Q: Something is chewing my lima bean leaves. What should I look for, and how do I treat it?

A: It’s probably Mexican bean beetles, which feed on all types of bean plants and can have between one and three generations per year in our area, so they’re active for a long period. Adults and larvae chew on all above-ground plant parts, but the larvae are most damaging to leaves.

Fortunately, bean plants tolerate a surprising amount of defoliation (up to about 20%) before damage significantly impacts their productivity and vigor. Look on both upper and lower leaf surfaces, and squish any eggs or larvae found. At first glance, bean beetle eggs can resemble ladybug eggs, so if you’re not certain which they are, either clip that leaf off and keep it in a container to see what hatches out, or leave them alone and watch closely for the newly emerged juveniles.

If you find a heavy infestation that cannot be removed by hand, you could resort to using an insecticide containing the active ingredient spinosad, neem or pyrethrum. Make sure the product is labeled for use on edible plants, and follow all usage directions.

In future seasons, covering the seedlings or transplants immediately after putting them in the ground can deny these and other pests access to the plants. Either floating row cover or insect mesh netting can be used; the latter allows for more light and better airflow to reach the plants, especially in summer.

Our Mexican Bean Beetle web page has more information, images for identification, and management tips.

Q: My dogwood tree developed weird leaves this year, looking stringy (slender, stunted, and wimpy) and not producing as much growth. How can I help it to recover?

A: Unfortunately this sounds like herbicide damage, and in that situation, no treatment is possible. We’ve received a few queries this year about dogwoods and other trees and shrubs succumbing to phytotoxicity, which is plant tissue damage caused by chemical exposure. That chemical can be any type of pesticide, fertilizer, or other substance, which you can learn more about on our new page “Phytotoxicity: Chemical Damage to Garden Plants.”

Trees surrounded by lawns are at higher risk of exposure since their roots cannot avoid runoff from a lawn application of weed killer. Some chemicals in commonly used herbicides are more hazardous to use around tree and shrub roots than others. Once absorbed, nothing can be done to flush them out of the plant’s tissues or counteract their impact.

Plants not heavily exposed, or which are not very sensitive to damage, might recover in time, but some cannot and will need replacement.

The risk of harming desirable plants is one of several reasons why herbicide use should be a last resort, and product directions for use always be followed carefully. Avoid treating an entire lawn for weed prevention if only certain areas are weed magnets. This will reduce the volume of herbicide needed, as will working on lawn improvements that increase its density and vigor, allowing the grass to out-compete most weeds on its own. Areas where you can’t get grass to thrive should be transitioned into other plantings (refer to our Lawn Alternatives web content). Take note that some areas – to date, Montgomery County and Baltimore City – also have legal restrictions on what herbicides are permitted on lawns.

Use a mulch buffer around the base of trees adjacent to a lawn to protect them from direct exposure (for chemicals that trunk bark can absorb) and from accidental injury from mowers or weed-whackers, which can also cause serious and untreatable tree damage.

If you need to replant, you can probably do so without having to wait too long, but the persistence period of any herbicide residues will depend on what was used. Early autumn is a great time to plant trees if you prefer to wait.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.