Scale Insect Pests of Tree Fruit
Table of Contents
Scale insects are serious pests of tree fruits in Ontario. Numerous species exist in Ontario including the Oystershell scale Lepidosaphes ulmi (Linn.), Lecanium scale Lecanium sp., San Jose scale (SJS) Quadraspidiotus perniciosus (Comstock) and the European Fruit scale (EFS) Q. ostreaeformis (Curtis) along with many other minor scale species. In Ontario, SJS and EFS are the most serious scale pests with both species found in all regions of the province in recent years. Each species 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. Records from the early part of the century revealed that high numbers of scale insects were a cause of tree mortality on tender fruit crops (peaches, nectarines, plums and cherries).
Although the type of injury caused by 515 and SJS is similar, the biology and the ability to reproduce differs between the species making effective control decisions important.
The SJS has a host range of over 700 plant species and was first recorded in Ontario over 90 years ago. There are two generations of SJS in Ontario each season. SJS overwinters as a first instar nymph beneath a thin waxy shell. This stage is referred to as a "blackcap" due to its physical colouration (Figure 1). 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. The actual insect is the soft bodied creature beneath the waxy shell which is wind and water repellent. While the waxy exuvia is present the scale is incapable of movement on or about the host. Just prior to bud break (late March/April) the scale begins to feed on tree sap and starts to exude additional layers of wax. By bloom, the scale has developed to the point where male and female scales can be distinguished. 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 (Figure 2). The outer shell of female SJS remains relatively circular with a central nipple while male exuviae is elongated with the nipple offset to one end (Figure 3).
Figure 1. Numerous blackcap scales on bark.
Figure 2. Male (left) and female (right) scales with shell removed.
Male emergence and flight lasts approximately 14 days with peak activity usually corresponding with full bloom of the McIntosh cultivar of apple. Females remain fixed in place on the host and the males must seek out the females for mating. About 4 to 6 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 4 to 6 weeks (late June to early August).
Figure 3. Mature female scale shown beside circular wax covering.
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 even birds. Generally most crawlers do not travel more than 1 meter from the parent female before settling into a feeding position. The establishment of the feeding position occurs within 48 hours to 72 hours of birth. Each crawler inserts 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 SJS nymphs will complete their development in six weeks giving rise to a new generation of adults in early August. Mating and gestation is 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 SJS population and greater potential for economic injury to the crop.
The European Fruit Scale has a life history similar to that of SJS but differs in having only one generation each year. The host range for EFS is more limited than that of SJS, infesting primarily Malus spp. (apple), Pyrus spp. (pear), and Prunus spp. (tender fruit).
Peak flight of male EFS occurs around petal fall (cv. McIntosh). Crawlers begin to appear in late June and continue to be released by the adult female for the remainder of the season (ie. late June until late October). Similar to SJS, each female EFS gives birth to about 400 nymphs.
Unlike the previous two species, the Oystershell scale overwinters as eggs beneath a wax scale secreted by the adult female the previous season. Egg hatch begins just prior to bloom (CV McIntosh) and continues from petal fall onward into the early summer. Crawlers move about for 48 to 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 females. Shortly after mating the females oviposit the over-wintering eggs; then die. There is only one generation of Oystershell scale (Figure 4) each season and this scale species is considered a sporadic minor pest.
Figure 4. Oystershell scale on bark.
For most growers the observation of scale damaged fruit the previous harvest is the first method of recognizing 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 skin colours (Figure 5a and Figure 5b) such as Loring peaches, Mutsu apples and Shiro plums the spots are easily visible. Since scales are not readily mobile, it is possible to identify locations or "hotspots" 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. On peaches, severe scale infestations may induce the tree to produce a gummy exudate that can be mistaken for infection by peach canker. Using a sharp instrument one can remove the bark and reveal a discolouration of the wood beneath (Figure 6). SJS can be identified by the purple stain, while EFS causes more of a water soaked brown stain. On apples the best cultivars to examine are Delicious and those having Delicious parentage (Empire, etc.). This occurs because scales have a tendency to establish on hosts where superior oil is not regularly applied.
Figure 5a. Scale injury and discolouration on a peach.
Figure 5b. Scale infestation and discolouration around the calyx end of an apple.
Figure 6. Bark removed to reveal discolouration of cambial tissue of wood by SJS feeding.
Sampling to establish the presence of scale involves taking twig and bark samples during the dormant season or at time of pruning. For apples bark samples measuring 5 sq cm (l/2" x 22") should be taken from at least 10 trees per block from the scaffold limb/trunk area. For tender fruit trees and dwarf apple trees examine 10 to 20 fruit spurs from the top interior section of 10 trees (ie. 1-2 spurs per tree) in areas with suspected scale infestation.
Monitoring for crawler activity is possible but is not recommended as there is little useful information gained for making a control decision. Some research papers indicate that crawler activity can be detected using black electrician's tape reversed with the adhesive side out and banded around the scaffold limbs of trees with known infestations. These bands should be placed out by mid-June and replaced every 2 to 3 weeks throughout the season. Crawlers are identified as their yellow bodies contrast with the black tape.
There are no economic thresholds for scale insects on fruit and in most cases any fruit injury at harvest will warrant corrective control the following spring.
The use of superior oil every few years is suggested to prevent the buildup of damaging populations. Where fruit injury is detected the use of oil in 2 or 3 consecutive seasons will be needed to achieve control. Scale insects reside below rough bark thus making high water volumes essential to get thorough coverage. 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 effective). Incomplete coverage will allow the scale to breathe and survive.
Superior oil should be used just before the tree breaks dormancy when the scale will only have a small 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. Proper nozzling of the sprayer to achieve good coverage in the top centre of the trees is important as the scale populations are highest in those locations. Superior oil works best when applied at temperatures above 4°C. Calm conditions and a slow travel speed will improve chances of successful scale control. Be sure to read the manufacturer's label for complete directions and warnings about potential bark injury on certain apple cultivars.
For apples additional products are available with different timings for scale control. Refer to the current OMAFRA Publication 360, Fruit Production Recommendations, for recommended materials.
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