City of Minnetonka

Is something “bugging” your tree?

If you are like most people, when you notice an insect in your house, you want to get rid of it. At the same time, you’re probably curious about the creepy crawly creatures on your trees and whether you should be doing something about them.

Insect CSI

After spotting an insect, do you launch a full-scale attack or watch and wait? By knowing the broad categories of insect damage to trees, you can better understand the risk to the plant and whether it is worth your time and resources to act.

Keep in mind that most insects are specialists and only eat one or two specific plants, which is why tree species diversity is so important. For example, emerald ash borer only inhabits… you guessed it, ash trees.

In order to manage plant-damaging insects, many arborists, horticulturists and farmers follow principles of integrated pest management (IPM) to guide treatment decisions. It takes into account multiple factors, including growing conditions, insect life cycles and all of the mechanical, cultural, chemical and biological pest control options available. The mere presence of an insect doesn’t justify treatment. Instead, the plant is monitored regularly and treated only when damage reaches a negative economic or aesthetic threshold.

You can do this in your own yard, helping you save money and toxic chemicals. In fact, as much as 10% of a plant can be damaged by insects before the average gardener even notices. The most noticeable and prevalent types of insect damage to trees are leaf damage and branch, trunk and root damage.

Leaf damage

Insects that damage or consume leaves show hundreds of telltale signs. Some feed in irregular patterns, some eat between leaf veins, others pierce leaves and suck the nutrients right out of leaves. The worst insects in this group are those that consume all the leaves on the tree. The most recognized leaf feeding insects are in the butterfly family, beetle and sawfly insect families.

Leaves are a tree’s food factories, and they naturally repel certain insects with chemical compounds unique to each plant. A tree’s energy budget can handle some leaf damage by native insects. As long as the tree is established and healthy it can usually recover, even if it doesn&rst look exactly as you think it should. Keep these items in mind:

• Trees use a lot of their energy budget in early spring to produce new leaves, so complete leaf consumption during this time of year can weaken trees, especially if the tree has to produce a second set of leaves or deal with another stressor.
• Avoid over fertilizing your trees. The rapid tender growth that follows fertilization can be more attractive to insects and can use extra energy that the tree would normally use instead to produce insect defense chemicals. Perform a soil test first to see if the soil is deficient and consider a slow release fertilizer.
• Consider the type and extent of damage before you treat. Does it affect the appearance only or does it damage the plants’ ability to function? For example, a large aphid population can cause discoloration of flower petals and leaves and lead to wilting. If you wish to control them, you can apply insecticidal soap to reduce aphid populations. This product is effective against aphids but has little effect on natural enemies, like ladybird beetles (true lady bugs). However, this isn't necessary as even a severe infestation is not likely to kill the plant.
• If the insect is non-native, and doesn’t have a native predator, there is a greater chance the population will grow unchecked. In these instances, there is a good chance that the tree can become weakened and killed after leaves are fully consumed for many years in a row. Sometimes trees produce another set of leaves, but this takes extra energy from their budget.
• An example of this type of insect is the Japanese beetle and the gypsy moth from Europe. While gypsy moth isn’t established here yet, the state conducts spot control when it is found in Minnesota in order to delay its establishment as long as possible.
• Only 10% of insects can eat leaves of several types of plants. Gypsy moth and Japanese beetle fall into this group because they eat many types of species.

  

  Japanese beetle consuming leaf

  Who

  Japanese beetle

  What

  Non-native invasive insect accidentally introduced from Japan and a serious pest of turf and 400 species of ornamental plants and trees.

  How to control

  Reduce the size of your lawn to limit the population (the baby beetles, called grubs, live under your turf), hand pick beetles and use pheromone traps. Chemical treatments must be carefully timed to be effective.

  How to prevent

  Consider planting species that are not susceptible like red and white oaks, red maple, balsam fir, Canadian hemlock, yew and others.

• Insect feeding on pine needles is a problem because once needles are consumed; they will not grow back in the same area the following spring. Instead new needles appear on the branch tips. The damaged tree will start to look bare on the inside. The European sawfly inflicts this type of damage by feeding on the previous year’s tree growth. When the insects are full grown at the end of June, the majority of the pine needle consumption and damage to the tree has occurred. Treatment at this time of year is not effective.

  

  European sawfly feeding on pine needles

  Who

  European sawfly

  What

  Non-native insect that can be a serious pest when population is high, and populations have been high the past few years.

