An annual spraying of backyard citrus trees with horticultural oil to control for citrus scale should be tops on the May garden chore list.
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It’s not always easy to identify adult citrus scale; they’re immobile and look like innocuous hard brown tiny bumps on twigs and leaf axils. What is easy to spot is their sticky ‘honeydew’ excretions and the ants that feed on the honeydew.
The hard shell of mature scale protects them from predators as well as from pesticides. Citrus trees infested with scale should be sprayed with horticultural oil in May when the young “crawler” stage scale hatch and move into their lifetime position on the tree. The crawlers are soft-bodied during the transition and can be suffocated by the horticultural oil.
Most horticultural oils are highly refined petroleum products, 92 percent to 99 percent pure, distilled to remove any toxic compounds. Plant-based horticultural oils are also available, including neem oil, soybean or cottonseed oil. These products are less refined, often less effective and may be more likely to burn plant tissue.
Make sure to apply summer-weight oils, sometimes called “supreme” or “superior” oils, rather than heavy-weight dormant oils. Summer-weight oils can be applied during the growing season when plants are in leaf; dormant oils can’t.
Horticultural oils smother soft-bodied pest insects including aphids, mealybugs, spider mites and whiteflies – and their eggs. The oils work only on contact with the insects, which means that all surfaces of the infested plant must be drenched with the oil. After the oil dries, it will not kill pest insects, beneficial insects or pollinators.
Although horticultural oils are considered a lesser-toxic product, they can cause skin and eye irritation to humans and are toxic to fish. Spray oils in early morning or late evening when bees are not actively foraging. Spray timing gets a little tricky in May; horticultural oils should not be sprayed when temperatures are in the 90s.
Here’s another reminder to check for the presence of the Asian citrus psyllid on your citrus trees at the same time you’re checking for citrus scale. The psyllid carries a fatal bacterial citrus disease, huanglongbing or HLB, which is a serious threat to the citrus industry in California. Citrus trees in the Central Valley are now under quarantine. There is no known cure for HLB and the disease is spreading as psyllids fly from diseased trees to feed on healthy trees. HLB can kill a citrus tree within 5 years. Yellowing leaves with asymmetrical blotching, branch dieback on one side of the tree, smaller fruit that remains green and has a bitter taste, and eventually extensive twig and branch dieback are all symptoms of HLB as the disease progresses.
Look for tiny brown winged insects feeding on flushes of new leaf growth in a “bottoms-up” position. New leaves may be twisted and folded. Also look for white waxy tubes produced by the psyllids to carry away excess honeydew. If you suspect that your backyard citrus is showing symptoms of HLB, call the CDFA Exotic Pest Hotline at 800-491-1899 or the Fresno County Master Gardeners hotline at 559-241-7534.
Elinor Teague: firstname.lastname@example.org