Natural Enemies: Predators & Parasites
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Excerpt from Fruit Crop Ecology and Management, Chapter 2: Managing the Community of Pests and Beneficials by Larry Gut, Annemiek Schilder, Rufus Isaacs and Patricia McManus
Ecological concept: Natural enemies help keep pest populations in check.
Putting it into practice: Preserve natural enemies.
Integrated pest management emphasizes the importance of interactions between pests and the natural enemies that prey upon them. When broad-spectrum insecticides are applied, pest and non-pest species are killed and the balance of the community is disrupted. For example, pesticide use in pear orchards to control codling moth can also destroy the natural enemies of pear psylla. In the absence of its natural enemies, pear psylla can reach high densities and cause significant damage to the fruit.
There is good potential for biological control of several fruit pests if populations of predators and parasitoids are preserved and enhanced. For example, biological control of plant-eating mites is achieved in many fruit production systems by conserving predatory mites, as well as predacious beetles and bugs. To avoid killing natural enemies of mites, pesticides must be selected carefully. Two classes of insecticides that are highly toxic for mite predators are the pyrethroids and the carbamates. Biological control of apple leafminers by parasitoids is common in Pacific Northwest orchards, and has potential in other areas. To conserve these important allies, avoid using moderately and highly toxic insecticides from mid-June to early July when adult parasitoids are most active.
Use control tactics that are the least harmful to natural enemies. Avoid insecticides that are highly toxic to predators and parasitoids. If one of these materials must be used, spray when natural enemies are least vulnerable. In general, broad-spectrum insecticides applied early in the growing season, before many natural enemies have become active or moved into the crop, tend to be less disruptive than those applied later in the summer. Use spot treatments or delayed applications when it appears natural enemies might be able to provide control. View images, more information about natural enemies.
Natural enemies are divided into two main groups: predators and parasites. A predator lives by capturing and feeding on another species. Predators are usually larger and more powerful than their prey. Many of the most common predators in fruit production systems attack a wide range of pest species and help regulate pest population densities. Several are listed here with some of their prey in fruit crops.
- Amoebae: Soilborne fungi, bacteria
- Anthocorid bugs: Spider mites, thrips, aphids, pear psylla, young scale, various insect eggs
- Bigeyed bugs: Lygus bugs, aphids, leafhoppers, spider mites
- Collembola: Fungi
- Ladybird beetles: Aphids, scale insects, pear psylla, mealybugs, other soft-bodied prey
- Lacewings: Aphids, scale insects, mealybugs, pear psylla leafhoppers, thrips, mites
- Mirid bugs: Spider mites, aphids, leafhoppers, pear psylla, scale insects
- Mycophagous mites: Fungi, eg. grapevine powdery mildew
- Nematodes: Soilborne fungi, bacteria, other nematodes
- Predatory mites: Plant-feeding mites
- Spiders: Pear psylla, aphids, leafhoppers
- Syrphid flies or flower flies: Aphids, scale insects
A parasite lives in, on, or with another organism and obtains food and usually shelter at the host's expense. Parasitic insects and microbes are important in the biological control of many pests. Plant pathogens may be considered parasites that cause disease symptoms in plants.
An insect that is parasitic on other insects during its immature stages, but is free-living as an adult, is called a parasitoid. Most parasitoids are small flies or wasps. Parasitoids are often common in flowering plants such as fruit crops and therefore are potentially very beneficial allies of fruit growers. Some parasitoids are specialists, attacking one or a few host species, while a few are generalists and use a wide variety of other insects as hosts. The free-living adults often feed on the nectar provided by flowers. The female parasitoid finds a host and lays eggs. The parasitoid larva develops inside or on the host. At first the larva feeds only on fatty tissues, allowing the host to continue to grow and develop. As the parasitoid nears the end of its development, it consumes the host's vital organs, killing it. The parasitoid larva pupates and later emerges as an adult.
