Of over 500 species of weevil native to the British Isles, only one is a serious pest of cyclamen, the black vine weevil (Otiorhynchus sulcatus). Since the withdrawal of aldrin, damage from vine weevil has become an acute problem, as this pesticide provided effective control in commercial nurseries and farms (reducing the overall population of the pest and the chances of infestation from purchased material)
Adult weevils feed at night, consuming leaf edges and leaving characteristic semi-circular notches where they have fed. Particular favourites include Rhododendron, vines and their relatives such as Rhoicissus rhomboidea and Cissus antarctica. Although of significance to commercial growers because of the unsightly appearance of damaged leaves, the adults do not cause serious or lasting harm to the plants they feed on. They tend to drop off and play dead when the plants they are feeding on are disturbed, making them difficult to detect. Modified legs enable the adults to grip any surface and they can easily cross walls and ceilings to lay eggs in hanging baskets etc. Although they are unable to fly, distribution of adults can be widespread thanks to this gripping ability, with plants and material dispatched from nurseries the commonest method of "hitching a ride".
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Reproduction is by parthenogenesis, that is, unfertilised eggs developing into larvae, with male vine weevil unknown. Each adult female will lay around 500 eggs, but up to 1600 have been recorded. Many of the eggs fail to develop, but despite this the population can increase significantly where a source of food is plentiful.
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The larvae cause most problem, feeding underground on roots and tubers until the plant is unable to take up water and collapses through moisture stress. They remain under the surface of the compost or soil throughout their development, and can easily tolerate being frozen solid during the winter months. larvae are active from late summer until April, when they pupate and begin to emerge as adults. An earthen cell is constructed in the soil or compost as a protection while metamorphosis is under way, with the very short-lived pupal stage triggered by rising ground temperature in spring. Adults then emerge by digging themselves free of the surface, where they remain for some time while the chitinous exoskeleton hardens. They are vulnerable at this point and for this reason emerge only at night. Egg laying starts about two weeks after emergence, with the eggs laid on the surface of container compost from May to October, and outside between June and September. On hatching, the new larvae burrow down to the roots to begin feeding.
Traditionally this was achieved using aldrin, a toxic organochlorine compound which was withdrawn from commercial use in 1990. Since that time, control using chemicals has been difficult and only recently have new pesticides been cleared for use in the UK The use of HCH (hexachloro-hexane or lindane) was recommended following the withdrawal of aldrin, and some success has been achieved using this chemical as a soil drench two or three times a year. However, many older pesticides which include HCH have also now been withdrawn; there is some concern about the safety of all organochlorine pesticides, including HCH.
Zeneca (formerly ICI, and through a management buy-out, now Miracle Products for amateur products) have promoted the use of fonofos as a control. Their product Cudgel is formulated as a granule and is incorporated into composts or cultivated into the soil for outdoor crops such as blackcurrants where vine weevil is a particular problem. Fonofos is restricted to professional use only and is classified as a Part II Substance, requiring specific safety measures to be taken in its use, including the use of respirators and full protective clothing.
Dursban (a formulation of chlorpyrifos manufactured by Dow Chemicals), and Spannit from PBI, are also widely used on commercial crops along with Yaltox (from Bayer, and based on carbofaran). The latter can be used only on outside crops and cannot be used to treat pot-grown plants. It does not persist very long in the soil and needs rain or irrigation after application to dissolve the granules.
More recently, a new formulation of chlorpyrifos has become available for professional growers in Britain. Developed in Australia, and previously available in continental Europe for a couple of years, this is suSCon Green, a controlled release formulation which is incorporated into potting compost Under the correct conditions it can control weevil for two years with a single application of 75g per cubic metre of compost. Its use as a compost additive means that the user must wear protective gloves while potting, and take care not to contaminate clothing. Mixing with the compost ingredients must be thorough, since phytotoxicity has been recorded in some genera where overdosing occurred through inadequate mixing. Use as a top dressing will not be effective since chiorpyrifos does not move in soil or composts, and for this reason use when potting on is also ineffective, as weevil larvae can survive in the untreated compost surrounding the plant SuSCon Green is restricted to professional use, and trials have not yet shown that in the recommended dose it is harmless to cyclamen.
One amateur product is available in the UK containing chlorpyrifos, and that is Chlorophos. Based on a mixture of chlorpyrifos and diazinon, this product will control weevil and other biting and sucking insects.
The most widely used biological control is based on nematodes (eelworms), which are found naturally in most soils. The particular nematodes used for this purpose are in the genera Steinernema and Heterorhabditis, with control of vine weevil relying on bacteria carried in the eelworm's digestive system. On contact with weevil larvae, the nematodes enter through orifices or directly through the cuticle, and their cargo of bacteria is released. The bacteria cause septicaemia and the internal breakdown of weevil larvae tissues, which is then consumed as a "soup" by the nematodes. They are thus parasites and can survive only by infecting larvae. Although native, Steinernema and Heterorhabditis become inactive at temperatures below 12ºC (54ºF) and have therefore been most widely applied where crops are in frost-free glasshouses.
Nemasys was the first commercial nematode control in the UK' and is based on Steinernema bibionis. The nematodes are supplied by Applied Genetics Company, MicroBio division, in polythene packs with a moist sterile carrier, which remain usable for 28 days when stored below 10ºC (that is, in a fridge). Each pack contains sufficient to treat around 1000 pots and should be applied at 3 and 9 weeks after potting. With most crops this would be in spring, but for cyclamen a late summer application would be more appropriate. Effective control has been achieved at Wakehurst Place using this product, with two applications in autumn (August and October) and one in April. The nematodes become ineffective during winter, but control achieved in autumn reduces the population sufficiently to minimise damage until temperatures rise again, when another treatment is given. At Wakehurst the weevil problem has been confined to glasshouses; though these are not all heated to the 12ºC recommended, timed applications appear to be effective.
