Identifying Insect Pests: What's Eating My Plant?

Scouting for Pests

Before you go scouting, remember: may insects are beneficial so don’t assume all insects are trouble! Remember the myth about peonies and ants. Also, never reach for an insecticide spray! Many of these insects aren’t affected by sprays and you’ll only be killing the beneficial insects and the helpful predators. Plus, if you kill the bees and pollinators, you’re only digging your garden’s grave.

• Leaf miners are fly larvae that tunnel between leaf layers in chard, spinach, lamb’s quarters, and beet greens leaving a light green meandering trail behind. Often you don’t notice them until you cook up the greens and find a lot of tiny worms. Yuck! See the Almanac’s solutions for leaf miners.

• Cutworms are one of the many critters that do their dastardly deeds at night. They are moth larvae and emerge from winter to eat young seedlings and new plants. In the morning you’ll find plants chewed off at the soil line leaving only stubs behind. Sometimes if you dig around the base of the plants you’ll find a fat cutworm sleeping it off and you can remove him from the premises in any way you choose.

• Another nighttime visitor is the slug. If large ragged holes are appearing in your hostas and petunia leaves, the slugs have been dining there. The holes should have irregular shapes due to the slugs’ file-like mouthparts. Small seedlings can be consumed entirely. Slugs can digest tissues from most plants, but you might find them especially liking plants with broad, delicate leaves, like beans, lettuce, cabbage, bok choy, and tomatoes. The only tip they leave are the silvery trails of dried mucus they slid out on. 

• We’ve all seen green aphids on plants in the springtime, but every summer we get a crop of red aphids on our heliopsis plants. They are easy to dislodge with a strong spray of water and usually don’t come back.

• I love my swallowtail butterflies, but after hosting about 20 of their caterpillars, my dill patch is looking pretty ragged. Hopefully, it will have time to spring back before the cukes are ready to pickle.

Beetlemania

• Ever heard of the tortoise beetle? My sweet potatoes and morning glories share this pest. It has a hard shell and quietly rasps round holes in the leaves. Sometimes I find a few on the tomatoes and eggplants. In the past we have had the golden tortoise beetle which looks like it has been gilded with gold leaf—very shiny and too pretty to kill.

• Flea beetles are tiny black hoppers that eat small holes in the leaves making them look like they have been hit with mini-buckshot. Their larvae, white grubs with brown heads, are 3/4 inch long - much larger than the adults. They live in the soil and eat plant roots.

• There are two types of common asparagus beetles. One is black and white and appears very early in the spring. The other one is bright orange and it is active right now (mid-summer), laying eggs on the fronds which hatch into fat slimy worms that hungrily defoliate the plants. Like many beetles they tend to drop when when disturbed so stick a small container of soapy water under them and shake the frond a bit and they will fall right into it. The larvae cling on tighter and have to be squished. If you grow asparagus don’t ignore it over the summer. 
   
•  Cucumber beetles can be a challenge. You’ll see this striped fellow below, who likes to chew holes in leaves. If you ignore your squash plants, these beetles will completely defoliate a plant; you’ll see holes no the flower blossoms and even on the fruit. 
  
  The trick is to keep the young cucumber and squash plants covered with reemay to discourage the beetle from attacking the plants before they are strong enough to survive. I found our cucumber beetles at a loss, sitting on potatoes and tomatoes not finding much to eat. This kept the population down a bit so when we finally uncovered the plants as they started to blossom they were not overwhelmed with bugs.

• There are a few squash bugs starting to show up and lay eggs on the underside of the zucchini leaves. Squash bugs are sucking insects that suck the sap out of leaves. They are fond of the young seedlings of squash and pumpkins which are too young to withstand their attacks. If you catch squash bugs early, they cause little damage to plants in late summer and fall. The eggs hide under leaves; crush them or pick them and drop into a soaky bucket. Adult squash bugs are difficult to kill. Learn more about controlling squash bugs.

• Mexican bean beetles are something I have not had to contend with yet which is a blessing. One female can lay over 500 eggs! If your bean leaves are being skeletonized check the underside for clusters of yellow eggs and crush them. Handpick the beetles or knock them into a bucket of soapy water and squish or drown the spiny yellow larvae.

• We went a little overboard on seed potatoes this year so we have beds of potatoes growing all over the garden. No potato bugs so far but I keep checking. Colorado potato beetles will not only eat potatoes but other nightshades as well including tomatoes, eggplants, peppers, and petunias—that is most of my garden! Like the bean beetles they lay masses of yellow eggs on the undersides of the leaves. When the brick red larvae hatch they eat large amounts of foliage. By now you know the drill—handpick the adults and squish or drown them and their offspring.

Plant Diseases

While you are scouting be sure to check for signs of plant diseases, too. Hot, humid weather is perfect for encouraging fungal and bacterial diseases, so watch those tomato leaves.

• Powdery mildew will start showing up soon, too. It looks like it sounds—as if someone sprinkled talcum powder on the leaves. See how to control powdery mildew.
   
• If your cukes suddenly wilt and no amount of water revives them, you probably have cucumber wilt caused by the cucumber beetle. Remove and destroy these plants and start some new ones in another spot. There is still time for you to harvest some cukes before fall. Cover the young plants with a lightweight row cover until they start to flower. Late plantings often are not as bothered by the beetles as the early cukes are.

Be vigilant; if you stay ahead of the bugs, you will be able to keep them under control without resorting to using chemicals. 

For more pest and disease advice, check out our Pest and Plant Disease Library.