Although you can't see or hear them, aphids, mites, and scale are already plotting the overthrow of your fruit trees. They are holding secret meetings and planning their strategy in tiny snack-filled bomb shelters around your garden.
Most aphids, mites and scale overwinter on trees in the egg stage, explains Jerry Goodspeed, Utah State University Extension horticulturist. As the weather starts to warm, and the leaves and flowers begin to grow, they emerge prepared for their onslaught.
"Some people don't think it makes sense to control these insects and pests now, when most of their damage is done later in the year," Goodspeed says. "However, now (mid April) , and in the next couple of weeks, is the best time to reduce the populations of these pests so they won't multiply and cause problems later in the summer."
He says the best control method this time of the year is to apply a dormant oil spray to the trees just as the buds start to swell and show a little color. Dormant oils, sometimes called "horticulture oils," are highly refined petroleum oils specifically designed and manufactured to control nasty critters.
"Motor oil, cooking oil, or any other oil is not recommended for application on trees," Goodspeed quips. "I once knew someone who applied motor oil to their tree thinking it was the same as a dormant oil. What a mess. Luckily the tree didn't die, but it sure wasn't happy."
Dormant oil should be applied to trees just as the buds are beginning to break, he says. This is why it is also called a "delayed dormant oil application." Peaches and cherries blossom first, so the emerging color will be pink. With apples and pears it's the green tips of the leaves that first appear.
"Dormant oil works as a physical poison, actually smothering the overwintering stages of aphids, mites, and scale. An insecticide such as Diazinon or Malathion can be added to the dormant oil which increases its effectiveness. The dormant oil penetrates any waxy barrier on the outside of the insect, allowing the insecticide to kill the pest," Goodspeed says.
To be truly effective, he explains that dormant oil needs to completely cover the area of the tree where the pests are found. This includes the entire canopy of the tree, including all branches and twigs.
"Do not apply the dormant oil to the trunk of the tree below the lowest branches. Good, helpful mites, called predator mites, overwinter on the lower part of the tree. These guys help control any bad mites that venture onto the tree later in the year," Goodspeed says.
Dormant oil may not kill all the pests out there, but it reduces their numbers and reduces the rate at which they can multiply, he says. Aphids, left untreated in early spring, multiply quickly and protect themselves by curling the leaves around the area they are eating. Most sprays never reach them as they eat in the protected comfort of a curled leaf.
For more information, contact your local USU County Extension office.
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Issued in furtherance of Cooperative Extension work, Acts of May 9 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Robert L. Gilliland, Vice-President and Director, Cooperative Extension Service, Utah State University, Logan, Utah. (EP/09-98/DF)