Tree Diseases and Pests

What is an aphid?
Aphids are tiny, soft bugs about 2mm long. They are usually light green in colour, but can come in many colors.

What kind of damage do aphids cause?
There are many types of aphids, and they can eat various kinds of plants. You might see them on trees like American elms and Manitoba maples. Aphids don't usually make plants sick, but they can change the way leaves look and grow. Aphids make a sticky liquid called honeydew that can stick to things like sidewalks and cars.

How do I control aphids?
Typically aphids don't require treatment with chemicals. There are sprays you can use, but always follow the instructions on the label. Sometimes, ladybugs can be used to eat and control aphids naturally.

What is an ash leaf cone roller?
The ash leaf cone roller is a new pest that bothers ash trees in cities. It's a small brown moth with a wingspan of 1.2-1.4 cm. These moths lay eggs on young ash leaves. When the eggs hatch, tiny caterpillars appear and eat the leaves from the inside, which is called leaf mining. After eating, the caterpillars float to other leaves using silk threads and roll the leaves into cones to hide and grow. The adult moth comes out in summer and rests until fall, then finds a safe place to stay during winter before waking up in the next year.

How do I control ash leaf cone caterpillars?
The damage from these caterpillars isn't big enough to need special treatments. It is important to keep your tree healthy through watering and pruning.

What is an ash plant bug?
Ash plant bugs are small oval bugs between 0.5 to 1.7 mm long. They have pale yellow to brown or black bodies with pink markings, while the young ones (nymphs) are green with black spots.

What kind of damage do ash plant bugs cause?
These bugs feed on ash tree leaves by piercing and sucking out nutrients, which kills the nearby leaf tissue and leaves a spotty look. The damage is not harmful to the tree, but it can be unsightly.

How do I control ash plant bugs?
If you have a small number of these bugs, you can often knock them off the tree with a strong spray of water.

What is a maple bug?
Maple bugs are small insects that suck nutrients from plants. They are about 1.0 to 1.5 cm long and have black bodies with red markings.

What kind of damage do maple bugs cause?
Maple bugs mainly feed on seeds of Manitoba maple (boxelder) and sometimes green ash. They are not harmful, but they can be bothersome when a lot of them gather on sidewalks, garages, or house walls.

How do I control maple bugs?
You can easily get rid of maple bugs by sweeping them into a garbage bag or using a vacuum when there are a lot of them.

What is a cankerworm?
Cankerworms are caterpillars that come from either the spring cankerworm or fall cankerworm moth.They feed on the leaves of trees that lose their leaves in the fall. These caterpillars hatch in spring and grow to be 20 to 30 mm long. They can be light green to brown and move in a looping way, like inchworms. Cankerworms hang from branches with silk and can create large webs. The adult moths are gray-brown.

What kind of damage do cankerworms cause?
Cankerworms eat the leaves of many trees, especially elm, maple, and ash. They can sometimes eat all the leaves off a tree, but the tree usually grows new leaves by July. If trees lose leaves for several years in a row and face other problems like drought, the tree can decline.

How do I control cankerworms?
Outbreaks usually last 1 to 4 years, and most trees stay strong during this period. If there are a lot of cankerworms, they can be annoying. You can spray them off trees with a strong blast of water from a hose. Make sure to water trees during dry spells to help them stay healthy.

Banding (insulation wrapped in saran wrap and Tanglefoot applied to the surface) may be used on private trees in the months of March and September. Banding at this time targets the wingless females before they lay eggs on the trunks of trees. Do not leave the bands on all summer, as this will promote mold and rot, which is more harmful to trees than feeding damage.

What is Dutch Elm Disease?
Dutch Elm Disease is a vascular disease caused by two kinds of fungus. These fungi block water in elm trees, eventually killing them. The disease is spread by elm bark beetles and through roots of nearby trees. No elm tree species is completely safe from it. Trees can get infected by:

• Elm bark beetles that carry the disease
• Roots connected between trees
• Contaminated pruning tools

What does a tree with Dutch Elm Disease look like?
At first, an infected tree will have leaves on some branches that start to wilt and droop. The leaves might turn yellow, brown, and then shrivel up. As it gets worse, branches can die, impacting more nd more of the tree. If a tree gets infected in late summer, leaves may look like they're changing to fall colours early.

How is Dutch Elm Disease spread?
Elm bark beetles spread the disease. They can fly up to two kilometers searching for elm trees. Little spores of the disease stick to the beetle's body. These beetles then carry the disease to other elm trees.

