Choosing a good release site
Adult field horsetail weevil
A critical success factor for the successful establishment of new weed biocontrol agents is choosing good release sites. This is even more critical when following the strategy of loading up just a few sites, because the agents are not readily available in large numbers, as described for the Darwin’s barberry seed weevil (Berberidicola exaratus) in the story above. Here are some tips to consider.
It is best to choose a site that is not intensively managed, or likely to be so in the future. While biocontrol agents can sometimes be established in conjunction with intensive land management activities such as spraying and mowing, this takes careful thought to achieve and for most sites will be too difficult. In a farming situation the best places to make initial releases of biocontrol agents are therefore usually not on the best flat land but in steep gullies or rough areas that machinery can’t get too. Conservation land, including covenanted private land, is often ideal since intensive management is unlikely.
Talking to the owners of potential release sites to determine their likely plans for an area is therefore important, as is gauging their interest in trialling biocontrol. If the landowners meet the criteria it is also important to involve them at release time so they know what the agents look like, where they were released, how to manage the site and what to expect. For example, it may be important for them to realise that only subtle signs an agent has established may be present for 5 years following release, so they don’t write it off as a failure too soon!
Marking the site with a peg or tape can be useful, especially if follow-up visits may be made by others, and a sign can educate or pique the interest of neighbours or passers-by. Keeping good records, including photos and GPS readings, will also help to locate sites in the future. Fencing off a release site is rarely recommended because it can change the dynamics of plant communities in unwanted ways (e.g. lack of grazing or mowing can allow grass to out-compete some weeds at a time when you want a healthy weed population).
It is also worth checking out the potential for natural hazards to wipe out your release site. While some of these can be difficult to predict in advance, such as accidental fires or slips due to large earthquakes, a propensity of an area to flood is usually well known by locals.
The size of the weed infestation is not too critical, although a really tiny infestation involving only a handful of plants should be avoided. However, small, slightly isolated infestations (around 10–20 m2) can be useful as nursery sites if harvesting and redistribution are planned for the future. Agents may be easier to collect in good numbers at these sites, provided they do not rapidly disperse. A large infestation offers plenty of choice for the agent, and possibly different micro-climates, but it may take longer to detect establishment or be able to begin harvesting.
The health of plants at potential release sites is a more important consideration. Healthy plants will provide better nutrition for the agents, and are more likely to produce flowers and fruits or pods for those agents that need them. Sites where plants are stressed because they are already being heavily attacked by other biocontrol agents should be avoided. As a general rule, warm, sheltered sites with adequate moisture will allow both the weed and control agents to do better than cold, exposed, excessively wet or dry areas, but there can be exceptions.