The 2006-07 scale insect infestation: is it poised to recur in 2017?

Photo of scale insects (Parthenolecanium corni) on maple twigsHow had we missed them? They must have been there before! About 10 days after the leaves came out last spring (so in mid-May, 2006) members of the Macoun Field Club whose study-trees were Sugar Maples began noticing that the twigs were abundantly beaded with reddish brown insects that we had not seen before. They had essentially no legs, wings, or eyes (when flipped over they feebly waved useless little legs and a long, tapering hair-like beak). We recognized them as scale insects. There seemed to be about 100 on every maple twig, even on tall forest trees, which we estimate have 2500 twigs, and therefore 250,000 insects each. They were splashing sticky honeydew (excess tree sap) on everything underneath. (To keep things in perspective, they are rather small, and packed together this number would make up a new imaginary animal no bigger than a small porcupine. Compare its sap-sucking effect with that of a human hanging three sapbuckets up on the same tree.)

A friendly entomologist offered an identification: the European Fruit Lecanium, Parthenolecanium corni. He calmly suggested that the infestation of our trees would flare up for a year or two, and then subside. This introduced species comes from Europe. All those we normally see are females, which reproduce by parthenogenesis (hence the scientific name).

Photo of female scale insects (Parthenolecanium corni) and egg mass

By the end of May, the rain of stickiness stopped. Now when we flipped the insects over a white dust spilled out. Under magnification, the dust proved to be tiny oval eggs — we estimated 1000 for each female. (The picture at left shows an undisturbed female on the twig, and another lying upside-down below it, with a loose mass of eggs that she left behind.)

Photo of a scale-insect (Parthenolecanium corni) crawler

At the end of June, the eggs began to hatch all at once. Over several days, minute yellow crawlers by the millions released themselves into the air and floated like pollen. They landed on everything — plants of every kind, rocks, leaf litter. Those that landed on Sugar Maple leaves crawled around to the lower surface, stuck in their feeding equipment, and began to suck sap.

Most of those that did not reach a Sugar Maple on their first try probably perished. But many of them sampled other types of trees, and sometimes found themselves on a new species that would also sustain scale-insect life. We have occasionally found crawlers on ash, ironwood, basswood, oak, sumac, and leatherwood, with one or two, or even twenty crawlers per leaf.

Photo of (Parthenolecanium corni) crawler-infested leaf

How many were there on Sugar Maples? In most cases, about 1000 crawlers on the underside of every leaf. We thought one leaf had closer to 4000! Here’s the central lobe of such a leaf — you can make your own estimate.

All summer long, and into the autumn, the crawlers sucked leaf sap, passing enormous quantities through their systems in order to get the particular nutrients they need, which are scarce, and excreting the excess. In May, the honeydew drops from the big adults had fallen straight down, but now a mist of tiny droplets floated out from the trees and coated surfaces of all kinds.

People who lived around infested Sugar Maples had to wash their car windshield each day before setting out (plain water worked just fine). But wherever the honeydew stuck, a black mold began to grow, gradually darkening the forest foliage, unwashed cars, and house walls.

Photo of crawlers (Parthenolecanium corni) in early fall colouration

In late August, more than a month before the leaves would fall from the trees, some of the crawlers got up on their little legs and moved from the leaves to the twigs. Most waited until the autumn colours were well developed. Though still less than a millimeter long (about 1/30th of an inch), they had grown considerably (use the background pattern of the leaf to compare these crawlers with the one shown earlier), and had developed an orangy colour. Finally, in their thousands they moved off the leaves. Perhaps because there were too many to fit on the twigs, they often crawled far down the branches, and even 20 feet down the trunks of small trees. They preferred the underside of branches, and where they could, they settled into protective irregularities in the bark. (Those that could penetrate the bark evidently continued to feed, because car windows were still being misted with honeydew weeks after the trees had become bare.)

Photo of crawlers (Parthenolecanium corni) in early winter colouration

By early winter the crawlers had darkened still more, and where they were resting in abundance they reddened whole branches so strongly that their presence could be guessed at 10 and 20 feet away. Would they survive the coming cold? In our area, the nighttime temperatures often went to -25 degrees C (-13 F).

