Catch 'em napping

I hear a lot about dormant oil sprays, but I've never used one, and I don't know anyone who does. If they're effective, why don't more people use them? — via the Internet

Spray oils' effectiveness is limited primarily to scales, aphids, mites and a few other pests. Even in these groups, not all species are susceptible, so their appropriateness as a pest control is restricted. Nevertheless, it's probably fair to say that pest control operators are less familiar with oils than they should be.

Oils control pests by suffocating them under a layer of oil. They are contact products, and so are only as effective as the coverage is complete. With no residual activity, initial control is probably going to be less than what you're accustomed to with more standard pesticides. However, spray (paraffinic) oils have low toxicity, are nearly odorless, have few negative effects on beneficial insects and best of all, they even work on eggs. So they definitely have their upside. Oils can be useful in chemical rotation programs and are often tank-mixed with conventional insecticides to add residual control.

In my personal experience, spray oils work about as well as conventional pesticides on spider mites during the growing season. It may be that what they lack in initial control they make up for in the fact that they are effective on eggs, a real benefit when dealing with mites.

Winter has traditionally been the recommended time to apply oils because of the phytoxicity that could result from hot weather applications. Plus, without foliage, you also can get excellent coverage of twigs and branches, which you need when you're targeting certain scale insects. As a dormant spray, some experts suggest that you get the best control by applying oil in early spring rather than the dead of winter — after weather begins to warm, but before plants leaf out. This is typical of fruit tree growers, who routinely use dormant sprays.

Newer formulations are more highly refined and can be safely applied in most weather conditions (except for the hottest summer days). This concept is relatively new, and still catching on. One caution: If you're shopping for a spray oil, you're likely to run across both types of oil — those that can be used most times of the year, and those that are strictly for dormant use. Be sure you read the labels!

Bad gas

I know I'm not supposed to use old gas in my equipment. But I'm also not supposed to dump it. So what do I do? — Indiana

Try contacting local recycling and waste disposal agencies. Often, you'll hear of local hazardous waste collection programs that make disposal facilities available for a few days each year. That is the suggestion of the Alliance for Proper Gasoline Handling (; 617-443-1321).

The Environmental Protection Agency (EPA) offers tips for reconditioning gasoline. This involves straining the gasoline through filter paper or cloth, then mixing the old gas with new gas in a 1:5 ratio. Octane booster — available at most auto parts stores — can be added to the mix if necessary (in addition to the deposits it leaves, a problem with old gas is that it doesn't burn as readily).

The EPA offers more details at

The recommendation of mixing old gas with new is one I have read in other places as well, so perhaps it has merit. Robert Sokol, a certified ASE Master Mechanic and editor of the ABOS Marine Bluebook and Recreational Vehicle Bluebook value guides, agrees that dilution is a relatively harmless way of ridding yourself of old fuel. However, Sokol suggests that a greater dilution ratio, such as 1:20, may be a bit safer.

It deserves repeating, however: To avoid the problem in the first place, use fuel stabilizer! It's cheap, simple to use and extends the shelf life of gasoline to many months.

Hydraulic oil change intervals

What is a good interval for changing hydraulic oil? — via the Internet

The short answer is, whatever the service recommendations tell you, or as needed. Regarding the latter, it's possible that hydraulic fluid can go bad before the recommended change interval if your equipment has some problem that causes overheating or contamination.

One common type of contamination is water. Water in your hydraulic fluid will appear milky (not unlike the appearance you'll see if air is finding its way into the system). To confirm the presence of water, take a small sample in a can and heat it over a flame. If it pops and crackles, you have water in the system.

Hydraulic fluid can lose its lubricating qualities if it overheats badly enough. Check the temperature of the fluid periodically during operation to see if it is excessively hot. You might also be able to smell a burnt odor in the fluid. These are signs that you may have problem in need of a fix. Once you do so, change your fluid as well.

Eric Liskey has a bachelor's degree in ornamental horticulture and a master's degree in botany. He has been licensed for pesticide advising and applications in California and Missouri, and has more than 10 years combined professional experience in landscape installation and maintenance, nursery retailing, pest management and botany.

If you have a question about landscape or turf management, write to “Researching Maintenance,” Grounds Maintenance, P.O. Box 12901, Overland Park, KS 66282-2901, or send you question by e-mail to

Questions are selected on the basis of current general interest. Unfortunately, we are unable to guarantee a response to individual letters.

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