Beneficials

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

1. Predatory mite biology
2. Predatory mites in Ontario apple orchards
3. Conservation predatory mites
4. Beneficial insects
   • What are the benefits of natural enemies in the orchard?
   • Aphid predators
   • Generalist predators
   • Parasitoids
5. Soil beneficials

 

Predatory mite biology

Many growers and consultants are familiar with plant-feeding (phytophagous) mites causing damage in the orchard. One of the most effective uses of biological control in apple involves the management of pest mites in apple orchards. There are several important predatory mite species that, under the right conditions, are present in sufficient numbers to keep phytophagous species below economic thresholds. Monitoring programs for pest mites always includes counts of beneficial species.

Predatory mites in Ontario apple orchards

The most recent published survey of predatory mites in Ontario apple orchards (1991) identified 16 species of predatory mite species, 12 phytoseiids, 2 stigmaeids and 2 erythraeids. This level of diversity is also observed in inventories of predacious mites in commercial apple orchards in Quebec (2006), though the species composition and abundance differs between the two provinces. Differences observed in both surveys may be a function of climate and habitat, or the result of changing pest management programs occurring between 1991 and 2006.

Three common species of predatory mites in managed Ontario apple orchards include Balaustium putmani (erythraeidae), Zetzellia mali (stigmaeidae) and Neoseiulus (= Amblyseius) fallacis (phytoseiidae). Other species identified include the phytoseiids Typhlodromus pyri and T. caudiglans, and the stigmaeid Agistemus fleschneri. Relative abundance of these and other species in any given orchard is dynamic, and varies with pest management practices, habitat (including groundcover), competition, predation by other mite species and overwintering survival ability.

Monitor for all species of predatory mites and insects during regular scouting activities and note their abundance. Predatory mites are distinguished from pest mites by observing their speed. Most move faster than pest mites. Predator abundance in the orchard is strongly affected by pesticide use.

Balaustium putmani (Smiley)

Description
Balaustium putmani is a large (0.3-0.7 mm) velvety-red, tick-shaped mite (Figure 4-208) visible to the naked eye. It is fast moving and is commonly seen on leaves and fruit in apple orchards.

Figure 4-208. Balaustium spp.

Interaction with host
These mites overwinter as eggs underneath bark and other protected areas. Nymphs hatch in early May and begin feeding on mite motiles and eggs, as well as scales and small soft-bodied insects (leafhoppers). They remain in fruit trees throughout the season but are more important as early-season predators. They may eat up to 40 adult twospotted spider mites per day.

Zetzellia mali (Ewing) and Agistemus fleschneri (Summers)

Description
Both stigmaieds are relatively slow moving, compared to other predatory species. Z. mali is smaller (0.5 mm) than other mite predators commonly found in the orchard. Z. mali is a lemon-yellow mite with a diamond-like shape (Figure 4-209), though its gut may take the colour of prey following feeding. There are four generations of this mite per year. Populations do not increase as rapidly as other predatory mites (Amblyseius). It is the most widespread stigmaeid in Ontario.

Figure 4-209. Zetzellia mali, red in colour due to feeding on European red mite (Dr. Art Agnello, Cornell University)

A. fleschneri measures 0.3-0.4 mm as an adult. It is yellow-orange with yellow legs. This species is distinguished by the presence of a honeycomb pattern is visible on the posterior half of the back.

Interaction with host
Z. mali overwinters as an adult female under bark and becomes active early in the spring, feeding on overwintering European red mite eggs and rust mites. It can be fairly effective in controlling low-density pest mite populations. Adults and nymphs are predaceous and feed on all life stages of mites (both European red mites and twospotted spider mites), but newly hatched larvae attack mostly apple rust mites. Z. mali persists in the orchard when prey are scarce, surviving on pollen, sap and fungal spores. It is less mobile than other predatory mites, and slow to travel to new feeding sites in search of prey. Z. mali is not likely to provide effective biocontrol on its own - it prefers to feed on apple rust mite and does not require many prey items to survive. As with other mite predators, Z. mali also feeds on the eggs of its rivals. The biology of A. fleschneri is similar to that of Z. mali.

