Balancing the risks and benefits of biological control

Biological control against introduced pests in New Zealand have been used constantly since the late 19th century. However, it is much harder to introduce new agents now due to regulations set by the New Zealand Governments Environmental Protection Agency (EPA), previously known as the Environmental Risk Management Authority (ERMA). Biological control agents have been used to target arthropods, weeds and vertebrates that have significant negative effects on the species native to New Zealand and also pests/weeds that cause problems  for primary industries. The risks and benefits of biological controls both need to be examined and evaluated when introducing a new control agent. Reliable research should be undertaken to understand potential problems before jumping to conclusions, Emberson (2000) is a good example of this.

Gorse with biocontrol spider mite infestation (Tetranychus lintearius)
The New Zealand weed gorse with biocontrol spider mite infestation (Tetranychus lintearius). Retrieved from Esteve Conaway, Flickr, Creative commons.

Many species that are weeds now were introduced by Europeans wanting to make their garden look like ‘home’ along with them being used as hedges and shelters. I suppose this is understandable. Imagine leaving home for a country you know nothing about. Many of these plants have escaped into the wild and have naturalized, becoming problematic. For example, gorse (Ulex europaeus), once used as hedges, is now a major weed in farmland. To control these weeds, it can be a good option to use a natural enemy of the weed (usually from its country of origin). There have been a number of invertebrates introduced to control gorse including thrips, spider mite, soft shoot moth, seed weevil pod moth and hard shoot moth. Another recent introduction is of the thistle beetle (Cassida rubiginosa) to control the introduced agriculture weed Californian thistle.

There is some controversy on the use of biological control agents and Emberson (2000) evaluated this.  There have been a number of unsuccessful early introductions of biological control agents, where the agent was not examined closely enough and they have gone on to have negative effects on species they were not meant to affect (or that people did not care about at the time). A good example of this was the introduction of mustelids (stoats, weasels and ferrets) to reduce the out of control rabbit population which ended in negatively impacting bird species. Introductions of new species is a sensitive matter, whether it will affect non target species, conflicts of interests between different parties, or concerns include whether these introductions will increase the competition for resources between natives and exotics or provide alternative hosts affecting native populations.

Stoat with its intended prey as an introduced biological control agent
Stoat with its intended prey as an introduced biological control agent. Retrieved from Andrew, Flickr Creative Commons.

The insect fauna of New Zealand has high endemism, so most of New Zealand’s insects are not found elsewhere. There is concern that introduced invertebrates may impact the endemic species of New Zealand.  This had the potential of putting a stop to the use of specific biological control agents which may be disappointing as this is often the best control option for some weed species. To see if these concerns were actually real Emberson (2000) looked at the numbers of insect species endemic/native, introduced and purposefully introduced for biological controls in New Zealand.

The number of wild introduced vascular plants in New Zealand is larger than the number of natives with the total introduced plants at 2436 and total indigenous plants at 2158, along with an estimated 24,700 exotic species in cultivation. It is fair to say that there is a significant number more exotics than natives/endemics in New Zealand. The insect fauna of New Zealand is not exactly known but  has been estimated to be around 20,000 species and other predictions have been up to 40,000 species. The number of introduced insect species is also not well known with an estimation of around 2600 species that have established since European colonization. This number can be used to look at biological control introductions and put it into perspective if it will really impact on natives. Emberson’s (2000) paper showed that, at most, 2.5% of exotic insects (0.35% of New Zealand’s insect fauna) were from insect introductions for biological controls.

Ladybugs are often encouraged to kill aphids.
Ladybugs are often encouraged as a biological control to suppress aphids. Retrieved from Dano, Flickr, Creative Commons.

Species that are introduced for weed biological controls are highly regulated and are selected because they are host specific and target the problematic weed. EPA is the regulator of these rules. This is an important consideration as accidentally introduced species are generally not host specific and often have economic impacts along with impacts on biodiversity and other organisms. The effect made by insect biological control agents on impacting endemic species seems minor when considering the amount of impact caused by accidentally introduced species. This is a really good result as it means the practice of biological control of weeds is still viable.

Embersons (2000) paper helped advances in weed biological control by relieving concerns that had the potential to stop the introduction of biological control agents. It concluded that through proper research biological control will not affect the high endemism levels found in New Zealand. This paper has also been used in studies since the year 2000 to show the benefits of using biological controls and to help assess the risk of introducing new species. There are of course still major risks when introducing new species into New Zealand but the high regulations by EPA help to reduce the possibility of risks occurring. Reliable research like this is important to help weigh up the risks and benefits of potential biological controls to help the continuation of this practice.

The author Bethanne Smith is a postgraduate student in the Master of Applied Science taught at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

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