Identify pest before applying chemical pesticide

Q: I thought my prayers had been answered several years ago when I came upon Bayer Tree and Shrub. I've applied it to ornamental plum and cherry when the leaves plainly showed that something was voraciously munching on them. Does this practice have an adverse effect on bees (colony collapse)?

Also, does this product work on the light brown apple moth? In my nasturtium and rosemary, I am always finding lime-green leaf curlers. Are these apple moths? You have previously suggested using Bacillus thuringiensis kurstaki to control the moth.

Also, I have an Italian cypress whose branches are being attacked and killed, one at a time. The cypress has what looks like two trunks and the one suffering the branch loss secretes sap. I cut out what had died and then aggressively sprayed, yet the death continues. Also, the ailing trunk is a bit sunlight-deprived.

A: Before you apply any pesticide, you should identify the pest, and then see if it is listed on the pesticide label. Because you haven't identified either of your pests, your efforts may be useless. But before I deal with whether you have the right chemicals, I will answer your question about bees.

I see from the fine print on the Bayer product label that the active ingredient is imidacloprid. This chemical is definitely toxic to bees. Because it is systemic, it will be in every cell of the treated plant. Bayer claims there isn't much of it in the pollen, but the jury is still out on this one. Bee die-offs are being studied, and the contribution of imidacloprid is being considered. Researchers feel that colony collapse is probably due to a combination of disease and stress factors such as pesticides.

Imidacloprid in soil is highly toxic to the tiny creatures that live in water ecosystems, serving as the base of the aquatic food chain. It is water soluble and persists a long time in soil or water. It would be particularly likely to pollute nearby water when used as a soil drench, as in the Bayer product.

Now, is the Bayer product helping your trees? Its label lists many insects, mostly ones that suck plant sap, but not caterpillars. If the leaves have large holes or rolled and chewed young leaves, evidence of caterpillars, your efforts are in vain. If by "voracious munching" you instead mean areas where the leaf has been eaten, leaving only the veins, you could have a pest that the Bayer imidacloprid product does kill, the pear slug. It's the black, slug-like larva of a sawfly. (However, a hard spray of water is often enough to significantly reduce its damage without resorting to pesticides.)

The caterpillars on your rosemary could be light brown apple moths, but I found no reference to the moths eating nasturtium. In any case, if damage is light, the best course of action is just to pick off infested leaves and caterpillars. Use the biological pesticide Btk only for severe infestations. It is harmless to bees. It is most effective while caterpillars are young.

Your cypress is probably infected with cypress canker. Italian, Monterey and Leyland cypress are highly susceptible to this fungus disease, especially when grown in warm summer areas, which in the Bay Area means away from the coast. The fungus plugs the conductive tissue of the branches, causing their death. You need a diagnosis before you try to solve the problem, but, if I am correct, your best option is probably to prune the tree well behind any diseased tissue. In your case this may mean removing the diseased trunk altogether, before the fungus reaches the other trunk.

I strongly urge you to have a certified arborist assess the tree. She or he can identify the disease, decide if the other trunk is healthy, consider all options, including radical pruning, decide if the tree will be structurally sound with the diseased trunk removed, and, if so, remove it safely.

If you can save an uninfected part of the tree, be sure it has adequate summer water. Also, check to see if the bottom of the trunk flares out a bit. If not, it is possible that the lower trunk was buried when you planted it, which makes a tree more susceptible to disease. If there is a flare just below ground level, you can dig soil away to expose it.

Inland, rather than planting any kind of cypress, gardeners should consider Western red cedar (Thuja plicata), which is immune to cypress canker. Near the coast, Western red cedar and incense cedar (Calocedrus decurrens) are safe options.