Gardening with Allen: Identify, manage garden pests
By Allen Wilson
Published: July 30, 2022, 6:05am
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This rhododendron leaf shows discoloration, along with lace bugs and their excrement. (Allen Wilson)
I have noticed chewed and discolored leaves on my shrubs but I don’t find any bugs. What could be causing this damage?
It could be that you are not looking at the right time or the right place. Or the damage may be caused by fungus or other diseases. Two of our most common chewing pests are slugs and snails, which feed at night and are not present in the daytime. Most sucking insects are on the bottoms of leaves. Then there are flying insects that feed and move on to the next shrub.
If you notice discoloration, always check the bottoms of leaves. Some pests are so small that you need a magnifying lens to find them. One of our most common bottom feeders are lace bugs, which cause yellowing of rhododendrons and azaleas. Plants deteriorate over a year or two and eventually die if the pests are not controlled.
Aphids (also known as plant lice) are easily noticed because they are concentrated on new growth, especially roses. They are usually green, but can also be pink or black. Aphids exude a sweet juice that drips on the ground. Ants are often associated with aphids because they use the juice for food. If you notice a few ladybugs present, don’t be too quick to spray. They will probably take care of the problem for you.
Mites cause mottled yellowing of leaves. Hold a white piece of paper under affected growth and shake. The plant has mites if you see tiny, moving specks on the paper. One of the best ways to control them is to wash them off by adding a little detergent to water in a sprayer. This works for aphids also. Once they are on the ground they can’t find their way back up on the plants. However, there may still be eggs that are more firmly attached that will hatch in a few days. Make a second application after 10 days to catch this next generation.
Leaf spots and rust or mildew discoloration are fungus diseases. They favor moist overnight conditions on the leaves, which can be caused by irrigation in late afternoon or early evening.
Snails and slugs are best controlled by baits. Metaldehyde baits are very effective. However, slug baits containing iron phosphate are safer to use around children and pets.
Two of my favorite organic pesticides that are very safe to use are neem oil and spinosad. Neem oil controls many common sucking insects and also controls most leaf diseases such as mildew, rust and black spot. Spinosad controls many chewing insects, including apple worms and cabbage worms that feed on all cabbage-family vegetables including broccoli and cauliflower. Repeat applications at biweekly intervals are necessary in both cases. Spinosad is effective on lace bugs and also has systemic properties so it can reach inside plant tissue and kill insects like borers.
Both products are also toxic to bees and should not be applied when they are pollinating flowers. There is little bee activity in the late afternoon and early evening.