Neoseiulus (=Amblyseius) fallacis (Garman)

Description
Neoseiulus (=Amblyseius) fallacis is a commonly found mite predator in Ontario, particularly in July and August (Figure 4-210). Adult A. fallacis have a broad abdomen (pear or tear-shaped), are 0.30-0.35 mm in length (slightly smaller than European red mites) and are very fast moving. They can be clear (pale yellow or translucent) or acquire the colour of prey items they eat (European red mites, two spotted spider mites), such as a mottled brownish-red. Eggs are almost transparent, oval or pear shaped and slightly larger than the rounded, reddish eggs of European red mites.

Figure 4-210. Amblyseius fallacis adult (Dr. Art Agnello, Cornell University)

Interaction with host
Adult females overwinter near the base of trees or surrounding ground cover, or where prey items are abundant in the fall, on the tree in protected areas. They become active in the spring, moving into tree canopies in June and July where they feed on European red mite eggs, nymphs and adults, and other mite pests (twospotted spider mite, rust mites). There are four to six generations per season. They prefer to feed on spider mites, but also feed on apple rust mite in absence of preferred prey.

A. fallacis is capable of rapid population increases under favourable conditions, and is a very effective natural control agent for pest mite populations. Studies show a ratio of one A. fallacis to 10-15 pest mites gives effective biological control. When prey items are scarce, A. fallacis leaves the tree in search of other food sources. Research shows dispersal into the canopy is affected by degree day accumulations, initial density of predators in the ground cover and prey density in the tree. When adequate prey is available, they appear in trees after 333 +/- 55 DDC (base 12ºC) after January 1. Spring frosts or freezing rains suppress activity early in the season. They can travel to other trees when prey is low by being carried off on a breeze.

Galendromus (=Typhlodromus) pyri (Scheuten) and Galendromus (=Typhlodromus) caudiglans

Description
T. pyri and T. caudiglans are both very similar in appearance to A. fallacis and the three species are not readily distinguished without careful examination of small taxonomic features.

Interaction with host
T. pyri is another phytoseiid mite species found in apple orchards. It overwinters as a mated adult female on the tree, emerging around the time the first green tissue is present. It is not as voracious a feeder and is not capable of the rapid population increases seen with A. fallacis, limiting its usefulness in managing pest mite population explosions. Unlike A. fallacis, it is generalist feeder and able to survive on alternate food sources (pollen, mildew, insect eggs, other mites) and remains in the tree canopy when primary prey items such as European red mite, twospotted spider mite and rust mites are scarce. The feeding biology of T. caudiglans is similar to T. pyri.

Conservation predatory mites

Populations of predatory mites are dynamic, and affected by the availability of food sources and pest management programs. A number of orchard pesticides are toxic to beneficial mites and other natural enemies. Pyrethroids, for example, can be extremely disruptive to some species of predatory mites, and their use is discouraged in apple pest management programs, particularly after petal fall. Populations of predatory mites adversely affected by pesticides are often slow to recover.

Many of the newer reduced-risk insecticides and some miticides available to growers are less harmful to beneficial mites - others have direct acute toxicity or sublethal effects on populations. Oil sprays applied before bloom to manage overwintering European red mite have little effect on the beneficial mites since they are hidden away in crevices on the bark of the tree, or found on the orchard floor at this point in the season. Consequently, they are important tools in conservation of predatory mites in the orchard.

Beneficial insects

When developing any IPM program, remember the valuable contributions natural enemies make in reducing pest populations. Many orchard pests are attacked by beneficial insects or mites and by various fungal, bacterial and viral agents. Often these natural enemies provide good suppression of pest populations, particularly indirect pests (e.g, aphids, mites, leafminers) - those that feed on or in leaves rather than on the fruit itself.

There are three general groups of natural enemies of orchard pests - predators, parasitoids and pathogens. Predators overcome, capture and consume their prey items. Most predators are large relative to their favourite prey items. A predator usually consumes many prey items over the course of its lifetime. While some predators are specialized, many are generalists and found in diverse habitats.

