Garden weevil in vineyards

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Garden weevil (Phlyctinus callosus) was accidentally introduced into Western Australia from South Africa. This weevil is now a severe pest of grapevines, deciduous fruit and other horticultural crops. Adults feed on any soft plant tissue, and the soil-borne larvae feed on roots.

Left uncontrolled, vigour, cosmetic quality, yield and growth of host crops can be adversely affected.

This article provides details on the identification, biology and management options for garden weevil in grapevines but much of the information is relevant to other crops attacked by this insect.

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 next to the head of a garden weevil adult
Garden weevil eggs next to the head of a garden weevil adult

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.

Garden weevil larvae have a brown head and are legless
Garden weevil larvae have a brown head and are legless

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.

Garden weevil pupae are white when newly formed
Newly formed garden weevil pupae
Weevil pupae darken as they develop before emerging as an adult
Weevil pupae darken as they develop before emerging as an adult

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.

Garden weevil adults are slightly bulbous and grey with a light V stripe across the top of the abdomen
Garden weevil adults
The tip of the abdomen of male garden weevils has a tuft of hairs and is squared off while in females it is pointed
The tip of the abdomen of male garden weevils has a tuft of hairs and is squared off while in females it is pointed

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.

Habits

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

Weevil larvae damage to roots
Weevil larvae damage to roots

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.

Garden weevil adult damage to grapevine leaves and stems
Garden weevil adult leaf and stem feeding damage
Garden weevil adult damage to grape berries
Garden weevil adult damage to grape berries
Garden weevil adult feeding can kill whole bunches by ring barking the stem
Whole bunch damage by garden weevil adults

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.

Monitoring

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

Monitoring for garden weevil - digging for soil stages and cardboard band for adults
Monitoring for garden weevil

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.

Adults

 

Single faced cardboard bands wrapped around a vine trunk to monitor garden weevil adults
Single faced cardboard bands wrapped around a vine trunk to monitor garden weevil adults

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.

Damage to water shoots of a grapevine by garden weevil adults
Damage to water shoots of a grapevine by garden weevil adults
Signs of garden weevil feeding on a young grapevine shoot
Signs of garden weevil feeding on a young grapevine shoot
Garden weevil leaf damage in the canopy of winegrapes in early summer
Garden weevil leaf damage in the canopy of winegrapes in early summer

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.

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 help control garden weevil
Poultry help control garden weevil

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.

Pupae of garden weevil - the grey pupa is infected with a parasitic nematode
Pupae of garden weevil - the grey pupa is infected with a parasitic nematode

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

Applying a polybutane sticky band to the trunk of grapevines will prevent garden weevil adults from entering the canopy
Applying a polybutane sticky band to the trunk of grapevines will prevent garden weevil adults from entering the canopy

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.

Artificial fibre bands on vines trunks and posts have given mixed results in preventing garden weevil adults accessing the vine canopy
Artificial fibre bands on vines trunks and posts

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.

Refined clay applied to trunks of vines in research to control garden weevil
Refined clay applied to trunks of vines
Refined clay applied to the canopy of vines in research to control garden weevil
Refined clay applied to the canopy of vines

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

Cultivating the inter-row of vineyards when garden weevil is in the delicate pupal stage has been effective in Victorian vineyards
Cultivating the inter-row of vineyards when garden weevil is in the delicate pupal stage has been effective in Victorian vineyards

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.

Spray return rig for applying trunk drench to control garden weevil adults
Trunk drenching spray rig

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.

Contact information

Alison Mathews
+61 (0)8 9777 0122