Scale Insects

Problem type: Insect

Name of problem: Scale Insects

Plant name(s): Nearly all plants can serve as a host for scale insects

Symptoms / Characteristics:
Scales are grouped into two categories: soft and armoured scales. Large amounts of honeydew are secreted from soft scale insects. This honeydew is clear and leaves a sticky residue on leaves, branches and other plants and objects nearby, and is also a tasty food source for ants to feed on. The honeydew promotes the development of a black fungus or sooty mold. Armoured scales do not produce honeydew. Heavy infestations of scale insects can cause leaf and needle yellowing, premature drop and can weaken or kill stems and branches. Light infestations do little harm to the host. Younger plants may be killed if infestations are severe over long periods. The pine needle scale is an armoured scale that attacks spruce and pine species making them appear to be splattered by white paint. The white flecks are the scale insects.

Scale insects are round to oval and flat. They can be orange-brown in colour or whitish, and sometimes covered in a powdery coating. Most species lay eggs in the spring (May), and the young hatch in June. Females can produce up to 1000 eggs each. The young are called crawlers, and travel only short distances after hatching. They can be found in masses on certain branches. The insects feed on the plant sap using their piercing-sucking mouthparts. The insects remain fixed in place over most of the summer with little growth development. They have a slightly convex shape and darken in colour as they mature. These nymphs remain on the plants through the winter. Growth resumes in the spring, scales mature and eggs are laid. Females enlarge, appearing round and swollen just prior to egg laying. The female then dies and the eggs develop and remain under the dead scale until the crawlers emerge.

Control / Preventions:
Immature crawlers are the most vulnerable to chemical sprays. Adults are protected by a hard body wall or cuticle that is developed not long after emergence. Application of an insecticide should be done just after eggs hatch (late June - early July). Most eggs hatch within a short period of time making control easier. Repeat applications may be necessary as some eggs may hatch at a later time.

A dormant oil or horticultural oil can be used before trees and shrubs begin to leaf out in the spring. Once crawlers emerge, a contact insecticide can be applied at weekly intervals. Make sure that the entire plant is covered including the inner branches. Summer oils or horticultural oils can be used in replace of chemical sprays during the summer but should be restricted to when crawlers are visible. Applying oils too frequently can clog the plants breathing pores which may damage leaves and bark.

Predators such as parasitic wasps and ladybugs are natural enemies of scale insects. Looking for these insects before applying chemicals should be done to avoid unnecessary spraying.

Keeping plants in generally good health by properly watering, fertilizing and pruning will help plants combat infestations. If population levels get too high, plant health can become affected. Infested areas may be pruned out if damage to the plant is severe and portions of the plant are dead or dying.