Biological control is where natural enemies (i.e. biocontrol agents) are utilised to reduce pest numbers. Natural enemies are typically invertebrates and pathogens, and almost always come from the native ‘home’ range of the pest, and therefore need to be imported into New Zealand.
Classical biological control offers a long-term, self-sustaining and environmentally friendly way of reducing wasp numbers. However, wasps would never be eliminated because the control agents depend on wasps for their own survival. We need an organism that will attack wasps and do well in New Zealand conditions but not harm other insects such as honeybees. Finding suitable biological control agents can be a long process because identifying an appropriate agent and screening it to make sure it attacks only wasps can take several years. Once an agent is set free in the environment, it may take many more years to have any noticeable effect.
Some biocontrol agents are less suitable for classical biological control but are more suited for Inundative Biological Control. These agents are usually released in massive numbers and ‘inundate’ the pest, suppressing it numbers. However, these agents generally have trouble in sustaining their large populations, and so need to be re-released at regular intervals.
Invertebrates as Biological Control Agents
In their native range, a Vespula wasp colony is a habitat for a wide range of organisms. Many types of species occur in their nests, some are predators, others feed on remains of dead wasps, some utilise mould or the paper nest structure; others are parasitic. Unfortunately there is very little information on what species attack wasps and even less information on the damage they exert on Vespula.
Biological control for wasps in New Zealand has received significant attention with the main focus on parasitoid wasps in the genus Sphecophaga. These are parasitoid wasps that attack the larvae of Vespula wasps (and a few other social wasps), laying their eggs on the outside of the developing larvae and pre-pupa. These eggs develop into larvae and consume the wasp host. Species of Sphecophaga are approximately 5–8 mm in size and are from the family Ichneumonidae (Order: Hymenoptera).
Despite being released at many sites throughout New Zealand, the species Sphecophaga vesparum vesparum, is only known to have established at two sites (Ashley Forest, north of Christchurch, and Pelorus Bridge in Marlborough) and there is no evidence it is reducing wasp numbers.
Several species have been proposed as biocontrol agents against wasps, but few have been thoroughly assessed in terms of their likely impact, or non-target impacts.
The two most promising agents include:
- Hoverflys. Two species of hoverflies (Diptera: Syrphidae) are known to be associated with the nests of Vespid wasps. These hoverflies, Volucella inanis, V. pellucens, are large (1.5 cm), brightly coloured, and relatively common across the UK and Europe. One species, Volucella inanis, seems to be an obligate ectoparasite on vespid wasps; they are very flattened so that they fit into the larval cells beside the wasp larvae on which they feed. Volucella larvae are mostly scavengers during their larval stage but they can be predators of wasp larvae and pupae, especially late in the season.
- Pneumolaelaps mite. Recently in Canterbury, mites have been discovered attached to backs of queen Vespula wasps. The mites have been identified as a species of Pneumolaelaps, but little is known about their biology. Overseas these mites seem to feed on nectar and pollen, and use the bumblebee or wasp nests to overwinter. However, in New Zealand the mites have been observed to feed on wasp larvae and could be responsible for spreading viruses to the wasps. It also seems the mites hitchhike on the back of the queen wasps for dispersal to new areas (an example of phoresy).
The potential for this ‘new discovery’ as a biological control agent is now being assessed with funding from the Sustainable Farming Fund.
Pathogens as Biological Control Agents
Pathogens (agents causing diseases) are a factor regulating many insect populations. For example, research into controlling honeybee diseases is of ongoing importance to the beekeeping industry. Pathogens are also beginning to be used to control social insects such as termites and fire ants.
Previous research has shown that a number of pathogens have been recorded from Vespula nests (and some other related social wasps), including 50 fungal, 12 bacterial, 5–7 nematode, 4 protozoan, and 2 viral species.
To date very few pathogens have been confirmed as actually pathogenic towards wasps, but pathogens remain significant biocontrol option for wasps; perhaps best suited as inundative control agents, or delivered in combination with other natural enemies (e.g. Pneumolaelaps mites, see above).
Wasps have a natural defence against some pathogens. They have antibiotics in their saliva and venom, and can quickly detect and remove infected individuals from the nest before disease spreads. However, it may be possible to overcome this cleaning behaviour by combining chemicals with pathogens.
Research is currently underway at Victoria University examining pathogens of wasps in their native and introduced ranges, and whether the lack of natural enemies results in successful invasion for wasps.