A parasitoid lives in or on the body of a single host individual, feeding on and eventually killing that individual over the course of its own development. Parasitoids are often smaller than their prey and their presence is easily missed by casual observations. Parasitized insects do not always die quickly. They may continue to feed and develop normally until they reach a certain stage of development.

Pathogens are often overlooked as biological control agents or natural enemies. They include bacteria, viruses, fungi and other microorganisms, as well as insect parasitic (entomopathogenic) nematodes. Bacteria and viruses must be ingested by the host, while fungi and nematodes invade through the cuticle or openings (mouth, anus, spiracles). Sickly looking individuals (sluggish, shrunken, discoloured) or those covered with a wooly mycelium may have been affected by a pathogen.

What are the benefits of natural enemies in the orchard?

Beneficial insects and mites delay or prevent the need for pesticide applications. IPM programs take advantage of the biological pest control provided by beneficial insects and mites by conserving or augmenting natural enemies in orchards. When chemical controls are necessary in an IPM program, pesticides recommended are those with minimal impact on beneficials naturally occurring in the orchard.

Although the impact of any one species of natural enemy may be minor, the combined impact of predators, parasitoids and insect pathogens can be considerable. Farming practices encouraging biological diversity support the conservation of natural enemies that help keep pest numbers down. The following are some common sense methods of conserving beneficial insects and mites in orchards

• recognize natural enemies and monitor their presence in orchards
  
• apply insecticides/miticides only when action thresholds are reached and use products less toxic to these predators
  
• establish native flowering plants and other ground covers in unsprayed areas - to provide nectar, pollen and shelter for many beneficial insects

Some of the more common beneficial insects include:

Aphid predators

Aphid midges - Aphidoletes aphidimyza (Cecidomyiidae)

Description
Eggs are oval, minute (0.1-0.3 mm) and orange. Larvae - the lifestage most commonly seen in orchards - resemble small (2-3 mm) orange maggots (Figure 4-210). Adult aphid midges are small flies (2-3 mm) with long, dangling legs and long antennae. They are seldom observed in the orchard. Food sources affect their larval colouring, which ranges from bright orange to red. Larvae use their strong "jaws" to grasp their prey.

Figure 4-211. Aphid midge larvae

Interaction with host
A. aphidimyza is found on a variety of crops including cabbage, apples, blueberries and ornamental bushes. It is recognized as an important naturally-occurring control agent of aphids and attacks more than 60 different species. Females lay eggs near aphid colonies. Larvae kill more aphids than they require to complete their development, particularly at high prey densities. In apple orchards, larvae are most common from mid to late summer in the field and are often found feeding in green apple aphid colonies.
Monitoring and management
Check for the presence of aphid midges in terminals when monitoring aphid populations. The presence of aphid midges and other aphid predators delays or prevents the need for insecticide applications for green apple aphids. Research in other areas indicates a ratio of 1 aphid midge larva to 15 aphids provides effective biological control. Females lay approximately 200 eggs, and a single larva kills up to 80 aphids. For more information on biocontrol of aphids, see the section on green apple aphids.

Hover or flower flies (Syrphidae)

Description
More than 25 species of flower flies have been identified in Ontario apple orchards. In general, larvae are slug-like and pointed at one end, with mottled brown and/or green colour patterns (Figure 4-212). They reach sizes of 5-10 mm. The adults of many species are dark brown with yellow bands, giving them a bee-like appearance. They have a habit of hovering in mid-air (hence the name) (Figure 4-213).

Figure 4-212. Hover fly larva

Figure 4-213. Hover fly adult

Interaction with host
Adult flower flies are important pollinators. They are not predaceous but feed on pollen, nectar and aphid honeydew. A female lays up to 400 eggs. Larvae are often found in aphid colonies feeding on aphids. They pierce the bodies of aphids and suck their fluids, leaving shriveled, blackened aphid bodies. Each larva consumes more than 500 aphids, and may also feed on scales, mites and caterpillars.
Monitoring and management

Monitor for their presence in terminals when monitoring aphid populations. The presence of aphid midges and other aphid predators can delay or prevent the need for insecticide applications for green apple aphids. For more information on biocontrol of aphids, see the section on green apple aphids.

