This August, two tree pests are having a field day in Denver – the Elm Leaf Beetle and the Honeylocust Spider Mite. Our wet spring set up conditions for the Elm Leaf Beetle to thrive and attack Elms across the city. Consecutive 90 degree days this summer is perfect breeding conditions for Honeylocust Spider Mites that have hit Honeylocust trees hard this month.
Yikes, There are Mites on My Trees
Tree Mite Lifecycles and Habits
The Spider Mite, an arachnid, is a relative of spiders and other eight-legged invertebrates. These pests thrive during hot, dry summer days. Spider mites’ colors range from red and brown to yellow and green but are challenging to see with the unaided eye. Tiny Spider Mites suck dry the nutrients and chlorophyll directly from plant leaves, usually on a cell level—the initial signs of their feeding are small brown or dry spots underneath the leaf. Once they begin reproducing, they can cover an entire leaf within a day. Any affected leaves will change color and turn dark yellow or brown; the edges will curl and eventually dry up.
Quick Facts from the Colorado State Extension Service…
- Spider mites are native to Colorado and common tree pests.
- Injuries include flecking and discoloration of leaves that can lead to leaf loss and even tree death.
- Natural enemies include Small Lady Beetles, Predatory Mites, Minute Pirate Bugs, Big-Eyed Bugs, And Predatory Thrips.
- Insecticides used to treat mites also kill their natural predators, which allows the mites to grow uninhibited.
- Healthy, well-watered trees can ward off mites and keep them under control.
After mating, female spider mites produce hundreds of eggs for about two weeks, leading to extremely rapid increases in mite populations. Dry conditions greatly favor all spider mites because their natural enemies require more humidity and do not thrive well in arid conditions. Furthermore, heat stress produces changes in plant biochemistry that make trees tastier to these arachnids. Click here for more information about Spider Mites from the Colorado State Extension Service.
The Spruce Spider Mite (Oligonychus ununguis) is another mite found in Denver yards. A related species, Oligonychus pratensis, attacks drought-stressed lawn grass. The Pine Spider Mite (Oligonychus subnudus) attacks Pines, while the Platytetranychus libocedri feed on Arborvitae and Juniper. Below is more information on the two most common Spider Mites in the Denver area.
Honeylocust Spider Mite
Honeylocust Spider Mite is similar in form to the Twospotted Spider Mite. After hatching, the young mites are orange, then turn a pale green as they mature.
Damage: These mites feed on the underside of Honeylocust leaves. Heavy infestations turn leaves an off-yellow color, and sometimes the whole tree becomes discolored and sheds leaves. Heat-stressed Honeylocust trees, like those planted along roads and parking lots, are susceptible to mite infestations.
Life History and Habits: Honeylocust Spider Mites overwinter as adult females hiding in bark crevices and under bud scales. The females start to lay eggs in the spring once trees begin to leaf out. Newly hatched nymphs molt two times before reaching their adult form. Populations build very rapidly in early summer and usually decline by mid-August.
Control: Healthy Honeylocust trees suffer fewer problems with this mite. Honeylocust Spider Mites are easier to treat with control products compared to other Spider Mite species.
Spruce Spider Mite
Spruce Spider Mites are 1/25 inch long with green bodies with orange markings. During heavy infestations, they may produce silk netting on needles.
Hosts: The mite feeds on most conifers but prefers Spruce and Junipers. Douglas-fir is also a known host.
Damage: The mites feed on the sap of trees. Infested trees become brownish-gray and may defoliate.
Life History and Habits: Spruce Spider Mites like conditions cooler than other spider mites. They overwinter as eggs attached at the base of needles, which hatch in mid-spring. First stage larvae start with only six legs but add two more in their nymph and adult stages. All stages feed on the sap of conifer needles. Since their lifecycles are only two weeks long, there are successive, overlapping generations. Peak populations often occur in spring, although late-season infections do occur.
Control: The most common natural enemy is the Black Lady Beetle. Miticides should be applied to control high mite populations.
How Do You Control Spider Mites?
Healthy trees experiencing little drought stress can fend off these pests themselves. To keep yard Honeylocust healthy, be sure to keep them watered and fertilized during the summer. Below is a list of ways to control spider mites.
Spider Mites have many predators, some with interesting names. Dark Lady Beetles eat up to 100 mites per day. Predatory Mites, Minute Pirate Bugs, Big-Eyed Bugs, and Predatory Thrips are important natural enemies.
It is important to remember that spider mites are not insects, so many insecticides are ineffective and may even aggravate the problem. For example, Sevin destroys many natural predators and can contribute to Spider Mite outbreaks. The same goes for Malathion. Soil applications of the systemic insecticide Imidacloprid have also contributed to some Spider Mite outbreaks. Miticides, such as Bifenthrin and Orthene, are specifically formulated for mite control. Since miticides do not affect eggs, we recommend repeat applications with 10 to 14-day intervals. Repeat applications of insecticidal soaps can be helpful. Dormant oil treatments in the winter kill eggs, so do suppress mite populations the following season.
Elm Leaf Beetles
Much like the Honeylocust Spider Mite, the Elm Leaf Beetle made its presence known this summer. Adult Elm Leaf Beetles chew holes in elm leaves. Their larvae feed on the leaf surface between the veins producing skeletonizing injuries. In spring, the surviving Elm Beetles move to trees to feed. The female beetles begin laying masses of yellow eggs on the underside of leaves. Once they hatch, the tiny, black larvae begin to chew little holes in the leaves. As they mature, they scatter through the tree, looking for more food. Within 3 to 4 weeks, the larvae are fully grown and begin to pupate. It takes about two weeks for adults to emerge, where they fly back into the branches to feed and mate, producing a second generation in a season. The next batch of larvae chews leaves from mid-July into September, then they pupate and overwinter as adults to start the cycle again the following spring. Click here for more tips on caring for Honeylocusts.
Control of Elm Leaf Beetles
Like most evasive species, these beetles have no natural predators in Colorado to keep their numbers in check. Denver’s wet spring this year produced large beetle populations, which is why they are a nuisance this year. For heavy infestations, one option is to spray right before the adults lay eggs, but it is hard to estimate treatment timing.
Systemic Product Control of Elm Leaf Pests
Ross Tree recommends systemic products to control Elm Leaf Beetles, a practicable alternative to spraying trees. Trunk injections and root drenches ensure that only the tree gets the product, not the surrounding environment. There is no need to close windows, cover furniture, or keep pets indoors when injecting trees to control tree pests. When injected into the tree’s root zone, the solution is taken up by the roots and eventually distributed to all parts of the tree, where it stays active for a year or more. When present in the leaf tissue, it kills any feeding insect.