In early March, Macoun members put twig samples under bright lights to look at the insects through the microscope — and the crawlers began to crawl! At the end of the month, just when the sap was beginning to run in Sugar Maples, the crawlers were waking up and ascending the trees.

Will scale insects be your problem this year? Our study area is on the western outskirts of Ottawa (Canada’s capital). In 2006 we found the infestation to be very uneven, but trees were heavily infested just east of the city, and also an hour’s drive to the west (White Lake). The scales were present in small numbers much more widely across eastern Ontario — and on a very local scale, we saw the crawlers appear where no adults had been seen in the previous generation.

Some surprises in 2007

Although we saw overwintering crawlers wake up and move out the branches, we later saw that they had left many dead behind. On most Sugar Maple trees, the number of surviving adults actually seems to be lower than last year. On other host species, however, such as Leatherwood, which had been only sporadically infested, the numbers of scales approaches that seen on Sugar Maples in 2006. And this is where it gets really interesting.

Photo of second-year crawlers (Parthenolecanium corni) in spring

On Leatherwoods (and Ironwoods, genus Ostrya) the scales developed a silvery sheen in late May, much as they had just before moulting in April. But instead of shedding their skins, the shiny, almost transparent coverings on these scales began to lift away from the bodies developing underneath. Most of them also sprouted a pair of white, thread-like tails that were sometimes longer than the body itself. (In the picture at left, you can see how much they had grown by May 24th, because one of the crawlers that died during the winter, still small, flat, and reddish, is in their midst.)

Photo of male scales (Parthenolecanium corni)

Lifting the canopy-like cases reveals reddish brown, winged insects — male scales. This species reproduces mainly by parthenogenesis, and males are said to be rather rare, but on these particular host plants (which were not the favoured species last year), as many as 70% of the crawlers have turned out to be males.

Photo of female crawler (Parthenolecanium corni) sprouting waxy curlicue from posterior

On Sugar Maples, by contrast, almost all the crawlers that survived developed into females. Females are much bigger, with strongly domed bodies, no wings, eyes, or usable legs. They don’t have the white tails, either, but many are sprouting little curlicues of white waxy material from the posterior end, which we don’t remember seeing last year.

As for our very first question, at the top of this page — How had we missed them prior to mid-May 2006? — we had learned that the scales change from reddish to a more cryptic chocolate brown and stay small right into May, and then suddenly plump up to noticeable size and adult colour in about a week.

They’re still growing, too, tossing out ever bigger splashes of honeydew. The males, though appearing fully developed, aren’t quite ready to take to wing yet.

The cycle repeats, but the population stumbles

The males took wing and were not seen again. The female scales produced eggs at the beginning of June. Just before the end of the month, the crawlers began to hatch out — being first noticed as a coarse yellow “pollen” on car windows.

Photo of white fungi sprouting from a scale (Parthenolecanium corni)

But not all females were successful. Typical predators seen that spring include Red Squirrels, Hairy Woodpeckers, and predaceous beetle larvae. And there are more subtle attackers. On some trees, most of the domed shells look a little drab and shrivelled, often with a small round hole bored through the top. Parasitic wasps had grown up inside the scales’ bodies and escaped. And the bodies of many scales have sprouted white, club-like fungi, as shown here.

The surviving crawlers were still numerous and dispersed. Right at the beginning of the hatch, the leaves of most host trees in areas of infestation were already dotted with crawlers that had settled in to feed. But none made it through to 2008, at least not in our Study Area.

Coming back from the wilderness, 10 years later?

In 2008, we did find some of these insects deep in the Pakenham Hills, an hour’s drive west of our Ottawa study area. And there they reappeared in 2015 — with both males and females on just one Myrica gale shrub by a stream. In June 2016, there was an even higher proportion of males on the very same shrub, and 10 miles away (at Rob Lee’s) a patchy infestation of females on — once again — Sugar Maples. They were numerous enough to coat the vegetaton below with sticky honeydew. By mid-June, they stopped feeding and produced thousands of eggs.

All images obtained in the field or under a microscope by Rob Lee. Created March 23, 2007, and modified June 29th of that year. Concluding update added June 22 2016.