Lacewings (Order Neuroptera)

Description
There are several species of green (Chrysopidae) and brown (Hemerobiidae) lacewings commonly found in apple orchards as eggs, larvae and adults. Eggs are less than 1 mm in length, oval and pale (Figure 4-214). Depending on the species, eggs are laid individually either at the end of long silken stalks attached to leaves or without a stalk. Mature larvae are 0.6-1.2 mm in length, with a long pointed body and a mottled appearance. They have well-developed legs and heads with large sickle-shaped mandibles (Figure 4-215). Lacewings have three larval instars. Mature larvae spin a white silken cocoon in which they pupate. Adults are 12-20 mm long, with long antennae, bright golden eyes and large veined wings (Figure 4-216). Lacewings have three to four generations per year in Ontario.

Figure 4-214. Lacewing eggs

Figure 4-215. Lacewing larva

Figure 4-216. Lacewing adult

Interaction with host
Common species include the green lacewings Chrysopa nigricornis (Burmeister), Chrysopa oculata (Say), C. carnea (Stephens) and the brown lacewing Hemerobius humilinus (L.). C. carnea is considered the most abundant. Both adults and larvae are predaceous generalist feeders.

Lacewing larvae are voracious predators and feed on aphids, phytophagous mites, lepidopteran eggs and other lacewings. Larvae use their mandibles and maxillae to pierce prey and suck out their body fluids. Occasionally, they pierce plant tissue for moisture or food. Consumption rates of 44-105 aphids/day and 1,000 mites/day have been reported. Adults require aphid honeydew and plant nectar as food prior to egg laying.

Monitoring and management
Lacewings are active in orchards from late May until early September. Check for the presence of lacewings by visual examination of terminals when monitoring aphid populations. The presence of lacewings and other aphid predators can delay or prevent the need of insecticide applications for green apple aphids. For more information on biocontrol of aphids, see the section on green apple aphids.

Lady beetles (Coccinellidae)

Description
There are several different species of lady beetles in Ontario apple orchards. Two of the most common include the multi-coloured Asian lady beetle Harmonia axyridis (Pallas) and the seven-spotted lady beetle Coccinella septempunctata (L.). Both of these lady beetles are introduced species.

Lady beetle eggs are 1.2 mm in length, oval, bright yellow and are laid in clusters (Figure 4-217). Mature larvae are 8-11 mm in length and black with prominent bright yellow-orange patches that help to distinguish the species. Larvae are covered in spines and often described as having an "alligator" shape (Figure 4-218). Adults are broad, oval-shaped and convex in form. Adult sizes range from the very small Stethorus spp. (1.4-1.6 mm) to the large Anatis spp. (8-10 mm). Ladybird beetles are often brightly coloured with spots (Figure 4-219).

Figure 4-217. Lady beetle egg mass

Figure 4-218. Lady beetle larva

Figure 4-219. Multi-coloured Asian lady beetle adults

Interaction with host
Adults and larvae are efficient predators found in a variety of crops and other plants in the landscape. While some are generalists - feeding on aphids, small caterpillars, scale insects, mealybugs and mites - others such as Stethorus spp. feed almost exclusively on phytophagous mites. Lady beetles overwinter as adults and become active in the spring. There may be one or two generations per year in Ontario. Some species, notably the multi-coloured Asian lady beetle, consume many hundreds of aphids during their lifetimes. The number of prey killed is affected by pest density. In situations where prey is abundant, lady beetles may only partially consume their prey before moving on to the next.

Monitoring and management
Check for lady beetles by visual examination of terminals when monitoring for aphids. If populations are sufficiently high, these predators keep indirect pests such as aphids and mites below threshold levels where IPM programs incorporating reduced-risk products are practiced.

