Unexpected Late-Season Pests in the Landscape and Garden

As summer wanes, daylight hours grow shorter, and the nights become cooler, our thoughts of the garden and landscape lean towards that of harvest, clean-up and prepping for winter. Late in the season we often neglect to remember that certain insects and mites can still be active and create big problems if populations become high and our vigilance falters. A few of these late-season pests (and their management) are highlighted below.

Spider Mites

Many of our serious spider mite pests fall into the category of being warm-season spider mites, meaning that their activity begins in springtime but accelerates dramatically as summer temperatures become hot. Many of these, such as the two-spotted spider mite (Tetranychus urticae), can go from the egg stage to reproductive adult in as little as 5 days under ideal conditions, which usually means the daily temperatures are between 90° and 100° F. With each adult female mite producing 200 or more eggs, populations can increase exponentially over a matter of weeks, thus resulting in a "spider mite explosion" on specific affected plants.

Also, during extended hot and dry periods, plants may greatly slow their biological processes and produce fewer natural defensive compounds to defend themselves against mite and insect attack, thus allowing better chances of spider mite success. Predatory mites are known to greatly slow and even temporarily cease their foraging for spider mites when temperatures reach 90° F, thus removing more natural pressure from spider mite populations.

However, by this time of the year, warm-season spider mite species should be winding down their activity outdoors, but host plants that were attacked by them earlier in the season will continue to display signs of their feeding. Cool-season mites, such as the spruce spider mite (Oligonychus ununguis) have just begun to resurge now that the temperatures that favor warm-season mites is past and cooler days prevail. Spruces, firs, and hemlock are all favored hosts for this species and great damage can occur well into October when their numbers are high. This species mostly feeds at the base of older needles first, causing a yellow stippling. Heavy feeding will lead to needles that are off-color leading to a grayish yellow. This mite produces some silk but also spends the bulk of its life on the needles where it leaves its cast-skins from molting, silk, eggs, and empty egg shells, thus giving affected needles an almost dusty and dirty appearance along with highly stippled needles. Inspect susceptible host plants for signs of infestation and damage. Shake branches over a white piece of paper and inspect with a hand lens to see how many mites were dislodged.

Also note the presence and abundance of any predatory mites. If spider mite numbers appear to be at unacceptable levels, and especially if the number of predators is low, then treatments may be necessary. A spray of horticultural oil at the summer rate should provide the necessary level of control. Trees will require the services of a professional tree care company with a commercial sprayer. Avoid applying oil sprays if temperatures are below 45° F or if freezing temperatures will prevail within 24-48 hours after application.

Lacebugs on Evergreen Shrubs

Lacebugs are small insects that have a piercing-sucking mouth and they always feed on the undersides of the foliage. Despite nearing the end of the season, some of these species, especially those attacking Japanese Andromeda, azaleas, and certain rhododendrons, can continue to feed and create damage well into October if days remain warm and sunny. Also, susceptible host plants that are planted in full sunlight will be much more heavily favored and damaged by these insects.

Damage will appear on the upper leaf surfaces as finely stippled yellow spots. Plants that have been attacked for much of the growing season will now display foliage that is quite chlorotic (lacking chlorophyll). When such yellowed foliage is obvious, inspect closely for the thousands of individual yellow spots created by the lacebugs. Then inspect the undersides of the foliage for the lacebugs themselves as well as the dark fecal spots that they have deposited on the leaf undersides. If treatments are necessary, a well-targeted horticultural oil spray to the leaf undersides should work well. See the notes about applying oils at this time of the year listed above under Spider Mites.

Sawfly caterpillars

Caterpillar-like larvae that have 6 or more pairs of prolegs (abdominal legs) belong to the Hymenoptera order of insects, specifically the sawflies, which are wasp-like as adults. Moth and butterfly caterpillar larvae have 5 or fewer pairs of prolegs. The dogwood sawfly remains active into late September and even October if temperatures remain on the warm side. Dogwood sawfly larvae are yellow and black but they produce a white and waxy pubescence that covers their bodies at times. When they sit on the foliage in a curled position, they resemble a bird-dropping more than a caterpillar. These can be voracious feeders and usually consume all but the main veins of the leaves of many different species of dogwood. When damage is obvious, treat with a product that contains Spinosad as the active ingredient. This product works very well against caterpillar larvae of all ages.

The red-headed pine sawfly is also active at this time of year and can continue to feed well into October as well. It attacks many species of pines but is commonly found on mugo pine. This yellow caterpillar with spots and a rusty-red head capsule feeds in groups clustered on branches, consuming all of the foliage down to the stems. This pest is capable of completely defoliating smaller plants if not treated. Smaller sawfly larvae can be treated with an insecticidal soap spray while the older ones need to be treated with a product that contains Spinosad as the active ingredient.

Written by: Robert Childs
Revised: 10/2011