Correct Treatments Require the Correct Diagnosis

May-June 2003

ALL ARBORISTS STRIVE to correctly diagnose the problems we see on trees. Correct diagnosis is essential to connect life cycle information, timing for detection and successful and new treatments. With pressure to reduce the use of chemical pesticides, we are turning to the use of biological and bio-rational pesticides, which are low or non-toxic but almost always have a narrower range of pests that they will control.

Two pairs of pests are often confused with one another. Can you tell the difference between Oak Slug and Oak Leaf Skeletonizer? Or Spruce Sawfly and Spruce Budworm? Diagnosis between these pests will be easy with a few tips and some observations on your own. I will also explain why I think the differences are important – especially when it comes to treatment – and not provide more useless information for your brain!

Oak Slug Versus Oak Leaf Skeletonize
The Oak Slug is a sawfly, which explains its alternate name – Oak Sawfly. References (Rose and Lindquist, 1982 and Johnson and Lyons, 1991) indicate that several species of sawfly (Order: Hymenoptera) from the Genus Caliroa attack oak and cause similar damage. The insect overwinters in the soil and adults emerge in spring. Eggs are laid in leaf tissue and the larvae may skeletonize either side of the leaf.

The larvae’s thorax is enlarged, giving it a tadpole appearance and the body is cover with a slimy material, making it look like a slug. Full size, the larvae may reach lengths up to 13 mm (1/2 inch). The larvae do not appear to travel much so the resultant skeletonized tissue is in one large patch. There may be two generations per year. I have seen noticeable damage on pin and red oak although the other oak species can also be attacked.

Oak Leaf Skeletonizer (Bucculatrix ainsliella) (Order Lepidoptera) is primarily a skeletonizer of red oak. It overwinters in the pupal stage and emerges as an adult moth in the spring when oak leaves are maturing. The first instar larvae (just out of the egg) mine the leaves. When they are about to molt, they emerge from the leaf, spin a round silk pad and molt underneath. They normally skeletonize from the bottom of the leaf. The larvae travel around the leaf and their feeding patches are small and scattered. The mature larvae are yellow with a noticeably segmented body, which can measure 1 cm long. The insect pupates in a white ribbed pupa casing placed on leaves or bark. There are two generations per year.

If the larvae are seen, identification should be easy. If only damage is seen, look at the pattern of skeletonization. Are there large patches up to the size of a quarter? That indicates Oak Slug. Alternately, are there small, scattered patches with a few white silk clouds and very small, hardly noticeable mines? These symptoms implicate Oak Leaf Skeletonizer. The only time it should be difficult to tell them apart is when populations are heavy and most of the green tissue is gone. However, if populations are that heavy, the presence (or absence) of ribbed white cocoons will help you make the right choice.

Finally, why should you care to distinguish between the two? Sawfly larvae are controlled easily with insecticidal soap. Moth larvae are not. Moth larvae can be controlled easily with Bacillus thuringiensis kurstaki (B.t.k.), the biological insecticide. Sawfly larvae can not.

Spruce Sawfly Versus Spruce Budworm
The Yellowheaded Spruce Sawfly (Pikonema alaskensis) is from the Order Hymenoptera and has a similar lifecycle to the Oak Slug. It should attack any spruce species but it appears to prefer Colorado spruce. The adult sawfly lays eggs on newly formed tips and small groups of larvae strip the tips of growth. They are fairly clean eaters and do not produce any noticeable silk so there is not a build-up of half eaten clumps of needles. When finished feeding, the larvae drop to the ground to pupate. Don’t let the name of the pest confuse you when identifying the pest – all actual larvae seen and pictures in books show the larva with a red head. There is one generation per year.

The Spruce Budworm (Choristoneura fumiferana) is the better-known pest of the two as it is a major pest of balsam fir and white spruce forests (and therefore, well studied by forestry students). The adult moth lays eggs in August and the first instar larvae overwinter on the tree. The larvae resume feeding in the spring. Needles are sewn together with silk leaving the eaten tips with a dirty appearance. The larvae pupate on the tree.

When the larvae are seen, identification should be easy. The Spruce Sawfly has 7 pairs of prolegs (the leg-like extensions on the abdomen, right behind the three pairs of real legs) and dark longitudinal strips down a yellowish body. The Spruce Budworm is brown with rows of white dots. It should have five pairs of prolegs. (Putting the larva in a Ziploc bag with the air removed is an excellent way to count prolegs and check colour).

If the insect is off the tree, look at the pattern of defoliation and the amount of garbage left behind. The Spruce Sawfly feeds from the tips back and leaves no junk behind. The Spruce Budworm feeds on the tips but leaves behind silk threads holding dead needles and empty pupa casings on the tree. Most defoliation that I see (especially in the Richmond Hill area) is from the Spruce Sawfly. I can only remember one property (Aurora in 1998) when I saw noticeable damage by Spruce Budworm.
As with the oak pests, sawflies respond well to applications of insecticidal soap. Spruce Budworm should be killed with an application of B.t.k. Both pests will succumb to most contact insecticides. On small spruce, the sawfly could potentially be picked off by hand – just target the bare tips around the tree and squish.

Correctly diagnosing tree problems is an essential part of being a professional Arborist. We can only hope the days of the public accepting general statements like "you got some sort of bug" are gone.

— Patricia Thomson, B.Sc.F. Consulting Arborist, Kelly’s Tree Care Ltd. with thanks to Alex Bykov, City of Toronto Forestry, for his help in fact-checking my observations. Photos supplied by author.

Johnson, Warren T. and Lyon, Howard H. 1991. Insects that Feed on Trees and Shrubs, 2 ed. Comstock Publishing Assoc. Ithaca, New York.

Rose, A.H. and Lindquist, O.H. 1982. Insects of Eastern Hardwood Trees. Canadian Forestry Service Forestry Technical Report 29. Ottawa. 304 pp.

Rose, A.H. and Lindquist, O.H. 1985. Insects of Eastern Spruces, Fir and Hemlock. Canadian Forestry Service Forestry Technical Report 23. Ottawa. 159 pp.

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