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Apple blossom weevil – additional information

Recognition

Adult

A small beetle, 3.5-6.0 mm long, with a long snout, dark brown to black, covered with brown, greyish and whitish hairs and mottled, with a V-shaped mark across the elytra and a prominent whitish spot between the elytra and thorax.

Egg

0.7 x 0.5 mm, oval, white and translucent.

 

Apple blossom weevil pupa

Eggs are laid on an anther lobe inside the flower bud.

Larva

Up to 8 mm long, white with a dark brown head, leg-less, body yellowish when fully grown. Found in capped blossoms.

Pupa

4-5 mm long, pale yellow.

Found inside capped blossoms.

Damage

  • The main damage is destruction of blossoms.

Capped blossoms caused by apple blossom weevil larvae

  • Brown capped blossoms, formed after the larvae have nipped the petal bases to arrest their development, are characteristic of this pest.
  • Infested flowers mostly fail to develop and later fall from the tree.
  • Light attacks may be of little importance and the pest may have a beneficial thinning effect. However, heavy infestations may destroy most or all of the flowers and crops may be devastated.
  • On some varieties, the fruit of some infested flowers continues to develop until harvest. The fruits are malformed, flattened in shape and are distorted around the calyx.

 

Life cycle

Adults hibernate under tree bark, in cracks in posts, beneath debris or other suitable shelter. However, modern apple orchards offer few shelters and weevils often migrate in numbers to suitable places in adjacent woodland, hedgerows and ditches.

  • The adults emerge very early in spring, in late February or March, flying to apple trees at about the time apple buds are breaking. Little flight activity occurs below 12oC but most weevils migrate actively at 18oC.
  • The weevils feed on the buds on arrival. They are active mainly, though not exclusively, at night when most crawling, mating and egg-laying occurs.
  • After mating, eggs are laid singly from bud-burst onwards, each inserted through a small puncture in the side of a flower bud which the female first makes with her rostrum. The egg is inserted with the ovipositor through the puncture in the side of the bud and is placed in a groove made by the female on an anther lobe.
  • Eggs hatch within 10 days. The young larvae graze on the anthers and styles and then nip the base of the petals to form the familiar capped blossom.
  • The larvae continue to feed on the flower parts and are full grown after about a month when they pupate within the shelter of the capped blossom.
  • Adults emerge 2-3 weeks later. Newly emerged weevils reach their greatest numbers by mid-June feeding on the undersides of apple leaves for a few weeks.
  • They then disperse to their winter hibernation sites. A few adults from the parent generation may also survive the winter.

Monitoring

Adults

The normal method of determining the risk of damaging infestation by apple blossom weevil is to assess populations of adults shortly after emergence at bud-burst in March using the beating method.

  • For each beat sample, a branch is tapped sharply with a beater (a 0.25 m length of stiff hose to prevent damage to the branch) over a 0.25 m2 collection tray.
  • At least 25 (preferably 50) beat samples should be taken per orchard and the total number of weevils collected should be recorded.
  • The economic threshold is taken to be 5 weevils in a 50 beat sample, but this may be too high when the quantity of blossom present is light.

Capped blossoms

Once capped blossoms are visible, the damage is done and it is too late to control the pest with insecticides to prevent damage in the current year.

  • However, a watchful eye for capped blossoms should be maintained, especially at the edges of orchards next to woodland or near unsprayed apple trees.
  • The pest is absent from most commercial orchards sprayed with conventional insecticides and the appearance of capped blossoms will be the first indications that the pest is starting to establish.
  • The presence of damage one season should prompt detailed assessment of the pest in subsequent seasons.

Damage to fruits during the growing season, at harvest or during grading

  • If significant malformation damage to fruits is seen one season, treatment is likely to be justified the next season to avoid damage intensifying.

 

Forecasting

The migration of the flight of apple blossom weevil adults occurs in March on warm, sunny days when daytime temperatures reach 12-20 °C.