  How to prevent

  Early scouting in April and early May will help interrupt the pest before it does too much damage. Pay special attention in the years following bigger outbreaks.

  How to control

  Pest is usually noticed once feeding is almost done in June when controls aren’t as effective. At this stage, consider hand-removing sawflies before they drop to the ground to pupate or use a hose with a forceful spray to dislodge sawflies from tree.

• If you notice feeding is happening late in the summer, there is less chance the tree’s long-term health will be affected because its food factories are starting to wind down for the season. An example of this is fall webworm.

  

  Fall webworm tent on the branch end of a walnut tree

  Who

  Fall webworm

  What

  Native insect that is not a serious pest of shade trees. It produces a tent-like web in autumn on the branch tips of crabapple, walnut, birch, cherry, willow, and other species.

  How to control

  Remove branch tip with a pruner and burn, dispose or tolerate it if it is too high to reach.

Branch, trunk, and root damage

Insects that feed on branches, trunks and roots can cause the most long-term damage to a tree. Specifically, the insects that feed just underneath the bark can damage the living tissues that transport water and nutrients and thus are the most problematic for a tree’s long-term survival. To understand what is going on inside a tree that is under attack internally, consider the visual image of a garden hose that has been run over with a lawn mower.

• Look for dieback or wilting branches that could indicate the tree is not effectively transporting water and/or nutrients. Examples of insects that inflict this damage are the non-native invasive emerald ash borer and the native two-lined chestnut borer.

  

  Two-lined chestnut borer damage
  on a white oak showing dieback

  Who

  Two-lined chestnut borer, whose immature beetles (larvae) feeds under bark on upper branches

  What

  A native insect that attacks stressed or weakened trees, effectively finishing them off. The population spikes in drought years and for several years following a drought.

  How to control

  Two-pronged approach: restore health of the tree by watering and mulching, then treating for the insect.

If you decide to launch a full-scale attack

If you’ve decided that tree damage has reached an aesthetic or economic threshold, be sure that you or the person you hire has answers to these questions:

• Are you sure it is an insect doing the damage? Is it still causing damage or has the damage already occurred?
• Is the plant otherwise healthy, established and growing well?
• Is the damage aesthetic only or is it affecting the tree’s function/ability to survive?
• Are treatments being applied at the weakest time in the insect’s lifecycle when they are likely to be effective?
  • If you spray when the bug is resistant to treatments, you are wasting time and money. You could also be killing some of the beneficial insects that help keep your pest in check, making the problem worse with population resurgence of your pest.
• Is it a native pest susceptible to population peaks and dips or a non-native pest without natural predators?
• What time of year is the damage occurring?

Need more help?

What’s wrong with my plant?

What insect is this?

www.extension.umn.edu/Garden/

If you suspect a disease rather than a bug: University of Minnesota Plant Disease Clinic

pdc.umn.edu/

For a soil test

soiltest.cfans.umn.edu/

For a local certified arborist, start by searching this list

Hiring Tree Care Pros page on eminnetonka.com

Insects that are a human nuisance

A few insects bug us because they try (and often succeed) to get into our homes. They do not cause damage to plants, but they sure do get in the way. Examples include Boxelder bugs and Asian lady beetles.

Insect fun facts

• Insects are the most diverse group of organisms on earth, representing about 80% of all animal species.
• Insects keep other insect populations in check. When you use insecticides to kill off insects, you also risk killing many other insects that are beneficial to your landscape.
• Do you like birds? Insects are a very important source of protein and fat for birds, especially young birds.
• Did you know that insects do not emerge at the exact same time every year? If we simply looked at a calendar to predict when an insect will become active, our prediction would be too late or too early. Unlike humans, insects are cold blooded and will develop rapidly in warm temperatures and slower in colder temperatures. Degree days are a method for calculating heat unites, often used by power companies to calculate how much energy a customer needs to cool or heat a house. Each insect requires a specific number of degree days to develop and estimates can be made about their development by counting the cumulative degree days at or above 50 degrees Fahrenheit.
• Many native insect pests have a “check and balance” where the population builds and then crashes or is kept at lower levels by other insect predators. Exceptions to this rule are a landscape that lacks in native plant diversity (certain bugs only inhabit specific plants), and frequent pesticide use, which kills off beneficial and damaging bugs.

Questions or concerns?
Ask Minnetonka Mike!