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Background information

Biological control best practice

Possible interactions with existing biological control agents for the target host

When considering the introduction of a new biological control agent, it is important to be aware of existing biological control agents (see BCANZ []) whether they have been deliberately released in the past or arrived accidentally, or indeed if they are naturally occurring. This is something that is usually addressed by the EPA (and ERMA NZ before them) and taken into consideration when making a decision. When applying for a biological control release, it is advisable to take into account existing natural enemies and attempting to predict as far as possible what interactions are likely to occur. For example, in the case of the application to introduce the gall fly, Procecidochares alani to control mist flower, complementarity between this proposed biological control agent and the previously introduced fungus Entyloma ageratinae was considered to be a key issue. The following is taken from the decision [] published by ERMA New Zealand, and can be found on the EPA website:

"The Committee was concerned that the benefits of introducing P. alani over and above those provided by the existing mist flower biological control agent, E. ageratinae, were not clear. The Committee therefore considered a key issue to be the extent of complementarity between the two control agents.

It is noted that the degree of control likely to be achieved by a biological control agent is largely unpredictable and is a function of the particular biological control agent-host-environment combination. Evidence recorded to date in New Zealand suggests that the impact of E. ageratinae is likely to be uneven geographically, seasonally and from year to year. In particular, the requirement of the fungus for moisture to successfully infect leaves, means that E. ageratinae will probably be most active at wet periods of the year and least active in mid-summer. The optimum temperature for E. ageratinae is several degrees lower than P. alani, also suggesting the fungus would be more active than P. alani in spring and autumn.

P. alani is likely to be most abundant and damaging in mid to late summer, coinciding with peak abundance of mist flower growing points into which flies can deposit eggs. Field observations in Hawaii and laboratory evidence suggest that P. alani can suppress the growth of mist flower in the absence of E. ageratinae. But were both agents to occur at the same site, it is possible that the production of galls during summer by P. alani may have an additive impact on mist flower, if plants were already stressed from defoliation by fungal attack during spring. The Committee notes that research to measure the relative impacts of these agents and their interactions has not been undertaken in Hawaii, and would have to await release of both agents into New Zealand.

Based on their habitat preferences, life cycles and modes of attack, it appears likely that P. alani and E. ageratinae will work in a complimentary fashion. It is also likely that some sites will be more conducive to one agent than the other, depending on environmental conditions such as moisture, temperature and exposure to wind and sun. Therefore, release of P. alani may increase the number of sites where mist flower is subject to biological control. The Committee concludes that if P. alani establishes, it is likely to provide additional benefits over and above those of the existing biological control programme against mist flower."

This extract from the decision on mist flower is reproduced in full to illustrate the degree of detail that was required at the time by ERMA New Zealand to assess complementarity between existing and proposed biological control agents. Clearly it is not in the interests of NZ to introduce a new organism which is likely to jeopardise the activity of an existing natural enemy, and hence the EPA would have to consider this as a potential risk factor.