Releasing biological control agents
The release strategy depends on the biology of the agent and the host, and on how many agents are available for release. Characteristics such as high reproductive rate and dispersal will favour establishment from a low number of founding individuals, whereas more sedentary agents may need to be released in larger numbers. It will vary from agent to agent, and no two strategies will be alike. The following principles should be taken into account when planning the strategy.
How many release sites?
Every release site will vary in such attributes as vegetation, aspect and microclimate. Releasing at only one site risks failure not because agents cannot establish in New Zealand but because the particular site was unsuitable, or was destroyed by some stochastic event such as a rainstorm or fire. The more sites that are selected, the more likely it is that at least one will match the agent's requirements for establishment. Memmott et al. (1998) found that gorse thrips (Sericothrips staphylinus Haliday) established at only 33% of the sites where 10, 30 and 90 thrips were released initially, but at 100% of sites where releases numbered 270 or 810 thrips. If faced with limited numbers of agents to release, the first priority is to release sufficient agents to avoid Allee effects (where the reproduction and survival per individual is lower in small populations than in large populations) but otherwise maximise the number of sites at which the agent is released.
How many releases and how often?
There are several release strategies that can be employed. The simplest is to release as many agents as practicable at a release site at the correct time of the year. Alternatively, smaller releases may be made at regular intervals to make sure that agents are in synchrony with hosts, or to augment populations at intervals to overcome Allee and founder effects (Etzel and Legner 1999).
How many agents per release?
In Canada, Beirne (1975) found that on average, the more parasitoids that were released, the more likely it was that the agents established. For species where there were more than 800 individuals per release, 60% of species established, whereas for species where there were fewer than 800 individuals per release, only 15% established. In New Zealand, Cameron et al. (1993) found that where over 30,000 individuals were released over the course of a biological control programme, 80% of species established, but for programmes in which fewer than 1,000 individuals were released, only 44% of species established. The message from both studies seems to be that the more agents that can be released the better. However, release of large numbers is not a prerequisite for agent establishment. Cameron et al. (1993) record that Lathrolestes luteolator, an ichneumonid parasitoid of Caliroa cerasi established successfully in New Zealand even though only 27 individuals were released. The parthenogenetic braconid Pholetesor pedias Nixon was sent to Canada from New Zealand for the control of Phyllonorycter blancardella (F.), but only two females survived the journey (Richard Hill pers. comm.). These were released onto sleeved branches, and within 18 months years, 25.7% of the leafminer larvae in the orchard were parasitised and the agent had been detected 43km away (Laing and Heraty 1981).
What stage to release?
You may have a choice of releasing adults or immature stages. Your choice will depend on the biology of the agent. You may wish to ensure that adults exhibit natural host-searching, mate selection and dispersal behaviours by having them emerge in the field rather than the laboratory. Conversely, you may have few agents, and you may wish to ensure that adults are fertile by having them mate or feed before release. You may wish to hold adults until any pre-oviposition period is over to ensure that most adults lay eggs before dying in the field.
How to release?
Your chosen release method may be as prosaic as driving to a single release site and opening a box. It might be as spectacular as the cassava mealy bug programme in Africa, where agents were released from planes on a landscape scale.
You may choose open release or you may make your first release onto hosts in sleeved stems, or onto plants in cages. There are many cage designs, including "open" cages (delayed emigration). It is important to retain the released agents near the release point to foster local population development. For highly active and vagile agents it may be important to minimise the dispersal instinct of agents. If possible, do not release insects in strong direct sunlight as you risk agents dispersing towards the sun on release. Choose a cloudy day, and release early in the morning or late in the afternoon. Agents with a high propensity to disperse may require larger releases than more sedentary agents.
Some other logistic issues to think about are:
- Boxes: ventilation is often not essential. It is important to keep relative humidity high, but it is also vital to avoid condensation. Incorporate lots of absorbent paper in packaging to absorb free water.
- You may need to feed agents in transit with a honey and water solution delivered on cotton wicks, or sugar solution in solid agar drops.
- Keep insects cool, but not cold.
- Keep all insects out of direct sunlight. Even short exposure is fatal to boxed insects.
- Weather: do not release when temperatures are too low to allow insects to seek shelter, or too high to allow normal behaviours. Do not release in the rain.
Beirne B.P. (1975). Biological control attempts by introductions against pest insects in the field in Canada. The Canadian Entomologist 107: 225-236.
Cameron P., Hill R.L., Bain J., Thomas W.P. (1993). Analysis of importations for biological control of insect pests and weeds in New Zealand. Biocontrol Science and Technology 3: 387-404.
Etzel L.K. and Legner E.F. (1999). Culture and colonisation. Pp. 125-197 In: Handbook of Biological Control: Principles and Applications of Biological Control, T.S. Bellows and T.W. Fisher (Ed.) Academic Press.
Laing J.E. and Heraty J.M. (1981). Establishment in Canada of the parasite Apanteles pedias Nixon on the spotted tentiform leafminer, Phyllonorycter blancardella (F.). Environmental Entomology 10: 933-935.
Memmott J., Fowler S.V., Hill R.L. (1998). The effect of release size on the probability of establishment of biological control agents: Gorse thrips (Sericothrips staphylinus) released against gorse (Ulex europaeus) in New Zealand. Biocontrol Science and Technology 8: 103-115.
Timing of releases
Collection and redistribution