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.

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.

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.

Alternative food sources
Some predators and parasitoids use alternative food sources during the growing season. These include prey or hosts other than pests, and nectar producing plants other than fruit crops. If an alternative host is not available, the predator or parasitoid may not survive or stay long enough in the crop to control pests when needed. Predatory mites often feed on rust mites when their primary hosts, spider mites, are absent or in low numbers.

Parasitoids may require an alternative host to complete their life cycle. A small parasitic wasp, Colpoclypeus florus, can have a major impact on some leafroller populations in apple orchards. However, C. florus is often not present in high numbers early in the season because none of the leafrollers in the orchard overwinter as late instar larvae, the host size required to complete its development. Another leafroller host of this parasitic wasp overwinters as a large larva on wild rose, found in wild habitats around orchards. Larvae of the parasitoid successfully overwinter in this host on rose and complete their development early in the spring. They then emerge and can fly to colonize leafrollers in nearby orchards.

Many natural enemies require more than one kind of food to develop normally and sustain their populations. Syrphid fly adults supplement their diets by gathering and eating pollen from flowering plants. Where natural enemies have access to pollen and nectar, there is often more predation and lower abundance of pests. Similarly, parasitic wasps supplement their diets by feeding on nectar, aphid honeydew, and other sources of sugar. For example, Trichogramma species are tiny wasps that parasitize the eggs of moths, such as codling moth. Planting a cover crop that includes flowering plants is a good way to provide nectar sources for these beneficial insects, but care must be taken in selecting a cover crop that is not a host for other pests.

Some pathogens also need to find alternative hosts when a fruit host is unavailable. For example, the root knot nematode can also reproduce on dandelions in vineyards. This weed can also serve as a host for viruses that are vectored by the dagger nematode. Weed management is essential to reduce these types of risk. Cover crops that suppress weeds and nematodes have been used in fruit systems and are likely to play an even more important role in the future. Verticillium albo-atrum, a soilborne fungus that causes a severe wilt in strawberries, also attacks the roots of many other hosts, especially solanaceous crops like tomatoes, peppers, and potatoes. Growers are advised not to plant strawberries after Verticillium-susceptible crops.

Management influences on the community
Management practices change the dynamics of the community of pests and natural enemies within the crop. The positive effects like reduced pest numbers and increased yields are obvious. Certain management decisions, however, can have unintended impacts on the community.

Impact of cultural practices
Fire blight of apple and pear was once considered a sporadic disease that usually could be managed by combining cultural and chemical methods. Since the early 1990's, however, fire blight has plagued apple growers to a degree previously unknown. Indeed, modern fire blight epidemics have been an economic disaster.

The "new face" of fire blight has resulted from several changes in the apple crop habitat. Genetic, physical, and cultural factors have interacted to create ideal conditions for growth and spread of the fire blight pathogen:

• The number of trees planted per acre has increased dramatically. This means that fire blight can move more easily from tree to tree.
• More acres are being planted to highly susceptible cultivars, including Braeburn, Fuji, Jonathan, and Rome.
• Size-controlling rootstocks, many of which are highly susceptible to fire blight, are used to achieve high-density plantings.
• Trees are being pushed to bear earlier and training systems are being adopted that are very different from the way apple trees grow in nature. These modern orchards have a high density of apple tissues such as flowers and young shoots that are very susceptible to fire blight. Traditional, low-density orchards also have susceptible tissues, but they are interspersed with a lot of older, woody tissues that are much less favorable to the growth of the fire blight pathogen.

New tools are being developed to manage fire blight on susceptible varieties in high-density plantings. For example, there are a few size-controlling rootstocks that are relatively resistant to fire blight. Certain plant growth regulators reduce vigorous shoot growth and thereby reduce shoot susceptibility to fire blight. Also, larger trees planted at lower densities will not have the production potential of the more modern orchard systems, but they will be more likely to survive fire blight long enough to yield a crop.

Pesticide impacts on the community
Although pesticide sprays are generally targeted against one or a few pest populations, they often influence other pest and non-pest species. Some insecticides are very toxic to predators and parasitoids. Destroying these natural enemies often results in target pest resurgence or secondary pest outbreaks. Some pesticides have a greater impact on the natural enemies than the target pest. Target pest resurgence can result when the unfavorable ratio of pests to natural enemies permits a rapid increase or resurgence of the pest population. For example, biological control of twospotted spider mite by predatory mites is common in many fruit crops. Insecticides that are applied for control of pest mites and insects are often highly toxic to predatory mites. Some pest mites survive the spray, but most predators are killed. The population of twospotted spider mites is able to quickly rebound, reaching economically damaging levels before its natural enemy can re-colonize from unsprayed areas.

Pre-plant fumigation of the soil is often used for control of black root rot in strawberry. However, if black root rot pathogens are re-introduced in fumigated soil with infected planting material, the disease comes back with a vengeance, presumably because competitive organisms in the soil have been eliminated by the fumigation.

A secondary pest outbreak occurs when a pesticide that was applied to control one pest kills the natural enemies that were keeping a second pest population in check. For example, a complex of predators can be helpful in keeping aphid populations from reaching damaging levels. The broad-spectrum insecticides that are used to control key pests are highly toxic to these predators. As a result, applying one of these insecticides often leads to secondary outbreaks of aphid populations. Pesticide applications can also impact beneficial microbes leading to increased plant disease problems.

Secondary pest problems are not always associated with the destruction of natural enemies. Control of codling moth by mating disruption entails releasing enough sex pheromone into orchards to interfere with mate location, reducing reproduction and subsequent larval infestations. However, using this highly specific tactic in place of broad-spectrum insecticides can also have a significant impact on other potential pests. In disrupted orchards, leafrollers that were kept at non-damaging levels by broad-spectrum insecticides are now only suppressed by natural enemies. Unless natural controls provide sufficient suppression, leafroller populations will increase, sometimes reaching damaging levels.