JoAnne Skelly
Are your trees dripping a sticky goo? The reason is that trees are often infested with aphids, scale or other insects that suck out sap and then exude a sticky residue called honeydew. Not only can honeydew damage paint jobs on anything on which it drips, it also attracts ants, wasps and other insects.
What can you do at this time of year as temperatures rise to reduce the pests causing the honeydew? With aphids, start by washing the tree a couple times a week with a strong jet from a hose.
Do this often because aphids have many generations a year. Females can birth live offspring without mating. Scales can be more challenging, especially if they are armored scales with a turtle-like shell. Soft scales can be treated like aphids.
Insecticidal soaps can be used for aphid or soft scale management with the least amount of damage to natural aphid enemies: parasitic wasps, lady beetles and lacewing larvae, among others.
Summer weight horticulture oils can also work but need to be used with care during hot weather and intense summer light, so follow the label or you can seriously damage trees. Both of these products kill by smothering and only work on the insects contacted on the day of application.
Scales are often controlled by beneficial predators such as lady beetles, lacewings or parasitic wasps. If you can apply insecticidal soap or summer weight oil as the scale crawlers emerge in early summer, you can get some control of most species.
For really serious infestations a systemic insecticide that is absorbed and then spread throughout the plant may be necessary. However, be aware that these products can also kill bees and many other beneficial insects.
Control the ants. Ants feed on the natural enemies in order to prevent them from reducing the honeydew food source produced by aphids, scales, etc. You can apply a sticky substance available at nurseries to a 2-6 inches wide wrap around the trunk.
Reapply the goop periodically to the trunk wrap. Don’t apply the sticky substance where birds might roost or to the bare trunk. Always maintain plants properly, giving them the water they need.
Avoid too much nitrogen early in the season because aphids and other insects like the tender succulent growth that high nitrogen encourages.
Dormant oil sprays after the trees lose their leaves in the fall and winter can kill eggs and overwintering populations of insects. For information: ipm.ucanr.edu/PMG/PESTNOTES/pn7404.html.
JoAnne Skelly is Associate Professor & Extension Educator Emerita University of Nevada Cooperative Extension. Email skellyj@unr.edu.