Generalist predators

Flower bugs (Anthocoridae)

Description

Flower bugs are small (2-4 mm), oval-shaped insects. Their wing covers are partially membranous. One of the most common species found in apple orchards include the minute pirate bug Orius insidiosus (Say) and the insidious flower bug Orius tristicolor (White). Eggs are 0.55 mm long and clear. Nymphs are 0.2- 0.5 mm, yellow-orange to brown and teardrop-shaped. Adults are 3 mm long, oval with black and white wing patches (Figure 4-220). Both nymphs and adults have piercing-sucking mouthparts used to subdue and feed on their prey. O. insidiosus has two or more generations per year in Ontario.

Figure 4-220. Minute pirate bug

Interaction with host
Orius spp. are common predators in numerous agricultural crops. They overwinter as adults both within and outside of the orchard. Adults and nymphs feed on aphids, spider mites, thrips, insect and mite eggs, and small caterpillars. When prey populations are low, they will feed on pollen and plant juices. Both immature and adult Orius spp. consume 30 or more spider mites/day.

Monitoring and management
O. insidiosus appears in orchards in mid April and remains active through to October. Crop consultants monitor for the presence of flower bugs by visual examination of terminals when monitoring for aphids

Plant bugs (Miridae)

Description
Plant bugs are phytophagous, predatory or both. One of the more common species found in orchards is the mullein bug, Campylomma verbasci (Meyer). Eggs are 0.8 mm long, white and flask shaped. Mullein bugs have five nymphal instars. Nymphs are 0.5-2.5 mm long, yellow, with a pointed head and red eyes. Adults are 3 mm long, oval and light green to tan in colour (Figure 4-221). There are two to three generations each year in Ontario.

Figure 4-221. Mullein bug adult (with prey)

Interaction with host
Although mullein bug is considered a pest of apple for two weeks pre- and post-bloom, after this time it becomes an important predator, feeding on mites and aphids found in apple orchards throughout the summer. Mullein plants are an alternate host during the summer. Adult and nymphs are predaceous and feed primarily on European red mites and the green apple aphid, and may suppress populations of these pests in the early stages of an infestation. They also feed on caterpillar larvae, leafhoppers and other soft bodied insects.

Monitoring and management
Mullein bugs are often found in terminals and during tapping in regular orchard monitoring.

Assassin bugs (Reduviidae)

Description
There are several different species of assassin bugs and they vary greatly in size form and colour. The most abundant assassin bug in Ontario apple orchards is Acholla multispinosa (DeGeer). Adults are large insects (1-2 cm in size), dark brown to grayish brown in colour and have raptorial front legs and large beaks used for capturing, subduing and feeding on prey. Adults are the lifestage most commonly seen in orchards (Figure 4-222). Egg masses containing 24 eggs are laid on buds or twigs and limbs.

Figure 4-222. Assassin bug adult

Interaction with host

Both nymphs and adults are predaceous and feed on all life stages of green apple aphid, adult leafhoppers and some caterpillar larvae. Nymphs appear in early June and adults in late July to early August. There is one generation each year.

Monitoring and management
Adults and nymphs are found in terminals and during tapping in regular orchard monitoring.

Ground beetles (Carabidae)

Description
Ground beetles are common in tree fruit orchards and other agricultural areas. There are 59 species of ground beetles in apple orchards in southern Ontario, with Amara aenea (DeGeer), Harpalus affinis (Shrank) and Pterostichus melanarius (III) the most common.
Ground beetle adults are elongate, flattened beetles that vary in size from 0.3-8.5 cm. They are usually dark brown to black in colour, but may be yellowish or reddish brown with blue green of copper metallic hues (Figure 4-223). They are ground dwellers and are found under stones and debris on the soil surface.

Figure 4-223. Ground beetle adult (caterpillar hunter)

Interaction with host
Larvae and adults are nocturnal and predacous. Some species are phytophagous and feed on seeds, shoots, fruits and pollen of plants. Prey items within the orchard include apple maggot pupae, codling moth larvae, aphids and other soft-bodied insects. They are present throughout the growing season.

