By Hannah Fraser, Entomologist – Horticulture, OMAFRA and
Pam Fisher, Horticulture Fruit Specialist, OMAFRA
Scale insects seem to be an emerging pest in blueberries in Ontario and several states. We noticed at least three finds of scale insects last year and reports are coming in from new locations. It is possible the spray programs used to manage spotted wing drosophila are having an impact on natural predators and parasites that would normally help keep numbers down*.
There are several species of scale insects affecting blueberries in Ontario, including both soft (Azalea scale, Eriococcus azalea, and Lecanium scales, with multiple species including terrapin) and armored scales (Putnam scale Diaspidiotus ancylus). Scales have piercing-sucking mouthparts. Heavy scale infestations can weaken blueberry plants and reduce yield. Soft scales feed on woody tissue, stems, and / or leaves. Soft scales can produce large amounts of honeydew (attractive to ants), which results in the development of unsightly sooty mold. Putnam scale can cause defoliation, decline and death of blueberry plants if populations are sufficiently high. It will also feed on fruit (Figure 1), causing direct damage, and on leaves (Figure 2).
Figure 1. Putnam scale on fruit. Each dot is a single scale insect.
Figure 2. Putnam scale on leaves.
Mature Lecanium scales are often brown with a rounded or helmet-like appearance, 6-8 mm in length at maturity (Figure 3). Azalea bark scale is most easily recognized by the white cottony sacs (Figure 4) that protect the developing pink eggs underneath (Figure 5). Putnam scales are small (1-2 mm at maturity) (Figure 6) and have a flattened, plate-like cover that can be removed to reveal the insect underneath. When numbers are high, they appear crust-like. In contrast, the “helmet” of soft scales is part of the body wall of the insect and cannot be removed; if you flip off the cover you will remove the entire insect (possibly revealing eggs or newly hatched nymphs). Putnam scale is easy to miss on old bark but the small grey dots are fairly obvious on leaves and fruit, or on new growth.
Figure 3. Lecanium scale, unidentified species.
Figure 4. Azalea scale egg sac.
Figure 5. Azalea scale – pink eggs under protective cottony covering.
Figure 6. Putnam scale on new growth.
Check branches for signs of scale insects. Scales overwinter on older, woody canes. Look at new growth as well as older wood. Plan to prune well next winter – regular pruning of old canes is important in keeping scale under control – and apply dormant oil in early spring. Thorough coverage is essential. The third step for lecanium scale control is application of Movento insecticide, after bloom but before harvest (7 day phi). The optimum timing is when scale insects are producing “crawlers”, the mobile first instar nymphs that leave the protection of their (now deceased) mother’s shell. These crawlers spread to other parts of the plant, or nearby plants, in search of places to settle and feed. Crawlers are tiny can be difficult to spot; can use double-sided tape to help monitor their activity.