Scale Insects

Excerpt from Publication 310, Integrated Pest Management for Apples,
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Table of Contents

  1. Introduction
  2. San Jose scale
  3. European fruit scale
  4. Oystershell scale

Introduction

Scale insects were serious pests of apples in Ontario, and are now less common with the introduction of dwarf rootstocks, pest monitoring and improved spray technology. Numerous scale species exist in Ontario including San Jose scale, Quadraspidiotus perniciosus (Comstock), European fruit scale, Quadraspidiotus ostreaeformis (Curtis), oystershell scale, Lepidosaphes ulmi (Linn.) and other minor scale species.
San Jose scale is the most serious scale pest in Ontario, with less injury observed by European fruit scale. San Jose scale causes direct injury to the fruit and/or injures the host tree by sap feeding - reducing tree vigour and possibly causing death of limbs. Host plants of San Jose scale include apple, pear, quince, plum, apricot, sweet cherries, currants, gooseberries and numerous woody ornamentals.

San Jose scale

San Jose scale, Quadraspidiotus perniciosus (Comstock), is the most common scale seen in orchards, and recently became more of a problem, perhaps due to reduced spray programs.

Description

Females give birth to living young, and do not lay eggs. Crawlers are small, orange-yellow in colour and oval shaped. They have six legs and one pair of antennae. Adult females are nearly round and located beneath a waxy scale covering (about 1.6 mm in diameter) with a raised nipple in the centre of the covering (Figure 4-7). They remain under their scale coverings their entire lives. Adult males are small winged insects about 1 mm in length, and golden brown in colour. Males emerge in flights from beneath elongate scale covers once they have matured. Male flight usually occurs during apple bloom, with a series of three overlapping summer flights from mid summer through fall.
Figure 1. Female scale

Figure 4-7. Female scale (Alan Eaton, University of New Hampshire Coop. Extension)

Biology

San Jose scale overwinters on the tree as immature scales. They remain dormant on the bark until tree sap starts moving in the spring, and are full grown by late May. Active males (small winged insects) emerge from their scales and seek females for mating. Females are immobile and remain under scales throughout their lives. After mating, females begin to produce living young, usually at the rate of 9-10 per day. They reproduce for six weeks and bear from 150-500 offspring (crawlers). Crawlers have six legs and two antennae, and most crawl within 1 m of the parent female in the first few hours of life. Crawlers move to find a suitable place to develop (bark or fruit), and then insert their mouthpart through the bark and begin to feed on sap. Three weeks later they molt and shed their skin. Scales are attached to bark by their sucking mouthparts. As the insects continue to grow, they build a waxy shell around them that protects from dessication, predators and pesticides. Females develop through two nymphal instars to the adult, while males develop through four instars. Scale populations increase rapidly in hot dry weather, and females produce more than 300 million offspring each year. There are two full generations each year. Crawlers spread through orchards by wind, birds' feet, workers' clothing and on farm equipment.

Damage

Heavy infestations of scale insects - particularly on young trees - seriously reduce tree vigour, growth and productivity. Severe infestations can kill whole limbs. Scale is found on limbs (Figure 4-8) or fruit (Figure 4-9). Confirm San Jose scale injury by cutting away bark to reveal a reddish discolouration. San Jose scale feeds on host plant sap. Heavy infestations of scale kill young trees in two to three years. Older trees are more resilient, but can also be killed by severe infestations.

Figure 4-8. San Jose scale on limb

Figure 4-8. San Jose scale on limb (Alan Eaton, University of New Hampshire Coop. Extension)

Figure 4-9. San Jose scale on fruit

Figure 4-9. San Jose scale on fruit

San Jose scale feeding on fruit is most abundant around the blossom and stem ends of fruit, often resulting in small gray patches on apples, surrounded by a small red inflamed area. Fruit with more than two blotches is generally graded out by packers. Some packers accept no scale damage to fruit (zero tolerance), especially for export markets. Early season severe infestations may result in small deformed fruit.