 

Other pests with which apple blossom weevil may be confused

Tortrix moth caterpillars can occasionally form a capped blossom rather like the apple blossom weevil.

Apple bud weevil (Anthonomus piri) The adult of the apple bud weevil is very similar in appearance to the adult apple blossom weevil, but is a lighter, brown colour, and lays its eggs in the autumn and early spring.

  • The apple bud weevil is local and rare in the UK and until recently was only recorded on apple though a recent local outbreak has occurred on pear in north Kent.
  • It is an important pest of pear in continental Europe.
  • The larva infests buds which are hollowed out remaining as dead husks in which the larva is found.

 

Adult Rhynchites weevil

Apple twig cutter (Rhynchites caeruleus): A locally common weevil that occurs on apple but the adult is metallic blue in colour and appears much later in May and June causing characteristic shoot severing damage.

Apple fruit rhynchites (Rhynchites aequatus): The adult weevil is reddish brown occurring in May to July and sometimes feeds on developing apple fruitlets drilling small holes in the flesh causing characteristic damage symptoms. 

 

Control methods

Chemical control

The usual method of control of apple blossom weevil is to apply a spray of a broad-spectrum insecticide against adults at bud burst when most adults have migrated into the apple crop but before significant egg-lying has occurred.

  • The organochlorine insecticides DDT and HCH were traditionally used for this purpose and were highly effective but were withdrawn from use many years ago.
  • Pyrethrins (Spruzit) is the only insecticide specifically recommended for control of apple blossom weevil on apple.
  • Synthetic pyrethroids such as cypermethrin (various products) or deltamethrin (Decis) are also highly effective when used against other pests, but their use should be avoided as they are harmful to the orchard predatory mite Typhlodromus pyri and a wide range of other natural enemies.
  • Pirimicarb (Aphox etc.) and fenoxycarb (Insegar) are ineffective.
  • However, the eggs from female apple blossom weevils that have fed on buds treated with diflubenzuron (Dimilin) often don’t hatch. Thiacloprid (Calypso), though only recommended for rosy apple aphid control on apple, is also very effective against apple blossom weevil. Best results are obtained with this product when it is used in admixture with a non-ionic wetter (e.g. Activator 90).
  • The spray should be applied in good, warm weather conditions if possible. Medium to high volumes are recommended to achieve good cover of the bark.
  • A beat sample should be taken after treatment to ensure that good control of adults has resulted and that no further invasion has occurred.
  • It is probable that sprays of  thiacloprid (Calypso) applied in June and July to control other pests also incidentally control newly emerged apple blossom weevil adults feeding on foliage before they migrate to their winter hibernation sites.

Insecticide resistance

Resistance of apple blossom weevil to insecticides is not known and is unlikely to occur.

Cultural control

Young, dwarf apple trees have smooth bark, which provides few overwintering sites for the weevil in the orchard. Moreover, well-managed orchards have little leaf litter in which the weevils can overwinter, so most weevils overwinter outside the orchard in nearby woodland and hedgerows. The weevil is less of a problem in orchards that are not adjacent to woodland or hedgerows.

  • Loss of even quite a high proportion of buds may be of limited importance on apple trees that have high numbers of apple buds and on varieties where damaged flowers do not continue to develop into mature fruits.
  • Good tree management and nutrition will greatly reduce the impact of this pest.
  • In organic orchards where the pest is especially troublesome, providing adequate nitrogen to the trees is important to maintain fruit bud numbers and quality.
  • On a limited scale, it might be feasible to remove and destroy capped blossoms before the weevil completes its development and emerges. This could be done when hand thinning of fruitlets is done. This should reduce the risk of allowing damaged fruits to mature.
  • On a limited scale, it might be possible to provide artificial hibernation sites for adults (e.g. trunk bands) in June before the weevil moves to its hibernation quarters. The artificial hibernation sites could then be removed and the weevils destroyed. This approach probably hasn’t been tried and may not be very effective.

