Vine weevil in soft fruit: Effects of crop type and production system

Overview

Vine weevil attacks have become more frequent and damaging in recent years as a result of increased soft fruit production under protection, in containers and the widespread use of polythene mulches in soil grown crops.

In terms of crop production, soil-less production in substrate and the use of polythene mulches for soil grown crops have many advantages. However, such systems provide warm moist conditions which are ideal for vine weevil larval development and increase the risk of adult survival over the winter.

Protection for vine weevil

They also offer protection from predatory birds such as pheasant, partridge and some protection from predatory ground beetles, particularly where crops are raised on tabletops.

For soil-grown crops, the mulches also limit opportunities to effectively target control measures at both vine weevil larvae and adults.

Field-grown strawberry crops

Field-grown main season varieties are normally retained for two crops but sometimes a third, especially in pick-your-own (PYO) production.

Everbearer varieties are generally grown for one crop but occasionally a second.

Where either main season or everbearer varieties are kept for these further years, there is more risk of vine weevil damage as populations have had more time to build up.

Potential solutions to problems with field-grown crops include:

• Annual cropping: the shorter cropping time gives vine weevil populations less chance to build up to damaging levels.
• Growers who reduce their cropping cycles succeed in reducing the level of damage, although there can be economic implications in reducing the number of crops picked from one plantation.
• High density plantings in the first year of cropping - a 60-day crop can help to make up for any loss of fruit in subsequent years after grubbing.

Container produced strawberries

Many growers now produce both main season and everbearer varieties in substrate bags or troughs, which are laid on polythene-mulched raised beds or supported on tabletops under semi-permanent tunnel structures.

Where bags are removed after each crop and replaced with a new set placed on the existing polythene-covered beds, this increases the risk of vine weevil carry over into succeeding crops.

Where possible, it is best to avoid planting and establishment of such bags during mid-summer (May to July) when adult weevils can easily walk into new plantations.

It is also best to avoid re-planting new crops into old bags or substrate in troughs as this will increase the risk of the new crop being infested with vine weevils.

Modular tray and potted planting material for strawberries

• Growers should be aware of the risk of importing vine weevil onto a site when purchasing plant material.
• This risk has increased in recent years due to the use of modular tray and potted plants.
• Ideally, the propagator should apply entomopathogenic nematodes to the plants before dispatch.
• However, on arrival, a number of modules should be removed from the trays and the root systems checked carefully for signs of larvae or damage.
• If larvae are found, the propagator should be alerted.
• If control measures are necessary, nematodes should be applied to the modules before planting.

Cane and bush fruit – soil-grown crops

These crops often remain in the soil for 10 years or more, and it can be difficult to eradicate vine weevil infestations.

To help reduce this problem, growers should restrict the use of polythene mulches (which aid winter survival of weevils and lower predation) to the first few years of plantation establishment.

Cane and bush fruit – container production

Protected raspberry, blackberry and blueberry crops grown in containers are particularly at risk to vine weevil as both the tunnel structure and pots provide favourable conditions for the pest.

In the warmer conditions, adults are active for longer than in outdoor crops and may overwinter.

Larvae are able to move more easily through the substrate than in field grown soils and can feed on a greater proportion of roots as they are contained within relatively small volumes in the pots.

Where crops such as blueberries are kept in containers for a number of years the risk of vine weevil infestation is greater the longer they are kept.

However, on the positive side, control measures are easier to apply and are more effective than in field grown crops, and container-grown crops of cane fruit are often retained for only one or two crops, giving vine weevil populations less time to build up.

Original author/s

Jude Bennison, Janet Allen & John Atwood, ADAS

Tom Pope, Harper Adams University