Nemasys H is based on the nematode Heterorhabditis megidis which is supplied in sachets with an inert carrier, also from MicroBio. Each pack contains around 50 million nematodes which are applied as a compost or soil drench. Each pack will treat 100 m2 (1080 sq ft) of containers under glass or 50 m2 (540 sq ft) of open ground. They are active in the compost and seek out weevil larvae, subsequent generations perpetuating the effective control after initial treatment This nematode becomes less active as temperature falls, and control will not be successful where the temperature falls below 12ºC (54ºF) within 1~14 days of treatment However, if it is applied in autumn (August to early October), and again in spring (April to early June), control can be achieved outside or in unheated glasshouses.
Another product available commercially, and based on Steinernema carpocapsae, is Fightagrub from the Applied Horticulture division of Fargro Ltd. This is supplied in sponges which are immersed in water to form a stock solution. The 10 ml size contains around 10 million nematodes, sufficient to treat 20 m2 (220 sq ft) or 1000 pots, and is applied as compost additive or drench. Its shelf life is only 48 hours at 2-5ºC (fridge temperature).
Exhibit is also based on Steinernema carpocapsae and is supplied in three distinct products through Ciba Bunting. In two of the formulations Exhibit GSC-10 and Exhibit GSC-50), the nematodes are supplied in bottles of an alginate gel. An activator is supplied which is introduced to lukewarm water where the gel dissolves and the nematodes are released, to be diluted and applied as a compost or soil drench. As Exhibit FSC-250, the nematodes are supplied in paper pouches, each of which is divided into 10 channels containing the nematodes and a polymer carrier. The end of the channel is cut open and the contents poured directly into a sprayer for diluting and drenching in containers. The choice of carrier has allowed much greater storage life, with the alginate-based products viable after 5 months at 20ºC (68ºF). where scale of cultivation allows, such as on a commercial nursery, application can be via the irrigation system. The temperature should be 140C (570F) or above when applying the nematodes, or their effectiveness is reduced. where control is required in colder conditions, applications in May and August have been shown to control effectively through summer and autumn, and reduce the population through the winter. S. carpocapsae is less sensitive to pesticide residues than Heterorhabditis species, and can infect the adults as well as larval and pupal stages. The nematodes also have some effect in controlling other soil pests such as flea beetle and cutworm, but their impact on beneficial organisms such as earthworms is thought to be negligible.
Another species of nematode, Steinernema feltiae, is used in Entonem. This product is very similar to Nemasys, containing 50 million nematodes, and can be used on vine weevil and mushroom fly. It is produced in Holland by Koppert, and supplied in England through Monro South.
Chlorpyrifos is harmful to all the nematodes, so the combination of biological and chemical control is not therefore possible.
Regrettably, these nematode products are all sold through commercial growers' suppliers such as Monro Horticulture or Fargro, and can therefore be obtained only by professionals. However, the increasing interest in biological control has meant that it is now economical for companies to market to amateur growers. Zeneca are now promoting a range called Nature's Friends, with biological control for common glasshouse pests including vine weevil. Sold as card boxes through garden centres, these contain no active product, but a postal application form. The purchaser sends this off when ready to apply the control, to receive the nematodes by return. Packs currently retail at UK£8.95 for sufficient to treat an average greenhouse.
A product under development by Bayer, the German agrochemical company, is based on a fungus (Metarrhizium anisopliae). This acts by attacking the vine weevil larvae under ground and can infest and kill them in a few weeks. The fungus is active between 15 and 30ºC (59~86ºF) and is incorporated in compost as spores. Work is still very much under development, with the use of live fungus cultures for increased speed of efficacy under examination. Introduction of a commercial product is still several years away as production costs are not finalised and clearance under the Control of Pesticides Regulations must first be completed.
Carabid beetles are native predators of vine weevil and their encouragement is being researched. The biology of these beetles for control in container crops has not been examined; although they are sensitive to pesticides, their value in controlling vine weevil in soft fruit is well known and understood. Since carabids are particularly sensitive to methiocarb, the active ingredient in most slug pellets, their use decreases the beetle population. Recent development of a biological control for slugs, based on another nematode Phasmarhabditis hennaphrodita, promises the possibility of eliminating methiocarb, and thus allowing carabid beetles to increase. The nematode slug control is available to amateurs through the Nature's Friends range from Zeneca.
Although there is no cultural method of eradicating vine weevil, some steps can be taken to ensure there is no unnecessary build-up of the pest Removing alternative hosts from the vicinity of the cyclamen collection will avoid the build-up of adults, and this is particularly the case with Rhododendron which is a food plant for larvae and adults.
Scrupulous glasshouse hygiene should be practised, as any debris left in odd corners will act as a hiding-place for adults during the day. Similarly, when infected plants are discovered, if there is a possibility of eggs being present in the compost this should be sterilised chemically or with boiling water before discarding. This will prevent any possibility of adults subsequently returning from a compost heap.
During the short-lived pupal phase, overwatering container-grown plants can drown the insects, but the timing of this operation is crucial and could damage plant growth if pots do not then dry out.
"Bug-vacs" have been developed for control of Colorado beetle and Lygus bugs in Germany and the USA, and it is possible that a similar system could work at night on adult vine weevil. This has, however, not been tested and further work is required.
Note: References to Government controls, availability and prices in the above article, refer to conditions in the United Kingdom.