Monitoring and threshold

Monitor fruit at harvest for scale. There are no thresholds for scale insects on apple. In most cases, any fruit injury at harvest warrants corrective measures the following spring. Use black electrical tape (with adhesive side out) around the scaffold limbs of trees, in areas with known infestations, to detect crawler activity. Place bands in trees by mid June for 7-10 days after petal fall and replace every two to three weeks throughout the season. Crawlers yellow bodies will be caught on the tape. Other areas (Michigan) use pheromone traps to time sprays for San Jose scale.

Management

Successful scale insect control starts with prevention. Carefully examine all nursery trees prior to actual planting. If scale insects are present, discard trees or exchange for clean trees. Plant new orchards away from hardwood stands and from older plantings where scale has been a problem.

For established orchards with a history of scale problems, use Superior oil before the tree breaks dormancy when scales have only a thin wax covering. Delaying application until green tissue is present often results in poor scale control, because scales have produced a larger protective wax coating making complete coverage of the insect more difficult. Insecticides have recently been registered to manage crawlers in the orchard. Researchers in the United States apply insecticides when crawlers are active (based on monitoring) in mid June and mid August. For more information on the timing of products for managing scale, see OMAFRA Publication 360, Fruit Production Recommendations.

European fruit scale

European fruit scale, Quadraspidiotus ostreaformis (Curtis), is an occasional pest in Ontario orchards.

Description

Crawlers (nymphs) are bright yellow with long antennae. Females are immobile and covered with a circular waxy coating that is elevated at the centre. Adult males are brownish red with an elongated abdomen, long antennae and wings.

Biology

Mated and fertilized females overwinter as immature scales on twigs (Figure 4-10). Overwintering young begin to develop in adult females - located beneath thick waxy scale coverings - in May and June. Females give birth to live young. Crawlers move to the undersides of leaves, twigs and scaffold limbs to settle and feed. Wood older than three years is usually too thick for insect mouthparts to penetrate. Winged males appear in August, and die after mating with females. European fruit scale has a limited host range and infests primarily Malus (apple) spp., Pyrus (pear) spp. and Prunus (tender fruit) spp. There is one generation of this pest each year.

Figure 4-10. European fruit scale on limb

Figure 4-10. European fruit scale on limb (BC Ministry of Agriculture and Lands)

Damage

Scales suck sap from leaves and twigs in July and August. Young scales produce a large quantity of honeydew - a clear, sweet liquid serving as a medium for fungal growth. Scale causes round spots on bark and weakens trees. Scale appears as spots encircled by a red halo on leaves and fruit (Figure 4-11).

Figure 4-11. European fruit scale on fruit

Figure 4-11. European fruit scale on fruit (BC Ministry of Agriculture and Lands)

Monitoring and threshold

Use electrical tape to monitor for crawlers (refer to San Jose scale monitoring).

Management

Refer to San Jose scale management.

Oystershell scale

Oystershell scale, Lepidosaphes ulmi (Linn.), affects fruit in most apple-growing provinces and states in eastern North America.

Description

Eggs are elliptical, and young nymphs are very small, six-legged and white in colour. Males are small winged insects. Females are about 0.7 mm long, cream-white in colour, and lack legs and antennae. There is one generation of oystershell scale each season.

Biology

Scales overwinter as fertilized females with 40-150 egg masses under their scale. Eggs hatch in late spring, approximately two to three weeks after bloom, and young crawlers emerge. Crawlers are small white with six legs, moving to an appropriate site where to begin feeding. After a few hours of feeding, the scale begins to form. Mating occurs and females die shortly after they lay their last eggs.

Damage

Oystershell scale feeding weakens the plant and damage can also be found on the fruit (Figure 4-12).
Figure 4-12. Oystershell scale on fruit

Figure 4-12. Oystershell scale on fruit

Monitoring and threshold

Use electrical tape to monitor for crawlers, refer to San Jose scale monitoring.

Management

Refer to San Jose scale management.


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: OMAFRA Staff
Creation Date: 21 July 2011
Last Reviewed: 21 July 2011