Control options
In young vines low numbers of garden weevil can be important because they may chew out the growing tip and affect vine growth. Vines may be killed by low numbers. If vines are planted in paddocks where weeds such as sorrel and capeweed were abundant, garden weevil may already be present in large numbers. In such cases, control weeds before planting or apply control measures as vines start to shoot.
The first consideration is to decide whether garden weevil is a problem. If there was a problem the previous season or if monitoring has revealed the potential for a high number of adults, it is important to consider and apply timely control options.
Checking for leaf damage across blocks at regular intervals during the emergence period from spring to early summer is a good guide as to whether and when control is required. Often leaf damage and minor bunch damage may be tolerated.
Biological control
Poultry such as bantams, chickens, guinea fowl and turkeys at densities around 50 per hectare can reduce weevil numbers over time. These birds have the added benefit of reducing wingless grasshopper abundance. If birds are to be used, consider fox control options and mobile pens for chickens.
Entomopathogenic nematodes have been used to control certain species of weevils, for example black vine weevil, Otiorhynchus sulcatus. They infect and kill both the larval and pupal stages. Experiments in Western Australia with a parasitic nematode Heterorhabditis zealandica, that attacks the larval stage of certain soil insects, were unsuccessful in controlling garden weevil. Further research on parasitic nematodes is being conducted in South Africa.
In South Africa, a species of wasp, the fairyfly Cleruchus depressus has been recorded as an egg parasite of garden weevil. While the levels of parasism in the field have been high at times, the overall level is regarded as insufficient to exert any meaningful level of control. The locations chosen for laying eggs in soil and within plant debris, would be limitations for the wasp to locate eggs.
Cultural control
Garden weevil larvae and adults breed successfully on a range of weeds such as sorrel, capeweed, dandelion and dock. Removing these weeds will help reduce weevil survival and abundance.
Physical control
Repellent polybutane sticky bands placed on the trunks of vines and posts in vineyards will prevent weevils from entering the canopy. The material can be used either on plastic bands or placed directly on posts and vines.
This approach requires a lot of labour but may be an option for newly planted vines, intensive areas of production such as table grapes or in hot spots. The sticky material is repellant to garden weevil adults, but other weevils such as apple weevil get trapped in it and eventually walk over it.
Polybutane is phytotoxic to some plants when applied directly to the trunk. If garden weevil is a pest in other crops, test the material first by applying it to a small branch rather than directly on the trunk.
Research and commercial application of artifical fibre bands placed on the trunks of vines have given mixed results on excluding weevils from the vine canopy. Application of different substances can improve the exclusion effect of such bands, for example hot chilli.
Research on the use of refined clay particle preparations as both a trunk spray and foliar applications has given mixed results in protecting vines from garden weevil. As canopy grows during spring, follow-up applications are required to maintain cover of the material. Such products would be acceptable for organic vineyards. They should be tested in small areas before more widspread use.
Mechanical controls
Research in Victoria has shown that using a rotary hoe during the time of pupation of garden weevil in spring can lead to a large reduction in weevil abundance. The distribution of pupae in the mid-row at this time was such that this method was successful there.
In Western Australian vineyards, where most weevil larvae have been found in the drip zone of vines, such an approach would not be effective unless rotary hoeing close to vines was conducted; but this could damage vines.
Cultivation equipment is available that allows soil in the dripline of vineyards to be pulled away and later, to be thrown back. This disturbance to the area of the vineyard floor where the weevil is most likley to occur is used in some vineyards with success.
Hygiene
It is unusual for an entire vineyard to be infested. Avoid moving soil, fruit, prunings, machinery or other equipment from infested to non-infested areas. Observe these precautions when moving equipment and vehicles between vineyards.
Chemical control
Trunk drenching
Because garden weevil adults enter the vine canopy soon after emergence and generally remain there, early timing of application of an insecticide to the trunks of vines as a drench is important as a preventative control.
If using monitoring bands, apply the trunk drench when there is an average of around five weevils per band.
Trunk drenches have the advantage over canopy applications in being an earlier and more targetted application of insecticide. This may be less harmful to natural enemies that inhabit the vine canopy to help keep other pests such as mites, mealybug and grapevine scale under control.
The most efficient method for applying a drench is to use a recycle sprayer with coarse nozzles. Such a machine would drench the main access points for weevils from the ground into the canopy.
There are no insectcides registerd for this use currently. For updates on this, consult the Australian Pesticides and Veterinary Medicines Authority search portal.
Canopy spray
Insecticides with different modes of actio are registered for application to the vine canopy - consult the Viticulture spray guide for Western Australia for details. Insecticide should be applied after peak weevil emergence in spring - the later these applications are made and observing the withholding period, the greater the chance that the breeding potential of garden weevil can be reduced with the possibility that insectcide application will not be required the following season.
Daytime spraying is effective but superior control may be achieved if spray is applied at night.
The use of these products may result in outbreaks of secondary pests such as mites, mealybugs and grapevine scale.