Monitoring and management
Monitor for them during regular orchard monitoring.

Spiders

Description
Spiders are not insects. The body of the spider has two regions the cephalothorax and abdomen, and have four pairs of legs. There are several different types of spiders found within the tree canopy, on branches and on the trunk. Foliage hunting spiders (jumping spiders, crab spiders and sac spiders) hunt for prey in foliage by physically grabbing them (Figure 4-224). Foliage web builders (orb weavers, cobweb spiders, meshweb weavers and sheetweb spiders) all spin webs to trap their prey.

Figure 4-224. Spider

Interaction with host
Spiders feed on many different insect pests of apple.

Monitoring and management
Note the presence of spiders when monitoring in orchards

Parasitoids

Tachinid flies (Tachinidae)

Description
Adults are medium to large flies, similar in appearance to houseflies but covered with prominent bristles. Larvae feed internally within hosts and have a maggot-like appearance. Pupae are found outside of the host. They are brown, elongate, and lack any obvious external features.

Interaction with host
Tachinids overwinter as pupae in leaf litter. Adults emerge in spring and mated females lay eggs on the back of hosts, including obliquebanded leafroller. Development occurs within the parasitized larva. Mature tachinid larvae exit the host to pupate. Rates of parasitism are highly variable.

Monitoring and management
Monitoring for tachinids is not a normal routine practice during scouting.

Aphid parasitoids (Aphelinidae)

Description
Adults are small (0.7-3mm), black or metallic wasps. Larvae are found inside mummified aphids (Figure 4-225).

Figure 4-225. Dissected aphid mummy with parasitoid larva

Interaction with host
Females lay eggs inside aphids. Larvae develop inside the aphid host, eventually chewing their way out. Several aphids within a colony are often parasitized. Aphelinus mali (Haldeman) is an important parasitoid of woolly apple aphid.

Monitoring and management
Look for aphid mummies within colonies. Parasitized woolly apple aphids appear black.

Chalcid wasps (Chalcididae)

Description
Chalcids are small (2-7 mm) shiny wasps with enlarged femurs on the back legs and short ovipositors. Larvae are ectoparasites, feeding externally on the host. Pupae are found within the leaf mine and easily recognized by their mummy-like appearance.

Interaction with host
Female wasps lay their eggs on suitable hosts.

Braconid wasps (Braconidae)

Description
Braconid wasps are often dark, though some are brightly coloured. They range in size from 2-15 mm. Their wings contain very few veins. A common species in apple orchards is the native Pholetesor ornigis, a parasite that develops within its host. Larvae of leafminers, including the spotted tentiform leafminer, are among its hosts. Another species, Macrocentrus linearis (nees), attacks larvae of the obliquebanded leafroller

Interaction with host
As with other parasitoids, braconid wasps require a host for development.

P. ornigis overwinters as a prepupa within a cocoon in the leaf mine of the host. Females emerge in spring and lay eggs individually into leafminer larvae. Parasitized larvae remain active until they reach the final instar or pupal stage, after which they are killed by the developing wasp larvae. The wasp larva exits the leafminer larva and pupates within a cocoon, which appears as a pale tighly woven tube-like structure with a dark band. The wasp biology is well-synchronized with the host species. There are several generations per year. These wasps are effective biological controls of spotted tentiform leafminer in Ontario apple orchards.

Monitoring and management
Check for the presence and abundance of braconid wasp cocoons while monitoring for spotted tentiform leafminers. The lifecycle of the tentiform leafminer and P. ornigis are synchronized, with adults emerging in spring just as the first sap feeders appear, typically around petal fall. These parasitoids are sensitive to many broad-spectrum insecticides. Select reduced-risk products when spraying for other pests where options exist.

Soil beneficials

In biologically active soil, the soil life accounts for more than 4 tons per acre (0.5%) in the plow layer. This represents a large portion of the soil organic matter pool in some soils. Active and diverse soil life is crucial to soil aeration, nutrient cycling and to maintaining the balance between beneficial and pest soil organisms.

