Scale Insects in Orchards

Table of Contents

1. Introduction
2. Description
3. Damage
4. Biology
   • San Jose Scale
   • European Fruit Scale
   • Oyster-Shell Scale
5. Management
6. Monitoring
7. Economic Threshold
8. Control

Introduction

Scale insects were serious pests of apple in Ontario in the past but have become less common with the introduction of dwarf rootstocks, improved spray technology, and the use of Superior oil applications for European red mite. Numerous species exist in Ontario including the oyster-shell scale, Lepidosaphes ulmi (Linn.) (Figure 1), lecaniom scale, Lecanium sp. (Figure 2), San Jose scale, Quadraspidiotus perniciosus (Comstock) (Figure 3), and the European fruit scale, Q. ostreaeformis (Curtis) (Figure 4), along with many other minor scale species.

Figure 1. Oyster-shell scale

Figure 2. Lecanium scale

Figure 3. San Jose scale

Figure 4. European fruit scale 

In Ontario, San Jose scale is the most serious scale pest. This scale is capable of causing direct injury to the fruit and/or injuring the host tree by sap feeding, thus reducing tree vigour and possibly causing death of limbs.

| Top of Page |

Description

These insects are soft-bodied homopterans. They protect themselves by living under a waxy shell (exuvia) that is wind- and water-repellent and protects the insects from predators. While the waxy exuvia is present, the scales are incapable of movement on or about the host (Figure 5).

Figure 5. Adult female San Jose scale with shell removed.

Female scales do not lay eggs but give birth to living young. Crawlers are minute, orange-yellow and oval, with six legs and one pair of antennae.

Sexes are differentiated by body form and shape of the wax exuviae. Beneath the waxy shell, males are elongated with pale yellow bodies, dark eyes and a long projection (caudal style) from the base of the abdomen.

In contrast, the females are dark yellow with an oval shape tapering to a point at one end. The outer shell of female San Jose scale remains relatively circular with a nipple, while male exuviae is elongated with the nipple offset to one end, while oyster-shell scale are elongated.

The females are nearly round and about 1.6mm across with a raised nipple in the centre, and they remain under their scale coverings their entire lives. The males are oval, about 1mm long and half as broad, with a raised dot near the larger end of the scale. It emerges form the scale as a small, yellow, two-winged insect (Figure 6).

Figure 6. Adult male San Jose scale.

| Top of Page |

Damage

Heavy infestations of scale insects (Figure 7), particularly on young trees, can seriously reduce tree vigour and even death of whole limbs. Injury from San Jose scale can be confirmed by cutting away the bark to revel a reddish discolouration (Figure 8).

Figure 7. Heavy scale-infested limb.

Figure 8. Discolouration of cambial tissue due to scale feeding.

More often encountered and more economically significant, is injury to fruit. Feeding by San Jose scale on the apple surface results in small red blotches with a lighter centre, most often near the calyx end of the fruit (Figure 9).

Figure 9. San Jose scale near calyx end of the apple.

| Top of Page |

Biology

San Jose Scale

There are two generations of San Jose scale in Ontario each season. This species overwinters as a first instar nymph beneath a thin waxy shell. This stage is referred to as a "black-cap" due to its physical colouration. These blackcaps are found primarily on rough wood or beneath the bark scales on scaffold limbs near the trunk, and on young, poorly sprayed wood in the top interior portion of the tree. Just prior to bud break (late March/April) the immature scale begins to feed on tree sap and starts to exude additional layers of wax. By bloom, the waxy covering has developed to the point where male and female scales can be distinguished.

Male emergence and flight lasts approximately 14 days with peak activity usually corresponding with full bloom on McIntosh. Females remain fixed in place on the host and the males must seek out the females for maturing. About four to six weeks after mating the female gives birth to numerous live young referred to as "crawlers". The crawlers are the only stage other than the adult male capable of moving about the host or from host to host. The adult female will continuously release live young (about 400 in total per female) over a period of four to six weeks (late June to early August).

The crawlers disperse over the host until they find a suitable feeding location (bark or fruit). Due to their small size and weight, crawlers may disperse to different host plants by wind, other insects or ever birds. Generally most crawlers do not travel more than 1 metre from the parent female before settling into a feeding position. The establishment of the feeding position occurs from 48-72 hours of birth. Each crawler insets their feeding tube (rostralis) into the host tissue, starts feeding and immediately begins excreting waxy filaments that will form the protective scale covering. The initial wax shell is visible within 24 hours of first feeding.

