With the arrival of warm weather also comes the arrival of summer threats: bugs.
While some people may hate insects just for being so buggy, for gardeners each year can seem like another new battle against the bugs as they determine what pest is destroying their plants and how to keep their crops from total destruction.
Here are some of Michigan’s most troublesome garden pests and how to get rid of them, said David Lowenstein, an integrated pest management expert and consumer horticulture educator at Michigan State University.
Saws
A particular problem in roses, sawfly larvae feed on the leaves with chewing mouthparts that cause “windowing,” an effect that occurs when the insect eats the material between the leaf veins, causing the veins and remaining plant tissue to break down into holes . .
Sawfly larvae look like caterpillars, but Lowenstein says it’s important to know the difference between the two in order to manage the pest. Insecticides that normally kill caterpillars have been shown to be ineffective on sawflies because sawfly biology tolerates the chemicals.
Pesticides such as insecticidal soap or neem oil can be used to kill sawfly larvae, Lowenstein says, or they can easily be picked directly from the plants and thrown into a bucket of soapy water.
Boxelder errors
Boxelder bugs, also called box bugs or maple bugs, are more of a visible pest than a harmful one, Lowenstein says. Boxelder bugs only feed on elderberry seeds and leaves, so they don’t actually cause damage to most plants.
“They show up in large numbers, which leads people to worry when you see 50, 60 insects on a tree or plant,” Lowenstein said. “They’re mostly swarming insects that don’t do any harm in the garden and I wouldn’t really do anything with them.”
Boxelder bugs can be more of a concern when temperatures drop in the fall because they can swarm outdoors or make their way into people’s homes. But again, they shouldn’t cause structural damage and can easily be vacuumed in the shop or sprayed from the side of the house.
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Slugs
Slugs that have just gone through the hatching season can be more visible throughout Michigan during this time, especially in cool or shaded, moist places, snacking on ornamentals and sometimes vegetables.
Lowenstein says there are several ways to control the number of slugs in your yard:
- Reduce crowding in areas of your garden that might be overgrown or otherwise crowded, giving slugs fewer places to hide.
- Aerate the soil in areas with poor soil drainage, otherwise wet soil will also be a favorable place for slugs.
- Sprinkle diatomaceous earth around your plants or other problem areas to destroy the exoskeletons of slugs and kill them on contact.
- Set beer traps by filling a small container or pan with cheap beer. The yeast in the beer attracts the slugs, then they fall into the trap and drown. However, Lowenstein recommends checking the traps regularly and replacing the traps if necessary if the container fills up with dead slugs or the amount of beer decreases.
Lily leaf beetle
Unlike some of the other pests listed, lily beetles are a bigger problem in southeast and central Michigan, mainly because they are invasive species that don’t have many natural predators.
Additionally, when in the larval stage, the bright red lily beetle becomes covered in feces to appear inedible to predators.
Leaf beetles can be hand-picked from plants, or gardeners can try to treat infestations with insecticides, but Lowenstein says if you already have leaf beetles, they’re likely to return.
“Unfortunately, this insect does not appear to be going away in southeast Michigan,” he said. I’ll have them next year too, so you might want to treat your lilies in May, that’s when they’ll start climbing.”
However, Lowenstein shared that he and an entomology professor at Michigan State University were working on a project modeled after an experiment that was successful on the East Coast to control lily beetle numbers by manipulating a parasitic wasp. The wasp lays its eggs in the larvae of lily beetles, killing the larvae. Lowenstein said they hope to release the wasps into areas with a high amount of lily damage next year.
Four row plant bug
Four-rowed bugs are what Lowenstein calls generalists: they will eat ornamentals, weeds, herbs and almost any other plant in your garden. However, they do not actually kill the plant.
The bugs have a piercing, sucking mouth that leaves brown circles when they feed on the leaves, and even if the damage is only cosmetic, the bugs move quickly, so it can appear as if the plant got sick overnight.
Lowenstein said it’s best to leave the bugs alone and let the plants grow, but if gardeners really can’t stand the spots, they can use an insecticidal soap to treat the plant. However, he emphasizes that it is better to treat plants that have not yet been exposed to four-row plants to avoid irreversible damage; unfortunately it is too late for plants that are already filled with brown circles.
Since the bugs feed on weeds, Lowenstein says going outside and pulling weeds to eradicate their food source might also help.
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Japanese beetles
The Japanese beetle is another common species, feeding on vegetables and leaving behind holes that look like skeletonized leaves on ornamental plants.
While the beetles don’t emerge until summer, they spend the spring living in the soil as grubs and feeding on plants in the lawn, which can cause browning no matter how much water is sprayed from your irrigation system.
“We get a lot of questions about ‘Why is my lawn not doing well?’ and often it’s because of a back injury,” Lowenstein said. “Another way people might realize they have ridges is if their lawn is dug up from wildlife that liked to eat the ridges. There will be lots of holes, and if you pull back the lawn and see more than 10-15 ridges per square foot track, that’s a sign that you might want to consider treating your lawn for bugs, and August through October is the best time to do that with an insecticide.”
Once the Japanese beetles are adults, Lowenstein says you can pick them off your plants by hand and otherwise place wire mesh or another similar barrier to keep the beetles out.
He says that insecticides should then only be used in extreme cases and pheromone traps should not be used, no matter how tempting. Traps do a very good job of attracting Japanese beetles, but once too many are poured in and the trap fills, you’ll end up with more beetles in your lawn than you started with. Lowenstein cautions that pheromone traps are better for commercial use or for multi-acre properties.
Tomato hornworms
As adults, tomato beetles turn into five-spotted hawksbills, but while in caterpillar form, the beetles can wreak havoc on your tomato plants.
Weevils can completely defoliate a plant and can chew holes in fruit and other produce.
Fortunately, tomato hornworms can be easily plucked from the plant and thrown into soapy water to kill them. Or you can remove the bugs from your plants and set them aside to sell; hornworms can be a good staple of live food for pet reptiles and amphibians, commonly sold at pet stores for over a dollar per worm.