The strawberry blossom weevil (Anthonomus rubi) is a significant pest in the UK, primarily affecting strawberry and raspberry crops. This article details how to identify this pest, where it is found, how to prevent its spread, and the methods to control its impact on your plants.
What Does the Strawberry Blossom Weevil Look Like?
The adult strawberry blossom weevil is small, typically measuring 2–4 mm in length, with a black body covered in fine greyish pubescence. The weevil has a long, narrow rostrum (snout) that constitutes about 40% of its body length and serves as a distinctive feature. When disturbed, these weevils may fold their legs and snout under their body, creating a ball-like appearance.
Larvae are around 3.5 mm long, cream-coloured, legless, and have a noticeable C-shape with a brown head. They are found inside flower buds, where they develop after hatching. Eggs are tiny, oval, and translucent, and are hidden within the buds of host plants.
Where Is the Strawberry Blossom Weevil Found in the UK?
The strawberry blossom weevil is widely distributed across the UK, particularly in agricultural areas where strawberries, raspberries, and other host plants from the Rosaceae family are cultivated. The pest can be found both in commercial farms and home gardens, as well as near wild host plants like brambles and wild roses. Weevils typically emerge from their overwintering sites in hedges, woodland edges, or other sheltered areas near fields and gardens in the spring.
What Is the Lifecycle of the Strawberry Blossom Weevil?
The strawberry blossom weevil has one generation per year in the UK. Adult weevils emerge in early spring, usually around April, and are most active on warm, sunny days. After a period of feeding on young leaves and flower buds, females begin laying eggs, typically in May, inserting them one at a time into closed flower buds of host plants.
After egg-laying, females partially sever the flower stalk, causing the bud to wither and either fall to the ground or remain drooping on the plant. The larval stage lasts around two weeks, during which larvae feed within the severed bud. Pupation occurs inside the bud, and new adults emerge in late summer, feeding on young leaves before overwintering in leaf litter or nearby sheltered locations.
How Can You Detect an Infestation of Strawberry Blossom Weevils?
Detecting strawberry blossom weevils involves looking for the characteristic damage they cause to flower buds. Signs of infestation include:
- Severed Flower Buds: The most distinctive sign of the weevil is drooping or withered flower buds, caused by females puncturing the stalk to lay their eggs.
- Holes in Young Leaves: Adult feeding creates small, irregular holes in the leaves, particularly in the folded young leaves around the flower buds.
- Adult Presence: Adults can be observed in strawberry patches on warm days, often along the edges of fields or near hedges where they overwintered.
How Can You Monitor and Prevent the Spread of Strawberry Blossom Weevils?
Monitoring is essential to prevent the spread of strawberry blossom weevils in fields and gardens. Here are the recommended strategies:
- Visual Inspections: Regularly inspect fields or garden beds for severed flower buds or feeding holes on young leaves. Early detection allows for more timely control measures.
- Hand Brushing: Use a white bowl or tray to catch adults by gently brushing plants over the container. This technique can help you identify the weevil population before significant damage occurs.
- Pheromone Traps: Male weevils release an aggregation pheromone to attract mates. In the UK, pheromone traps using this blend of chemicals can help monitor population levels and determine when to apply insecticides.
How Can You Control Strawberry Blossom Weevil Populations?
Controlling strawberry blossom weevils in the UK typically involves an integrated approach that combines chemical and biological methods, along with timely cultural practices.
Chemical Control
Applying insecticides can effectively reduce weevil populations, especially when timed appropriately. Suitable insecticides in the UK include:
- Bifenthrin: A broad-spectrum pyrethroid insecticide effective against adult weevils. However, it can impact beneficial insects and biological control agents, so consider using it sparingly.
- Pyrethrins (Pyrethrum): Derived from natural sources, this contact insecticide is effective when sprayed directly on weevils but has short persistence, allowing beneficial insects to return shortly after application.
- Thiacloprid: This systemic insecticide is effective against adults and larvae, controlling weevils within severed buds. Thiacloprid has become a preferred option for strawberry blossom weevil control in the UK, as it affects both adults and developing larvae.
Timing of insecticide applications is critical. To avoid unnecessary damage, apply treatments when the adults first emerge and before significant flower bud severing has occurred. Spraying during full flowering is generally ineffective, as most buds will have already been attacked.
Biological Control
Current research in the UK is exploring biological control options, primarily parasitoids from the Braconidae and Pteromalidae families, which attack weevil larvae. While studies on natural parasitism rates in the UK are limited, these parasitoids show potential for controlling weevil populations in conjunction with other strategies.
Natural Predators
Although little is known about specific natural predators of the strawberry blossom weevil in the UK, generalist predators like spiders, ladybirds, and certain bird species (such as chaffinches) may contribute to weevil population control. Encouraging biodiversity in garden or field borders can support these natural predator populations.
How Can You Manage and Minimise Crop Damage from Strawberry Blossom Weevils?
In addition to chemical and biological controls, cultural practices help manage weevil populations and limit crop damage:
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Hedge and Field Management: Clearing or trimming nearby hedges and removing debris can reduce overwintering sites for adult weevils. Ensure that field edges are well-maintained to limit migration from overwintering areas.
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Pheromone Trap Placement: Place pheromone traps around field margins or garden edges to intercept adult weevils as they emerge in spring. Using these traps in combination with early monitoring enables better timing of control actions.
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Resistant Varieties: Certain strawberry varieties can better withstand the severing damage caused by strawberry blossom weevils. For example, the ‘Elsanta’ variety has been shown to withstand lower weevil densities, making it a preferable option in infested areas.
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Timely Crop Monitoring: Consistent monitoring throughout the season, particularly during the early flowering period, allows growers to assess damage levels and identify when further action may be necessary.
What Damage Do Strawberry Blossom Weevils Cause to Crops?
Strawberry blossom weevils inflict direct damage on crops through their egg-laying behavior and feeding habits:
- Flower Bud Severing: Female weevils sever the stalks of flower buds after laying eggs, causing the buds to droop and die. This severing damage is typically more intense along the edges of fields or close to overwintering sites, resulting in patchy but significant losses.
- Adult Feeding Damage: Adult weevils feed on flower petals and young leaves, creating small holes that may contribute to reduced plant health and vigour, especially when populations are high.
The severity of damage depends on weevil population density, with high numbers potentially resulting in considerable yield losses. Studies on the ‘Elsanta’ strawberry variety suggest that crop yield may be compromised if the remaining flower buds cannot compensate for the loss in overall bud numbers.
How Do Pheromones Affect Strawberry Blossom Weevil Behaviour?
Strawberry blossom weevils are strongly influenced by chemical cues. In particular, females are drawn to host plants by compounds like monoterpenes and sesquiterpenes released by strawberries. Additionally, male weevils emit an aggregation pheromone that attracts both sexes, which is crucial for mass monitoring through pheromone traps.
To achieve effective weevil control, monitoring and timely action are key. With the proper combination of methods, from monitoring with pheromone traps to cultural practices, chemical applications, and possibly biological controls, UK growers can effectively manage the strawberry blossom weevil and limit its impact on their crops.