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.
predators and some of their prey in fruit crops
||Soilborne fungi, bacteria
mites, thrips, aphids, pear psylla, young scale, various insect
bugs, aphids, leafhoppers, spider mites
scale insects, pear psylla, mealybugs, other soft-bodied prey
scale insects, mealybugs, pear psylla leafhoppers, thrips, mites
mites, aphids, leafhoppers, pear psylla, scale insects
||Fungi, eg. grapevine powdery mildew
||Soilborne fungi, bacteria, other nematodes
psylla, aphids, leafhoppers
flies or flower flies
parasites and some of their hosts in fruit crops
|Bacillus thuringiensis (bacterium)
|Pseudomonas fluorescens (bacterium)
|Beauveria bassiana (fungus)
||Pythium, Rhizoctonia and other pathogens
|Ampelomyces quisqualis (fungus)
|Arthrobytris (nematode-trapping fungus)
Pasteuria penetrans (bacterium)
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.
|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.
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.
good, the bad and the ugly
Many kinds of organisms inhabit fruit production systems.
Monitor both pest and beneficial species.
|A good scouting
program is key to making ecologically sound pest management decisions.
The need for control and the impact of any action are determined
by monitoring pest and natural enemy populations. Sampling provides
information on the organisms present, their stages of development,
population densities, and the ratio of pests to natural enemies.
A scout must know how an organism develops because different life
stages may be monitored in different ways. For example, you would
sample fruit to look for grape berry moth larvae, but use pheromone
traps to monitor adults. In addition, a single life stage may move
to different plant parts as the season progresses. Oriental fruit
moth larvae attack terminal shoots early in the season, feed in
shoots and fruit in the middle of the summer, but only infest fruit
late in the season.
predators and parasitoids is as important as monitoring pest populations.
Higher pest densities can be tolerated when populations of natural
enemies are also high. For example, a mite spray may be warranted
in apple when there are two or three mites per leaf if there are
no natural enemies, but a grower might wait until there are five
or six per leaf if one predator mite per leaf is also present. Monitoring
parasitoid populations can be tricky as often it is only the signs
of their presence that can be readily detected. For example, aphid
parasitoids that feed within their hosts cause the aphids to become
puffy or mummified and tan, golden, or black in color. A round hole
can be observed where the wasp has cut its way out of the aphid
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
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.
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.
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. In this case, 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
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.
|Ants tending aphids
to benefit from the aphid-produced honeydew. Photo:
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
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.
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
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.
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.
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.
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
In similar fashion,
minor diseases that are not normally a problem can become important when
major diseases are controlled. Since pathogens often compete for space
and nutrients, removing one pathogen with a fungicide may benefit other
pathogens that are not affected by that particular fungicide. An example
of this is the increase in Alternaria infections of blueberry fruit in
fields that have been treated with a fungicide against anthracnose fruit