I am not ready to call it winter yet, but the onset of cooler weather brings the opportunity to complete some tasks best not performed in the heat of summer.
The classic example is applying oil sprays to woody plants suffering from scale insect infestations. There are a number of effective insecticidal oils available. They all work the same way, and all need the same close attention to application technique to make them work. When scale insects hatch in the spring and summer, the new “crawlers” search the plant and stab their hypodermic-needle-like mouths into the plant, searching for a sweet spot. If we really are good, we can scout for the crawlers and control them by applying one of several insecticide sprays, but the window to apply is short. Once the crawlers find a spot they like, they hunker down and cover themselves with a waxy coating that sticks them to the leaf and protects them from losing water and from water-borne pesticides.
The crawlers are vulnerable while they are walking around on the plant, but once they wax themselves into place they are impervious to nearly all the insecticides we mix in water in our sprayers. During the growing season, we get around this by applying systemic insecticides to the soil that the plant roots take up and deliver to the scale in the plant sap. With the systemics, we have to beware of developing resistance to the pesticide in the insect population. But the tried-and-true method of scale control is one to which scale likely will never develop resistance: horticultural oils. Volck is the oldest and probably the most-widely recognized horticultural oil, but there are several others that have excellent performance in their control niches.
Horticultural oils typically are dispersed in water and applied with a sprayer to the plant. The oil covers the insect and smothers it. The insect cannot breathe and dies of suffocation. No poisons are involved. The trick is to make sure every scale insect is covered by the oil. If the oil does not cover the scale, the scale escapes to breed more scale. Scale can be found all over the plat, not just the tops of the leaves. The underside of leaves is a favorite place to look for scale. However, inside leaf axils, next to twig buds and even down the trunk and along the roots under the soil can be home for scale.
Probably the worst scale we are facing is the cycad scale that assaults sago palm. This scale can cover the plant top to bottom and kill mature sago in as little as six months. The males look like little cigarettes under magnification, while the females look like little oyster shells. The two sexes tend to segregate themselves, with males on the underside of the frond and females on the top of the frond, at least while there is room. Heavy infestations will turn the sago into an entirely white plant. It will look like someone sprayed the plant with artificial snow used for decorations. A good oil spray in the winter can be the difference and save a plant with a severe infestation of scale.
Heavy horticultural oil sprays, like Volck, are applied in the winter because the oil not only smothers the insect, but also slows down water loss and gas exchange in the plants towhich it is applied. Plants have to lose water in order to stay cool in the summer, so summer applications of heavy horticultural oil sprays literally can make your plants cook in the heat.
Some of the newer, lighter oils and paraffin-based sprays are called “summer oils” because they do not interfere with the plants as much, so summer application is practical. But if I had a sago palm with cycad scale on it, I definitely would opt for a good, full-contact horticultural oil spray in the winter. This oil sticks really well, so move the car away from the shrubs before you spray them.
Gardner lives in Keller and is the UGA extension agent for Glynn County, serving South Bryan.