The emerging parasitoid often leaves behind telltale signs of its handiwork. When scouting for pests, also watch for parasitoid pupal cases or emergence holes in insect bodies. Try to choose management strategies that protect parasitoids such as using selective insecticides.
In the diagram, A. A wasp lays an egg in a host (in this example, an aphid). B and C. As the host feeds and grows, so does the wasp larva. D.The parasitoid kills then pupates within the dead host. E. An adult parasitoid emerges from the dead host.
The following are common parasites and some of their hosts in fruit crops.
- Aphelinid wasps: Aphids
- Tachinid flies: Caterpillars, beetles
- Trichogramma wasps: Moth eggs
- Bacillus thuringiensis (bacterium): Butterfly/moth larvae
- Pseudomonas fluorescens (bacterium): Fungi
- Polyhedrosis virus: Butterfly/moth larvae
- Beauveria bassiana (fungus): Many insects
- Trichoderma harzianum (fungus): Pythium, Rhizoctonia and other pathogens
- Ampelomyces quisqualis (fungus): Powdery mildew
- Arthrobytris (nematode-trapping fungus): Nematodes
- Steinernema (nematode): Insect larvae
- Pasteuria penetrans (bacterium): Nematodes
Parasitic microbes such as fungi, bacteria, and viruses can cause diseases of insects. Bacillus thuringiensis (Bt) is a well-known bacterium that kills insects with a potent toxin. Bt must be eaten before it can kill its host, so sprays should be timed to coincide with warm periods when the target insect is most likely to be feeding. Once consumed, the Bt toxin destroys the insect's gut. Infected insects become lethargic, stop feeding, and die.
Parasites also keep pathogen populations in check. For instance, the fungus Ampelomyces quisqualis parasitizes powdery mildew fungi on several fruit crops. A commercial formulation can be applied to slow disease development by reducing vigor and spore production of the mildew colonies. Also well known are Trichoderma species, which parasitize soilborne pathogens such as Rhizoctonia and Pythium.
Even more interesting are soil-inhabiting fungi that trap and devour nematodes with specialized structures that resemble lollipops and lassos. Bacteria are also known to parasitize nematodes. For example, the bacterium Pasteuria penetrans attacks the root knot nematode.
Mutually beneficial relationships
Some parasitic fungi have mutually beneficial relationships with plants. For example, mycorrhizae are fungi that live inside plant roots and generally have a beneficial effect on the plant. They use their extensive threadlike mycelia to absorb nutrients and water from the soil, passing them on to the plant roots. In return, the plant provides shelter and nourishes the fungus. Mycorrhizae may also protect plant roots against pathogen invasion.
Some insects live together to benefit one another and this can make pest management more challenging. Aphids can often be found living in a mutually beneficial arrangement with colonies of ants. The aphids produce honeydew and the ants harvest the sugary liquid for food. Worker ants can be seen running between aphid colonies and their nests in the soil. In return, the ants protect aphids from predators, and may even carry them to a better habitat if the plant starts to die. This interaction can lead to rapid increases in aphid populations because natural enemies are prevented from regulating the aphids.
Pathogen and vector relationships
Another important interaction among organisms in a fruit crop is the role of insects in spreading diseases. Blueberry aphids, which are pests in their own right, can also vector blueberry shoestring virus, which causes malformation of blueberry shoots and leaves and a decline in vigor and productivity. The mummy berry fungus forms a unique alliance with an insect that visits blueberry flowers daily. Bees are fooled into thinking that the shoots covered with fungal spores are actually flowers by the distinct UV patterns produced by the diseased tissue. The spores, which are produced in a sweet sticky matrix, easily stick to the bee's body and are delivered to the stigma where infection occurs. Below ground, certain nematodes are also vectors for plant pathogens. The dagger nematode can transmit the tobacco and tomato ringspot viruses to various hosts, including grapes. These viruses cause a slow decline of the grapevine.