Weevils

Description

Garden weevils are grey-brown and about 7 mm long and have a visible pale ‘V’ stripe at the base of their abdomen. Apple weevils are dark brown – black measuring 8 mm long, with a bulbous abdomen and are all females. Both species are flightless, and nocturnal.

Oval shaped eggs, which are golden in colour, are laid in sheltered positions. The eggs of GW are laid in clusters or small groups, while AW are laid individually. Eggs are resistant to drought conditions, hatching within 10- 20 days when conditions are favourable. Larvae are yellow and legless, with a brown head and can grow up to 9 mm long. Once hatched, larvae burrow to a depth of 25 cm into the soil and commence feeding on roots. As spring approaches, mature larvae migrate closer to the surface, eventually developing into pupae. Pupae are initially white, which darken as they mature. Pupation can take from 10 days up to 4 weeks depending on the species. Adult weevils emerge from the soil in late spring – early summer, climbing their way into the canopies where they feed on leaves and fruit. Generally, only one generation occurs each year.

Garden weevil adults feed on fruit causing scarring, often making produce unmarketable. Apple weevils feed on the fruit stalks, this can result in partial or complete ringbarking, which can produce reduced fruit size or fruit fall.

All adult weevils feed on the leaves and in severe infestations defoliation can occur, along with premature fruit drop, water stress and reduced fruit size.

Larvae feed on plant roots, which can cause reduced tree vigour, although this is rare in apple trees. However, in young grape vines this damage can cause stunted growth and water stress and under severe infestations, the death of young plants.

Control

Once established, weevils can be difficult to control, therefore preventing establishment and spread is essential. Utilizing a combination of cultural and chemical control options, in addition to vigilant monitoring is the best approach.

Weevils are easily spread, therefore avoid moving soil, fruit, prunings and equipment from infested to non-infested areas. Maintaining a clean weed strip under trees, as well as a neatly mowed inter-row is important, as trash and weeds such as dock and capeweed on the orchard floor provide alternative feeding sources and breeding sites.

In Victoria, good results have been achieved by rotary hoeing the mid row to disturb pupae. This method does have limitations though, depending on where the pupae are in the soil and the potential to disturb tree roots and trigger suckers in some species. Weevils live long periods in the soil and benefit from cracks which form during drought. The addition of compost and mulches to reduce soil drying will therefore help reduce weevil populations.

Smaller orchards have had success using poultry to control weevils, however monitoring is required to ensure their foraging doesn’t damage roots, or cause compaction. Several natural predators such as wasps, assassin bugs, praying mantises and parasitic nematodes can help keep weevil populations low. To preserve these natural enemies avoid using broad spectrum insecticides.

Sticky bands placed on tree trunks can be used limit the number of GW that climb trees into the canopy. It is important to know which weevils are present as this method isn’t effective on AW, instead placing fibrous bands around the trunks has proven to reduce the number of AW numbers climbing trees.

Monitoring for larvae and pupae should commence from late winter to early spring by examining a shovelful of soil at the base of trees. While monitoring for adult weevils should commence from late October to late November. Look for damage on the leaves of weeds as these are likely to be infested before trees. Cardboard traps can be used to record weevil populations and should be checked fortnightly.

If monitoring reveals a high number of adults, it is recommended a timely application of insecticide is made to control populations. Vayego® is a novel insecticide that provides fast cessation of feeding and good residual activity on a broad spectrum of pests. Vayego is registered for the control of GW, AW and FRW in pome and stone fruit and should not be applied more than three times in one season.

References

Hetherington, S (2009), Weevils: Fullers rose weevil, Garden weevil, Apple weevil, Fruit tree root weevil, Eucalyptus weevil’, Integrated pest management for Australian apples and pears, Apple and Pear Australia Ltd, integrated pest management for Australian apples & pears (nsw.gov.au)

Learmonth, S (1999/2000), ‘Weevil management in orchards and vineyards looks promising’, Journal of the Department of Agriculture Western Australia, Series 4, Vol. 41: No. 1, Article 17, Weevil management in orchards and vineyards looks promising (agric.wa.gov.au)

Learmonth, S and Fisher, D (2020), ‘Garden weevils in vineyards’, Department of Primary Industries and Regional Development (DPIRD), Garden weevil in vineyards | Agriculture and Food