At the microscopic level there are fungi, bacteria, actinomycetes and algae in most soils. Populations vary though depending upon soil type, pH, soil cover or crop rotation and tillage.

Fungi make up the largest amount of living material after plant roots. They are intolerant of intensive tillage and are favoured in low pH soils. Fungi are involved in decomposition of organic matter which helps to cycle nutrients. In particular, a group of fungi called vesicular arbuscular mycorrhizae (VAM) is symbiotically involved with plant roots, particularly in perennial crops like orchards and plants growing in undisturbed soils. The hyphae (root-like structures) of VAM help increase the absorptive area of plant roots by a thousand fold, particularly important in the uptake of phosphorous in low testing soils.

Bacteria number in the billions in most soils. Bacteria are an important part of the soil food web, cycling nutrients as organic matter decomposes

Actinomycetes are less well known but play an important role in the decomposition of soil organic matter. They are particularly abundant in low pH, droughty soils, similar to many highly productive horticultural soils.

Algae help to decompose organic matter. Most commonly seen in poorly drained soils, they often become visible when soils remain wet for extended periods.

Macroscopic soil animals include arthropods, earthworms and rodents.

Arthropods represent a wide variety of soil dwelling insects, spiders and mites.

Microarthropods are extremely numerous in undisturbed soils or soils with perennial crops such as orchards with sod cover, with populations of more than 1 million per square metre. In contrast, typical field crop soils have much lower levels (75,000/m2) due to the decrease in soil organic matter from tillage and changes in microclimate from cropping patterns. These microarthropods are involved in the decomposition of plant residues, and also stimulate fungal growth and microbial activity. A few of these creatures are pests, some also provide biocontrol of nematodes.

The earthworms in Ontario are not native. The glaciers that receded more than 10,000 years ago, scoured and deposited the parent materials for our soils, leaving little or no life behind. Earthworms came with the early settlers on tools, seeds and plants and as earthen ballast from ships. Earthworm populations vary greatly with soil type and drainage, crop management and location within Ontario.

There are essentially three types of earthworms that are defined by their burrowing habits:

• shallow horizontal burrows filled in by the small earthworms, usually within the top 8-10 cm of soil
  
• horizontal permanent burrows created by shallow dwelling worms that are small, may be red, grey or almost transparent, usually found in the top 10-15 cm of soil
  
• vertical, permanent burrows created by the larger, deep soil dwelling worms such as Lumbricus terrestris

Earthworms can number in the thousands per square metre under a pasture, or less than 10/ m2 under a typical field crop rotation. Earthworms - particularly dew worms - burrow extensively in the soil creating long vertical burrows or macropores which improve drainage. Soil passes through the gut of the earthworm and nutrient-rich feces are plastered along the burrow walls. Plant roots often make use of these burrows, due to the ease of rooting, good aeration and abundant nutrients. On the soil surface, dew worms burrows are easily identified by the pile of castings and crop residues also called middens. In perennial crops like orchards, earthworms play an important role in incorporating leaves and other organic materials, increasing fertility and reducing scab inoculum.

Orchard soils are a little different in terms of soil life and activity. The reduction in tillage with a perennial crop and the leaf litter and shading create a much different environment than a corn crop. Populations of most soil plants and animals are generally higher due to the lack of disturbance. Concerns over pesticides are often expressed for soil life. Studies show, however, that earthmoving and tillage has a far greater effect than most pesticide applications. Ground cover management can have an impact though. Frequently tilled row middles have lower numbers of earthworms due to damage to the worms from tillage implements. Cover crop middles tend to have slightly higher earthworm numbers as a result of the shading and return of organic materials. In contrast, earthworm numbers under dense sod middles are lower. Monitoring of earthworm numbers in some orchards in southwestern Ontario indicate significantly higher numbers of earthworms in the herbicide strip immediately under the trees. Leaf litter, shading and in some cases irrigation may have helped increase these numbers.