What is biological control?

Biological control is the use of beneficial organisms to help control pest organisms.  These beneficial organisms are often referred to as “biocontrol agents” or “natural enemies.”  Weeds, pest insects, and plant diseases are some of the frequent targets of biological control in agriculture.  Natural enemies can themselves be predatory/parasitic insects (targeting pest insects), herbivorous (plant-feeding) insects (targeting weeds), or diseases (“pathogens”) of pest insects, weeds, or even other harmful plant pathogens.  Most of my comments below are directed at the use of predatory/parasitic insects to control pest insects.

A good overview of and background information about biocontrol can be found at the following site, sponsored by Cornell University. http://www.nysaes.cornell.edu/ent/biocontrol/toc.html

Links to other quality biocontrol pages can be found at: http://www.entomology.wisc.edu/mbcn/bcweb.html

And also at the following address.  This page contains several links to information pages about biocontrol projects that currently underway around the country: 

http://www.nysaes.cornell.edu/ent/biocontrol/websites.html

A history of biological control as practiced in this country, and some of the original “classic” success stories, can be found in the document below, written by Mark Hoddle of the University of California Riverside.  Classical biocontrol has been practiced in this country since the 1800’s.  http://www.agattack.com/basics.htm

We usually divide the practice of biological control into three categories: augmentation biocontrol, conservation biocontrol, and classical biocontrol. 

1) Augmentation/innundative biocontrol [sometimes considered to be separate] is adding natural enemies, usually in large quantity, to a specific area where we are trying to manage pests, usually not with the goal of long-term establishment.  This is the type of biocontrol with which the home gardener, for example, is most familiar.  You often see ads in garden supply catalogs for natural enemies that you can order in quantity for local release.  In the agricultural context, this is usually not a practical approach, given the physical scale of the areas needing pest management.  The greenhouse industry can often use augmentation biocontrol to good effect, however, with their smaller, contained environments.

2) Conservation biocontrol is employing practices to help protect, conserve, or attract natural enemies that already occur in an area naturally, to help maximize their benefit.

3) Classical biocontrol is the introduction of natural enemies from abroad, with the goal of long-term establishment.  Many of our insect pests are accidentally introduced from other places.  One of the reasons they become pests is that they arrive without their own natural enemies that help control them in their native range.  By introducing the natural enemies of a given pest here, we re-establish the predator-prey relationships. 

The introduction of biocontrol agents into the US is very strictly regulated by the federal government, through the offices of the United States Department of Agriculture.  Detailed environmental assessments must be done before a permit is issued allowing domestic release of an organism.  One of the goals of this environmental assessment is to determine what kind of impact the introduced organism would be likely to have on non-target (un-intended) species and the environment.  The required environmental assessments required for insect-targeting predatory/parasitic natural enemies used to be much less strict than that required for plant-feeding insects intended for weed biocontrol.  That has changed a lot in recent years, with standards for the environmental assessment of predatory/parasitic insects becoming much higher.  (See discussion of the Asian ladybeetle, below.)

More about Classical biological control efforts in Soybean

What are the different types of insects that provide natural control of other insects?

The two most common types of insects used to control other insects are predators and parasitoids (a type of parasite).  Predators (like ladybeetles) eat prey insects directly, and usually consume many prey in their lifetimes.  Parasitoids (certain types of wasps and flies) often don’t prey on other insects as adults, but instead lay their eggs inside or on a host (prey) insect.  The eggs hatch, and the immature form (larva, plural larvae) consumes the living host from within.

Generalizations can be dangerous because there are always exceptions to the rule.  However, as a rule of thumb, predators tend to be “generalist” in their feeding habits, meaning that they attack a wide range of insect species.  Parasitoids tend to be “specialists” and attack a very narrow range of insect hosts (often just one species, or a few closely related species).   At the following link, click on the icons to learn more about these different types of natural enemies: http://www.nysaes.cornell.edu/ent/biocontrol/index.html

How does biocontrol fit into Integrated Pest Management (IPM)?

There have been some tremendous and dramatic success stories from individual natural enemy introductions that have literally saved entire agricultural industries.  But at a more rank-and-file level, it is often the combined effect of many different predators and parasitoids acting in concert on a given pest that can help keep it to acceptably low levels.  However, biological control alone is often not enough to singlehandedly maintain these low levels pest levels in conventional agricultural systems.