These newly affixed San Jose scale nymphs will complete their development in six weeks, giving rise to a new generation of adults in early August. mating and gestation are much shorter, with this second generation of females giving birth to crawlers from mid-August until late October. In most orchards it is this generation of crawlers that infests the fruit, causing direct economic loss. Having two generations annually allows for a large increase in the San Jose scale population and greater potential for economic injury to the crop.

European Fruit Scale

The European fruit scale has a life history similar to that of San Jose scale, but differs in having only one generation each year. The host range for European fruit scale is more limited, infesting primarily Malus (apple) spp., Pyrus (pear) spp., and Prunus (tender fruit) spp.

Peak flight of male European fruit scale occurs around petal fall (cv. McIntosh). Crawlers begin to appear in late June and continue to be release by the adult female for the remainder of the season (i.e., late June until late October). Each female European fruit scale gives birth to about 400 nymphs.

| Top of Page |

Oyster-Shell Scale

Unlike the previous two species, the oyster-shell scale overwinters as eggs beneath a wax scale secreted by the adult female the previous season. Egg hatch begins just prior to bloom of McIntosh and continues from petal fall onward into the early summer. Crawlers move about for 48-72 hours, then affix into position.

By mid-August, the insects beneath the scales have matured into adults, with males emerging to mate with the nonmotile (immobile) females. Shortly after mating the females oviposit the overwintering eggs then die. There is only one generation of oyster-shell scale each season, and this scale species is considered a sporadic minor pest.

Management

For most growers, the observation of scale-damaged fruit the previous harvest is the first indication of a scale problem. Scale-injured fruit is identified by the red "measles" or spots often most numerous around the calyx end of the fruit. On fruit with light yellow-green skin colours such as Golden Delicious and Mutsu apples, the spots are easily visible.

| Top of Page |

Monitoring

Since scales are not readily mobile, it is possible to identify locations or hot spots within an orchard and apply corrective measures before the entire orchard becomes infested. Fruit cultivars with rough bark are often the first infested as the bark crevices provide excellent refuge for the scale insect from pesticides and predators. During pruning the grower can identify the presence of scale on the bark. Heavy infestations on the wood will give the bark a rough, sandpaper appearance. Using a sharp instrument one can remove the bark and reveal a discolouration of the wood beneath.

San Jose scale can be identified by a purple staining of the wood, while European fruit scale causes more of a water-soaked brown stain. On apples the best cultivars where Superior oil is not regularly applied.

Crawler activity can be detected using black electrician's tape reversed, with the adhesive side out and banded round the scaffold limbs of trees with known infestations. These bands should be placed out by mid-June for 7-10 days after petal fall and replaced every two to three weeks throughout the season. Crawlers are identified as their yellow bodies contrast with the black tape. The benefit of this effort is small as insecticide applications to control crawlers is not recommended.

Economic Threshold

There are no scientific-based economic thresholds for scale insects on apple. In most cases any fruit injury at harvest will warrant corrective measures the following spring.

| Top of Page |

Control

The first step in successful scale insect control is prevention. Carefully examine all nursery trees prior to actual planting. If scale are present, discard trees or exchange them for "clean" trees. Plant new orchards away from hardwood stands and from older plantings where scale has been a problem.

For established orchards, Superior oil should be used just before the tree breaks dormancy when the scales have only a thin wax covering. Delaying the application until green tissue is present often results in poor scale control, because the scales have produced a larger protective wax coating making complete coverage of the insect more difficult.

Superior oil works best when applied at temperatures about 4°C. Potential bark injury may occur with the use of Superior oil on certain apple cultivars, such as Red Delicious and Empire, especially when temperatures are below freezing. Annual Superior oil applicants, timed at tight cluster for European red mite control, may sufficiently suppress low-scale populations. If scale populations are high, then Superior oil spray timing should be adjusted to full dormancy to provide maximum control of scale. In problem blocks, the use of oil over two to three consecutive years may be necessary to reduce scale populations to a level where they are economically acceptable.

Since scale insects overwinter below rough bark, high water volumes to achieve thorough coverage are essential. Superior oil does not kill by chemical action; instead it must completely cover the scale and exclude air, thereby suffocating the insect (just as rust-proofing an automobile requires complete coverage to be effecting). Incomplete coverage will allow the scale to breathe and survive. Proper nozzling of the sprayer to achieve good coverage in the top centre of larger trees in important as the scale populations are highest in those locations. Calm conditions and a slow travel speed will improve chances of successful scale control.

| Top of Page |