Last week I wrote about soft versus armored scale insects. Soft scales exude honeydew, are protected by ants and can be controlled by systemic insecticides, some of which have the active ingredient imidacloprid. Armored scales do not emit honeydew and are not managed by an imidacloprid insecticide.

Imidacloprid is a systemic neonicotinoid insecticide that is designed to kill piercing/sucking insects via damage to their nervous system. Systemic means it is applied on the leaves, stems or as a soil drench and spreads internally throughout the plant. When an insect feeds on any part of the plant such as the pollen, nectar, flowers, fruit, leaves, or other plant tissues, it ingests the toxic chemical and dies.

Systemic insecticides can kill not only pests such as soft scales, but also their natural enemies and non-target/beneficial insects such as bees and other pollinators, including butterflies, moths and their larvae. They also can “adversely affect reproduction, growth, insect immune systems, learning, flying, or other attributes even at concentrations too low to cause death outright.” Native bees, who often nest in the ground, can be more susceptible than honeybees, particularly to soil drench applications. 

Systemic insecticides kill over time at a steady concentration, which can cause a longer toxic exposure. Although it is often suggested to apply in the evening when insects aren’t out, or after flowering season, harmful exposures can still occur. These chemicals can last for months or years in soil. They sometimes leach into groundwater but fortunately are usually only slightly toxic to fish. One application may provide season long control.

Chemical insecticides are always a last resort. The risks versus benefits must be carefully evaluated. A healthy thriving plant is one of the best defenses against scale or other insect damage. Good watering, the appropriate amount of fertilizer and maintenance keep a plant resistant to attacks. Predators, including lady beetles, bugs and lacewings, play an important part in pest management.

Plant a diversity of flowering plants to attract and feed natural enemies. Sometimes when you see scales on a plant, they may have already been parasitized by parasitic wasps. If a large number are parasitized, there are eggs in or on each scale eating away at them. Sometimes scales are no longer alive and do not require a pesticide treatment. Check before applying insecticides. Using tape traps to monitor crawlers will help you accurately time the use of horticulture oils, Neem oil or insecticidal soap. Dormant oil treatment on deciduous woody plants will help kill overwintering scale pests and reduce future populations.

JoAnne Skelly is an Associate Professor and Extension Educator, Emerita, University of Nevada Cooperative Extension. She can be reached at skellyj@unr.edu.

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