Description and life cycle
The garden weevil, Phlyctinus callosus, was accidentally introduced to Australia from South Africa where it is known as the banded fruit weevil.
Garden weevil eggs are laid in groups in loose organic matter on the soil or inserted into hollows in plant debris. Eggs are capsule shaped, small and almost impossible to find in the field. They hatch in 10 to 14 days.
Larvae of garden weevil are soil-dwelling, legless and have brown heads. After hatching, larvae immediately burrow into soil and feed on plant roots.
They are difficult to distinguish from other brown-headed larvae - apple weevil (Otiorhynchus cribricollis), spotted vegetable weevil (Desiantha diversipes), and sitona weevil (Sitona discoideus). But they can be distinguished from the white-headed weevil larvae of Fuller's rose weevil (Asynonychus cervinus) and whitefringed weevil (Naupactus leucoloma) and leaf feeding larvae of vegetable weevil (Listroderes obliquus). Presence of the more easily identified adult stage of weevils is a good guide to the species of larvae in vineyards or any horticultural crop where weevils are a pest. To distinguish between larvae of garden weevil and these other species, refer to the Department of Primary Industries and Regional Development information page Identifying soil beetle pests.
Larvae develop slowly during winter and more rapidly as soil temperature increases in spring. Initially they are off-white, and become milky as they mature and form a smooth-sided earthen cell in the soil in which they pupate.
Pupae are a delicate, soft-bodied stage and most abundant in September. They are white at first, and then darken. The duration of the pupal stage depends on weather conditions but it is usually completed in three to four weeks. The eyes become black and the body turns grey-brown just before they emerge as adults.
Adult garden weevils are grey-brown, about 7 millimetres long, and are flightless. They have a bulbous abdomen with a prominent pale white V stripe across the end of the abdomen. The tip of the abdomen in female weevils is pointed while that of males has a tuft of hairs and is more squared off .
The adult is similar in appearance to apple weevil, Fuller's rose weevil and vegetable weevil. Another weevil that may be found occasionally in vineyards is whitefringed weevil. To distinguish between adults of garden weevil and these other species, refer to the Department of Primary Industries and Regional Development information page Identifying soil beetle pests.
Adult weevils emerge from the soil in mid to late October and are most numerous during November and early December. Many adults survive until April with some present through winter.
As the weevils are most abundant in spring, they appear to only have one generation each year. However, in some situations there may be two generations.
Larvae of the garden weevil tend to occur in the soil in the drip zone of vines, with comparatively fewer in the mid-row, unlike observations in Victoria where inter-rows are more heavily infested.
Weeds such as sorrel, capeweed, dock and dandelion can support large numbers of garden weevil larvae and the presence of such weeds may influence the distribution and abundance of larvae across the vineyard floor.
After emergence, adult garden weevils enter the vine canopy and tend to stay there. Adult weevils feed at night. They are inactive during the day and shelter under bark, in the crotch of branches and the main stem and around posts or plant limbs supported by wires. They can also be found under plant debris on the vineyard floor and in curled up dead leaves and in grape bunches in the vine canopy. When disturbed they remain still, feigning death and may fall to the ground. Adult weevils climb into the vine canopy via the trunk, posts, trailing canes or where weeds touch the canopy.
Damage and loss
Larvae feed on the roots of vines and other plants growing in the vineyard. Larvae feeding on vine roots can severely damage young vines such that they become stunted and appear water-stressed. The root system of mature vines would be more tolerant of feeding by weevil larvae. Control of adults would help reduce the abundance of larvae.
Adult garden weevils attack foliage, flowers, buds and fruit. Leaves usually have distinctive round holes and scalloped edges. Adult weevils can scar grapes but also destroy bunches by ringbarking the stalk. Feeding around growing tips can kill them and this can affect the structure of vines and reduce the number of buds the following season.
Garden weevil is also a pest in deciduous fruit tree orchards, especially apples, nectarines and cherries, as well as strawberries, root vegetables, asparagus and ornamentals. This insect is not a major pest of vineyards in the Swan Valley although it occurs there. It is also found in the Perth metropolitan area.
Damage may be isolated, so inspecting a representative sample of vines and host weed species across the vineyard is necessary. Areas where garden weevils were a problem the previous season should be included.
Larvae and pupae
If monitoring for the soil borne larvae and pupae, commence in late winter to early spring as soil temperature increases and these stages become larger and more readily seen as they develop. Examining a spadeful of soil near the base of vines and under host weed species in late winter/early spring may reveal garden weevil larvae actively feeding on vine and weed roots around budburst. The ease with which larvae are found is a good indication of subsequent adult abundance and consequently the severity of damage by adults later in the season.
If half of the spadefuls of soil across a block have no garden weevil larvae or pupae while the other half of the soil samples have only one or two, garden weevil is unlikely to be a problem. If larvae are readily found - an average of five or more per spade of soil - the potential for a problem increases and control options should be considered.
Continued weekly checks near the base of vines and host weeds to determine the proportion of larvae and pupae should help to indicate the timing of emergence of adults: the higher the proportion of pupae the closer to adult emergence. As pupae approach emergence they become darker.
Vines may be monitored for adult emergence by wrapping a piece of single faced corrugated cardboard 10cm wide just below the crown of vines. Place bands in areas identified as hot spots. Inspect for adults sheltering in these bands every seven days.
Garden weevil adults may be monitored by scraping the soil at the base of vines, under bark and in the crotch of branches where they shelter during the day. Inspection at night may reveal the adults actively feeding.
Once weevils emerge, they commence to feed and the vines are the most favoured food. Weeds may also be chewed. Water shoots of vines will be attacked first followed by folaige near the crown before they move along cordons and attack bunches.
Characteristic signs of leaf feeding by garden weevil adults on water shoots and the crowns of vines are a useful guide to the extent and severity of attack across vineyard blocks.
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.
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.
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.
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.
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.
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.
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.
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.