Co. 409p., page 372 (1978)
(6) Borror, D. J., C. A. Tripplehorn, and N. F. Johnson, An Introduction to the Study of Insects, 6th ed. Philadelphia: Saunders College Publishing. 875 p. Page 702 (1989)
(7) Daly, H. V., J. T. Doyen, and A. H. Purcell III, Introduction to Insect Biology and Diversity, 2nd ed. Oxford University Press. 680p. Pages 579-583 (1998)

THIS IS ANOTHER VERSION OF THE SAWFLY MARK STEWART WROTE. THIS ONE APPEARED IN THE NEWSLETTER OF THE EAST TENNESEE CREATION SCIENCE ASSOCIATION (SEP/OCT 2001, PG. 3) (Karl)

The unique digestive ability of the Neodiprion could not have evolved step by step. The adult Conifer sawfly, or Neodiprion, is a serious pest of coniferous trees. A medium-sized insect, it is common throughout most of North America , except the Midwest . Sawfly larva (caterpillars) inhabit the forests of Europe also, and as mentioned, are notorious for defoliating various types of conifers and pines. The sawfly is aided in its curious dietary habits by a unique digestive system which allows it to regurgitate poisonous oils and resins found in these leaves.

On the surface, few people would suspect that one small insect could have an unsettling effect on the basic tenets of the theory of evolution. But the larva of the common sawfly does just that.

The sawfly larva, a rather ordinary looking creature as caterpillars go, has managed to carve out an ecological niche on the fragrant needles of the pine tree. At first glance this may not seem like a particularly noteworthy accomplishment. But in the small world of insects it represents a radical departure from accepted environmental norms.

To many small insects the pine tree is a virtual no-man's land. Its resins and oils, which may smell sweet to us humans, are highly toxic substances as far as the six-leggers are concerned.

But not to the sawfly larva!

This little critter not only can tolerate pine oil and resins, but it even "knows how" to chew them up without being poisoned.

While munching on its staple diet of pine needles, the larva somehow separates the poisonous oils and resins from the digestible pulp and stores them in two goiter-like sacks located at the sides of its oral cavity. If provoked, it instantly turns its head toward its aggressor and secretes a drop of its stored liquid. This foul-smelling brew is usually enough to discourage would-be predators such as spiders, ants, and birds.

How does the sawfly larva accomplish this phenomenal fact?

First of all, for the sawfly to do this the edible parts must somehow be separated from the inedible parts; secondly, each must go the right direction so that the resins and oils will not get into the digestive tract; and thirdly, the tissue of its storage sacks must be insensitive to the resin acids.

The larva of the sawfly is able to perform these functions quite well. Its storage sacks are covered with a chitinous membrane and are thus effectively protected. The muscle tissue of the sacks is so extraordinarily strong that one sawfly expert believes it helps in the separation of the digestible and indigestible ingredients. Exactly how the separation takes place - how the larva is able to let all resinous bits disappear in the sacks during the chewing process and eat only edible parts - is still a puzzle, but one which may be eventually understood.

The sawfly's unique digestive system is more than just an oddity. It presents a number of thorny questions as far as the theory of evolution is concerned. First of all, try to imagine the difficulties some ancient variety of sawfly larva would have encountered had it tried to switch from a normal leafy diet to one of pine needles. According to evolutionary theory, the changes necessary to enable it to digest the pine needles could only have occurred through small mutations in a step-by-step fashion. But in the case of the sawfly larva, such piecemeal transitions become totally impractical, and one is immediately confronted with one of evolution's basic problems of logic.

The sawfly larva could not live on pine needles until a completely developed mechanism which separated wholesome food from poisonous pine resin was working with perfect reliability. But such a mechanism, if it came gradually into existence by small mutations, would have developed only if the food already consisted of pine needles. Yet all the sawfly's intermediate transition forms (which evolution requires) would clearly have been unable to adequately cope with the poisonous effects of the pine needles. In that case the sawfly's evolutionary progress would have been brought to a standstill.

On the other hand if there were no pine needles around for the larva to feed on, there would be no stimulus for it to develop any mechanism for digestive separation and protection.