What is the life cycle of the disease?
Elm bark beetles spend winter hiding at the bottom of elm trees. In spring, they fly to the tops of healthy trees to eat. They go to sick or dying trees to lay eggs, and the new beetles feed inside the tree's bark. By fall, these beetles grow up, fly to new trees, and bring the disease with them if they're from an infected tree.

What is the best way to prevent Dutch Elm Disease?

• Don't keep or move elm wood. Provincial regulations prohibit the storing or transporting of elm wood. Elm wood must be disposed of at the City landfill.
• Avoid pruning elm trees during the provincial elm pruning ban from April 1 to August 31.
• Remove dead or dying branches quickly because beetles like to breed there.
• Clean your tools after working on elm trees.
• Report any sick-looking trees to the City.

More information on Dutch elm disease and the City's response plan can be found here.

What is the emerald ash borer?
The emerald ash borer is a very harmful insect that attacks ash trees. Its larvae eat under the tree's bark, eventually killing the tree. All types of ash trees can be affected. Even though the emerald ash borer hasn't been found in Yorkton yet, it would be a big problem if it arrived because about 25% of the city's trees are ash trees. It's hard to spot this insect, and often by the time it's found, many trees are already dying. Almost all trees die after it's detected.

What you can expect from us
The City of Yorkton sets up and checks traps to find emerald ash borers early.

How you can help us
Emerald ash borers spread when people move infested firewood. To help stop this beetle from spreading, don't move firewood from areas where the emerald ash borer is present.

What is a forest tent caterpillar?
Forest tent caterpillars are dark with white spots on their backs and can grow up to 5 cm long and are covered in long hairs. They have one generation each year, starting in June when they make a cocoon to turn into moths. The adult moth is yellow or tan with a furry body. While both forest tent caterpillars and fall webworms make webs on trees, the caterpillar webs look more like "tents" at branch nodes, while webworm webs are messier and cover the whole end of a branch.

What kind of damage do forest tent caterpillars cause?
These caterpillars eat the leaves of many trees, like ash, poplar, and chokecherry. They can sometimes eat all the leaves off a tree, but usually, the tree grows back its leaves. If trees lose leaves for many years in a row, they may decline.

How do I control forest tent caterpillars?
Outbreaks last 3 to 7 years, and most trees stay healthy. During a big outbreak, there might be a lot of caterpillars, making them annoying. You can spray them off the tree with a strong burst of water.

What is Fall Webworm?
Fall webworms are caterpillars similar to forest tent caterpillars. They are fuzzy and dark but don't have white spots on their backs. Fall webworms have one generation a year and eat at the end of summer into early fall. They mostly munch on leaves of fruit trees like cherry and crabapple, and other trees like poplar, birch, elm, and willow. Their webs are usually found high up on tree branches.

What kind of damage do Fall Webworms cause?
Fall webworms and forest tent caterpillars both make webs on tree branches. Fall webworm webs are messier and cover the whole end of a branch and its leaves, while forest tent caterpillar webs look more like tents. Though the webs can look unsightly, they usually don't harm trees because they eat leaves that would fall off anyway before winter.

How do I control Fall Webworm?
The City of Yorkton doesn't have a program to control fall webworms. If you're worried about how your tree looks, you can ask an arborist or a local garden center for help.

What is a gall?
Galls are unusual bumps that grow on leaves, branches, or roots. They are made by tiny creatures like nematodes, mites, or insects, and sometimes by bacteria, fungi, or viruses. Galls look different depending on what plant they're on and who is making them. Sometimes you can tell what caused the gall by how it looks and what color it is.

What kind of damage do galls cause?
Galls can change the shape of the tree parts they grow on, but they usually don't harm the tree's health.

How do I control galls?
Galls on trees generally don't need any treatment.

What is a leaf miner?
A leaf miner is a name for insect larvae that eat the inside of a leaf. In Saskatchewan, birch leaf miners (sawflies) and ash leaf conerollers (moths) are common, but trees like poplar, lilac, oak, and hawthorn can also get different leaf miners.

What kind of damage do leaf miners cause?
Leaf miners make winding tunnels inside leaves as they eat, but this usually doesn't hurt the tree's health.

How do I control leaf miners?
If only a few leaves are affected, you can pick them off. Usually, you don't need to do anything special for leaf miners.