 

Natural enemies

The apple blossom weevil is eaten by small birds such as tits but the most important natural enemies are two species of parasitic wasp. Because small numbers of apple blossom weevil can be tolerated economically, these parasitic wasps might be important natural enemies.

Parasitic wasps

The ichneumon parasitic wasp Scambus pomorum is a common parasite of apple blossom weevil.

  • The adult female parasitic wasp pierces the capped blossom with her ovipositor, stings the apple blossom weevil larva and injects a paralysing poison. She then deposits a transparent, elongate egg onto the larva.
  • The egg hatches in a few days and the parasite larva attaches itself to the dorsal side of the host larva with its mouth parts immersed within the tissue of the latter. The fully-grown larvae are elongate and fusiform in shape, narrowing towards the extremities.
  • There are four larval stages. Eight to ten days after hatching, the fully grown parasite larva leaves the remains of the host to spin a thin silken cocoon within the cavity of the capped blossom.
  • Pupation is completed in 4-5 days. The pupa turns black and the adult wasp emerges about 11 days later, about 2 weeks later than adults of the apple blossom weevil.
  • Up to 50% parasitism has been recorded. Scambus pomorum does not have a second generation. After emergence, the females appear to be in a state of reproductive diapause.
  • The adults feed on a wide range of types of larvae (moths, flies, sawflies) throughout the summer and overwinter.

The other parasitic wasp is the braconid Syrrhizius delusorius which parasitises the adult apple blossom weevil. This parasite is believed to have 2 generations a year.

  • It overwinters as a small (about 1.2 mm long) larva in the abdomen of the adult weevil.
  • In May, the full-grown larva leaves the host through a small hole it makes in the exoskeleton of its host.
  • The emerging larva is very active and spins a cocoon within 24 hours within which it pupates.
  • The adult wasp emerges a few weeks later in synchrony with the emergence of adult weevils.
  • The female parasite climbs onto the back of the weevil and stings into the abdomen under the wing cases.
  • Females become excited when offered a capped blossom, even when empty, indicating the way the host is located.

Biological control

No specific biological control measures have been developed. The natural enemies of apple blossom weevil, especially the parasitic wasps, should be fostered by avoiding the use of harmful insecticides (see above). Nesting boxes could be provided for tits which might help reduce populations.

Biotechnological control

It has been shown that the weevil is strongly attracted by host plant volatiles produced by bursting apple buds. Some of the terpenoid chemical compounds involved have been identified. It might be possible to exploit these for monitoring or control of the pest but such methods have not been investigated or developed as yet. Work is in progress at East Malling Research to identify a sex pheromone of apple blossom weevil

 

Further reading

Duan, J. J., Weber, D. C., Hirs, B., & Dorn, S. 1996. Spring behavioural patterns of the apple blossom weevil. Entomologia Experimentalis et Applicata 79, 9-17.

Kalinova, B., Stransky, K., Harmantha, J. Ctvrtecka, R. & Zd’arek, J. 2000. Can chemical cues from blossom buds influence cultivar preference in the apple blossom weevil (Anthonomus pomorum)? Entomologia Experimentalis et Applicata 95, 47-52.

Miles, H. W. 1923. Observations on the bionomics of the apple blossom weevil, Anthonomus pomorum Linn. Annals of Applied Biology 10, 348-369.

Morris, M. G. 1977. The British  species of Anthonomus Germar (Col., Curculionidae). Entomologists Monthly Magazine 112, 19-40.

Reijbroek, P. 1983. The relation between attack and damage by the apple blossom weevil, Anthonomus pomorum L. Mededelingen van de Faculteit Landbouwwetenschappen Rijksuniversiteit Gent 48, 287-291.

Zizp, J. P. & Blommers, L. H. M. 1992. Syrrhizus delusorius and Scambus pomorum, two parasitoids of the apple blossom weevil. Proceedings of Experimental and Applied Entomology, N. E. V. Amsterdam, 3, 46-50.