Solving plant problems requires knowing what the enemy is

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By Ted Manzer

Friday, September 14, 2018

A couple weeks ago somebody brought me a Ziploc bag full of blueish gray bugs with orange stripes and spots on them. He told me they were killing his crape myrtles. Many of the leaves were falling prematurely.

He also brought a few leaves that were curled, sticky and discolored. Some of the leaves had a powdery black substance on them. Others were almost totally black.

I immediately asked him if he sprayed or in any other way tried to kill these bugs. He hadn’t. He wanted me to tell him what to spray and if the trees were even savable.

The insect specimens he had were lady beetle larvae. Most people call them ladybugs. In another week or so he would have recognized them.

They weren’t what was causing his problem. In fact, they were in the process of partially solving his problem. Some folks buy lady beetles specifically to eat pests on their plants.

Using predators to kill pests is a type of biological control. Sometimes people use other insects like parasitic wasps to accomplish the same goal.

The insects responsible for the damage to his crape myrtles were aphids, most likely crape myrtle aphids. They are small, nearly clear and often hard to see. Aphids are a favorite food of ladybugs.

The sticky substance was honeydew produced by the aphids. It became the food for the black powdery stuff called sooty mold. While sooty mold is unsightly, it doesn’t greatly affect the health of the plants. It usually shows up in fall when trees lose their leaves anyway. The aphids weaken plants by sucking fluids from leaf cells. Aphids also can transmit viruses.

So often what we perceive are our problems really aren’t. Sometimes plant leaves and stems on our houseplants start to wilt. Many folks immediately assume the plant isn’t getting enough water. They feel the soil and it isn’t dust dry. However, they water the plants anyway, assuming they’d rather be safe than sorry.

Their beloved houseplant usually winds up dying. The problem was not dryness. It was lack of oxygen to the roots caused by wetness and usually compounded by insufficient light.

This summer I lost a mature white pine. Someone suggested it must have been too cold last winter. However, white pines live as far north as central Canada, where winter temperatures often reach 40 below. Cold wasn’t the problem. Too much water was. White pines don’t like wet feet.

Until the end of August, most places around here have been much wetter than normal. Prior to last week I haven’t been able to mow parts of my lawn since early June. Even then I got stuck a few times and had to pull the mower out with the truck and a chain. Try that without getting covered with mud.

This has been a crazy year. From the start, we have experienced extreme cold, unseasonably warm late winter weather, and a wet spring and summer. Farmers and homeowners have had many problems to contend with. Misdiagnosis only adds to the problems.

Ted Manzer teaches agriculture at Northeastern High School.