Oil sprays can help eliminate scale pests

I have a scale insect infestation on my gardenias and citrus. I have sprayed with an oil spray a number of times. I think I did some good, but I definitely did not eliminate the problem. What am I doing wrong?

I have a scale insect infestation on my gardenias and citrus. I have sprayed with an oil spray a number of times. I think I did some good, but I definitely did not eliminate the problem. What am I doing wrong?


Good question. Many times oil sprays are recommended for the control of scale insects. Here, a more complete explanation would be helpful.

Many pesticide labels will give a time range in which to reapply the material, for example, in five to seven days. The reason for repeat applications is that certain stages of growth, like the egg and pupae stage, are not susceptible to insecticides as are the other stages of development. Consequently, eradication of a scale population, as well as others like whitefly, may require as many as four applications. The five- to seven-day interval is to give time for the non-susceptible stage to develop on to a susceptible one.

Another factor is that horticultural oil is most effective when sprayed soon after the young scales, called crawlers, have emerged. They are susceptible whereas older, usually darker-colored adults are much less vulnerable to the oil. In fact, if the plant is sprayed when the majority of the insects are full grown adults, the results will be poor. Many of these adults may actually be dead, with just eggs underneath the shell. A quick crushing of the scale with your fingernail will show if the insect is alive or dead.

Be sure to apply a thorough coverage of the oil solution to both the top and underside of the leaves. In addition, it is prudent to make an application as soon as the insect pest is detected. Do not apply oil when it is over 90 degrees. Drought-stressed plants can be injured when sprayed with oil.

Concerning horticultural oils themselves, they are specially refined petroleum products, often called narrow-range or supreme oils. Some plant-derived oils are also available. One study has shown that the harsher organophosphate insecticides, like malathion, were no more effective than a thorough spray of oil or insecticidal soap alone, when properly timed. Another study found spraying oil on foliage once during the late spring was as effective as using a systemic organophosphate (acephate). This is certainly noteworthy!


Referring to my June 18 article on cardboard mulches, J.P. writes about another use:

“Read your article which talked about cardboard. I have yet another use. I use it both as a mulch/weed blocker/helpful seed starter. I take large boxes and flatten them, then make circular holes at the right spacing for larger seeds (okra, soybeans, etc.). I make sure the ground underneath is relatively level, place the cardboard on it, and plant the seeds through the holes at the right depth. Works like a charm. Have to say that first I have covered the area with some mulch, so the weeds are fairly well under control already. Keep up the good work.”


We have a jackfruit tree with fruit on it. The fruit is pretty large, but I don’t know if they are ripe yet. Can you tell me how do you know when to harvest the jackfruit?


The jackfruit (Artocarpus heterophyllus) is a relative of the common breadfruit (Artocarpus incises) and comes from India. It produces some of the largest fruits known: a record 144 pounds from India. In Hawaii, they are smaller, averaging around 35 pounds, with a record size of 79 pounds. They are borne along the trunk and older branches. Harvest season will vary, of course, depending on location. The fruit will mature six to eight months after flowering. Factors which indicate that harvest is near include a hollow sound when tapped, change of skin color, increased odor and a flattening of its spines.

Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at askthegarden guy@earthlink.net. You also can visit his website at www.gardenguyhawaii.com.