Integrated pest management attempts to combine different strategies for managing pests and to find ways to make them work together.  Biological control is often one component of an integrated pest management strategy.  Insecticides are another important component.  However, when certain insecticides are used to control pest insects, they also can kill the beneficial natural enemies that are in the fields.  How can you use the two strategies together?  These are the types of problems the IPM entomologists often work on.  Some pesticides are narrow-spectrum, and only kill the target pest – not the natural enemies.  So the use of narrow spectrum pesticides, when possible, is a good IPM strategy.  Another strategy is the use of economic thresholds to determine when a pest insect really needs to be controlled.  There are often certain low levels of pest insects that can exist in a crop without causing enough damage to warrant the cost of insecticide treatment.  By using insecticide only when pest levels exceed this threshold, and not before, the cost of an unnecessary insecticide application is saved, and this also helps conserve the natural enemies in the field that are providing a certain level of background control for free.  The economic injury level (EIL) is pest level that harms the crop plants enough to cause economic damage from yield loss equal to the cost of control.  On the other hand, the economic threshold (ET) (also called the action threshold) is the pest level at which we recommend a producer treats the field to keep the pest population from reaching the level where it will cause economic damage that justifies the cost of the treatment.  The ET is lower than the EIL -- not the point at which economic damage occurs, but the point to take action to prevent economic damage.

Is Biocontrol Safe?

What About the Asian Ladybeetle?

One of the greatest concerns when contemplating the introduction of a foreign natural enemy is the impact that it will have on non-target species, and the environment.  The standards for environmental assessment for predators and parasitoids of insect pests have increased greatly in recent years.  An important component of whether an organism is suitable for release/attempted establishment is how host-specific it is – how many prey species it can and will use.  Generalists that attack a wide range of species are no longer considered suitable for release.  The more species a predator attacks, the greater its ecological “footprint” is likely to be – not just in terms of which non-target species may be affected, but also in terms of the ripple effects that flow to associated species from those species being affected.  The Asian ladybeetle, Harmonia axyridis, now familiar to many homeowners and others in the Midwest, is an example of a natural enemy that (though quite beneficial in the context of soybean fields) would still never meet today’s standards for release.  This ladybeetle was not released for biological control of soybean aphid, but has in fact been in this country at least since the 1980s.  For more about the history of the Asian ladybeetle in the US, see:

What About the Asian Ladybeetle?

http://www.ars.usda.gov/is/br/lbeetle/

http://www.ars.usda.gov/is/AR/archive/mar95/001030.beetlemagstory.htm

What About B. Communis?

Back to Binodoxys communis, the parasitoid recently approved for release in the US against the soybean aphid.  What makes it different than the Asian ladybeetle?  The key difference, and the reason that B. communis has met the standards of environmental assessment set by the USDA, is that this species is much more host-specific in terms of the aphid prey species it can use (e.g., it has a much narrower host range, and can successfully attack very few aphid species – and can use no non-aphid species of any type).  This is a common attribute of parasitoids which, because of the parasitic phase of their life cycle as immatures inside the living host body, have much more intimate association with their hosts, and must be more closely adapted to the physiological particulars of their host species.  The researchers repsonsible for the host-range testing of Binodoxys communis tested 20 different non-target aphid species.  These were selected acording to various criteria.  One important criteria was their degree of relatedness to the soybean aphid – running a range from closely related to more distantly related.  The more closely related a non-target potential host is to the main, intended prey, the greater the chance that it will also be an acceptible host.  Testing a range from closely related to less related allowed the researchers to gauge the extent of the host range.  The aphid species for host-range testing were also selected according to which ones would be most likely to fall within the habitat orbit of B. communis if it should be released.  Particular attention was given to aphids that are found in Midwestern prairies, as the proximity of these habitats to soybean fields might put the aphids in them at a higher risk of parasitoid spillover.  Based on the laboratory host-range testing, one non-target species in particular posed the greatest concern, Aphis monardae, an aphid species in the same genus as the soybean aphid.  So B. communis underwent futher testing for this species, to determine if it poses a risk to A. monardae under realistic field conditions.  The researchers found that this aphid dwells mostly down in the flower heads of the plant where it lives and feeds, and is somewhat hidden from the parasitoids.  Futher, A. monardae is guarded by ants, which protect them from predation.  Two published, peer-reviewed journal papers dealing with these interactions (by Kris Wyckhuys, Robert Koch, and George Heimpel at the University of Minnesota, are attached).

The Environmental Assessment of B. communis prepared for the USDA is one of the most exhaustive evaluations of a proposed biocontrol agent ever.  Based on this formal Assessment, which predicts a negligible impact of the parasitoid on non-target species and the environment, the USDA has granted the permit for domestic B. communis release, with hopeful establishment.

Given that already-existing natural enemies in this country provide a meaningful level of soybean aphid control already, why is an introduced biocontrol agent needed?  Most of the natural enemies that currently attack soybean aphid are predators.  The ecological niche for parasitoids remains relatively unfilled.  Thus, a parasitoid would help fill out the complement of different natural enemy types acting against this pest species.

http://www.entomology.wisc.edu/sabc/