Biological Control: Another dangerous method of eradicating non-native species

We were recently reminded of the use of biological controls to eradicate non-native species when we learned that Australian insects may have been illegally imported to California to kill eucalyptus, which had been virtually pest free until 1983.  So, an article in the New York Times about the development of a fungus for the purpose of killing cheatgrass (Bromus tectorum) caught our attention.  The fungus has been given the ominous name, Black Fingers of Death, for the black stubs of cheatgrass infected with the fungus.

Cheatgrass is one of the non-native grasses that have essentially replaced native grasses throughout the United States.  It was probably introduced with ship ballast and wheat seed stock in about 1850.  As we have reported, native grasses were quickly replaced by the non-native grasses which tolerate the heavy grazing of domesticated animals brought by settlers.    Native Americans had no domesticated animals.

Biological controls have frequently caused more serious damage than the problems they were intended to solve.  Therefore, we would hope that their intended target is doing more damage than the potential damage of its biological control.   We must ask if the cure is worse than the disease.  And in this case, we don’t think the damage done by cheatgrass justifies inflicting it with the Black Fingers of Death.

The track record of biological control

Biological control is the intentional introduction of animals, pests, microbes, fungi, pathogens, etc., for the purpose of killing a plant or animal which is perceived to be causing a problem.  The ways in which some of these biocontrols have gone badly wrong are as varied and as many as the methods used.

Introduced species of plants are said to have an initial advantage in their new home because their pests and competitors are not always introduced with them.  This is the “enemy release hypothesis” popular amongst native plant advocates to explain the tendency of non-native plants to be invasive.  However, this is usually a temporary advantage which is exaggerated by native plant advocates who do not seem to recognize the speed with which native species can adapt to new species, and vice versa.

Therefore, a popular method of biological control is to import the predator or competitor of the non-native species which is considered invasive.  This is only effective if the pest is selective in its host.  There are many examples of such introductions which did not prove to be selective:  “For the United States mainland, Hawaii, and the Caribbean region, Pemberton (2000) listed 15 species of herbivorous biocontrol insects that have extended their feeding habits to 41 species of native plants…” (1)  Although most of the unintended hosts were related to the intended hosts, some were not.

Similar shifts from target to nontarget species have occurred for biocontrol agents of animal pests:  “For parasitoids introduced to North America for control of insect pests Hawkins and Marino (1997) found that 51 (16.7%) of the 313 introduced species were recorded from nontarget hosts.  For Hawaii, 37 (32.3%) of 115 parasitoid species were noted to use nontarget hosts…biological control introductions are considered to be responsible for extinctions of at least 15 native moth species [in Hawaii].”  (1)

There are also several cases of biological controls escaping from the laboratory setting before they had been adequately tested and approved for release.   A virus escaped the laboratory in Australia and killed 90% of the rabbits in its initial spread through the wild population.  Very quickly, the virus evolved to a less fatal strain that killed less than 50% of the rabbits it infected.  A second virus was then tested and also escaped its laboratory trial and has spread through the rabbit population throughout Australia.

A fly being considered for introduction to control yellow starthistle apparently escaped and damaged a major cash crop of safflower in California according to a study published in 2001, illustrating the risks of biocontrols to agriculture.

This is but a brief description of the diverse ways in which nature has foiled the best efforts of the scientists designing biological controls for non-native species of plants and animals.  The source of this information (1) therefore concludes, “…many releases of species have inadequate justification…The first goal of research must be to show that the introduced biological control agent will not itself cause damage.”  Given this wise advice, we will return to the question, “What damage is being done by cheatgrass and does that damage justify the introduction of The Black Fingers of Death?”

Why is cheatgrass considered a problem?

Cheatgrass is one of the many non-native annual grasses which have replaced the native grasses which were not adapted to the grazing of domesticated animals.  Cheatgrass is a valuable nutritional source for grazing animals when it is green and loses much of its nutritional value when it dries.

Grazing is only one of the types of disturbance which create opportunities for non-native grasses to expand their range into unoccupied ground.  Fire is another disturbance which gives cheatgrass a competitive advantage over native grasses because it uses available moisture and germinates before native grasses can gain a foothold on the bare ground cleared by fire.

Cheatgrass is said to increase fire frequency by increasing fuel load and continuity.  Unfortunately, increasing levels of CO₂ (carbon dioxide) in the atmosphere is increasing the fuel load of cheatgrass:  “…the indigestible portion of aboveground plant material [of cheatgrass] …increased with increasing CO₂.” (2)

Carbon dioxide is the predominant greenhouse gas which is contributing to climate change.  And increasing frequency of wildfires is one of the consequences of the higher temperatures associated with climate change.  Therefore, one of the causes of the expanding range of cheatgrass is increasing levels of the greenhouse gases contributing to climate change.  Rather than address the underlying cause, we are apparently planning to poison the cheatgrass with a deadly fungus.

If we are successful in killing the cheatgrass, what will occupy the bare ground?  Will native grasses and shrubs return?  Will whatever occupies the bare ground be an improvement over the cheatgrass which has some nutritional value to grazing animals?  The US Forest Service plant database gives us this warning, “Care must be taken with methods employed to control cheatgrass so that any void left by cheatgrass removal is not filled with another nonnative invasive species that may be even less desirable.” 

Recapitulating familiar themes

The project to develop a deadly fungus to kill cheatgrass is another example of the issues that we often discuss on Million Trees:

• Are the risks of the methods used to eradicate non-native species being adequately assessed and evaluated before projects are undertaken?
• Are the underlying conditions—such as climate change–that have contributed to an “invasion” being addressed by the methods used to eradicate them?  If not, will the effort be successful?
• Is the damage done by the “invasion” greater than the damage done by the methods used to eradicate the invader?  Is the cure worse than the disease?

We do not believe that these questions are being addressed by the many “restoration” projects we see in the San Francisco Bay Area.  Consequently, we believe that these projects often do more harm than good.

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(1)    Cox, George W., Alien Species and Evolution, Island Press, 2004

(2)    Ziska, L.H.; Reeves III, J.B.; Blank, R.R. (2005), “The impact of recent increases in atmospheric CO₂ on biomass production and vegetative retention of cheatgrass (B. tectorum): Implications for fire disturbance.”, Global Change Biology. 11 (8): 1325–1332,