Pear slugs, frost could be to blame for lack of fruit

We have two cherry trees in our yard and have rarely gotten any cherries from them because of a slug-type bug. We were advised to spray them with dormant oil, but we don’t know when to do that. Also what type of dormant oil do you advise?

We have one tree each of Bing and Black Tartarian and we do get a very few Bing cherries each year. It doesn’t seem to get as many of the slug-type bugs on it.

— Cathy

It sounds like you have a problem with a pear slug. Pear slugs are actually the larvae of a particular type of fly and they look like small slimy, dark green slugs. They feed on the leaf tissue of certain plants, leaving a dry brown network of veins, creating a “skeletonized” appearance.

Pear slugs are mostly an aesthetic problem. They rarely hurt the plant; they just make it look ratty. We tend to see them most often on pears, cistena plum, cherries, hawthorn and coton-

There tend to be two generations of this insect each year. The first and most “destructive” generation appears in late spring to midsummer. A smaller, second generation occurs in late summer or early fall.

Pear slugs are easy to control with just about any contact insecticide, such as Permethrin, Malathion, Sevin or Spinosad. Most people just keep their eyes open for pear slugs to show up and then react to their presence. One spray will usually clear up the problem, but keep an eye out for the second generation later on in the year.

Although I’m a big fan of using dormant oil sprays to control a wide range of insect and disease problems, in this case dormant oil spray isn’t going to be all that helpful. This insect overwinters as a pupa in the soil so spraying the twigs isn’t going to really do much for this problem. It may help with some other issues on cherries such as aphids.

If you want to do a dormant oil spray, the time to do it is mid- to late March (whatever you do, make sure that the spray is applied before bud break). Use a dormant oil formulation that you can get at any garden center and spray the branches and twigs of the tree, making sure to thoroughly coat everything.

You can heighten the effect of a dormant oil spray by mixing it with an insecticide or fungicide (or both), depending on the problem you’re targeting.

I’m a bit skeptical that the pear slugs are to blame for poor fruiting on your trees. Severe pear slug infestations year after year can reduce plant vigor, which can affect fruiting, but poor fruit set on a cherry tree is usually because of either the lack of a pollinizer, an inappropriate pollinizing variety or late frosts.

Most sweet cherries require another sweet cherry variety to properly pollinize and set fruit. For a Bing, the best pollinizers are Black Tartarian (such as yours), Stella, Van and Black Republican. All of these pollinizing varieties also will bear delicious cherries.

You already have a good variety, so I’m wondering if late frosts are affecting you. If you really want to cover your bases, consider planting a third cherry tree of a different variety from the two you already have.

Dennis Hill is the nursery manager at Bookcliff Gardens, Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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