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Vine Weevil and Control Options
The vine weevil is a stubborn pest of plant roots. In recent years, it has come to the fore as a major horticultural problem for several reasons.
Increased use of peat-based composts and polythene mulches, combined with the withdrawal of persistent soil insecticides seem the most likely candidates.Organochlorine insecticides, such as aldrin, were persistent in the soil and gave good control of vine weevil grubs but were withdrawn for environmental and health reasons. Within Northern Ireland horticulture, vine weevil is mainly a pest of strawberries, hardy nursery stock and bedding plants, whereas many gardeners will be familiar with wilting plants with eaten-away roots and the ominous presence of the white 'C'-shaped, legless grubs. Vine weevil are not fussy eaters and many different plants can be attacked but those in containers are particularly susceptible. Often, as with many soil pests, growers only become aware of the problem when plants start to look poorly, but by then the damage has been done. Though, in the case of vine weevil, the presence of leaf notching on lower branches caused by adult weevil feeding could act as a warning of what is to come.
The adult vine weevil is a dark grey, fairly large weevil (c. 1 cm in length), recognised by its pronounced rostrum or snout and orangish flecks on its back (Figure 1). The adults are nocturnal, but can often be found walking around the base of plants or, more noticeably, climbing up walls. Although a good climber, vine weevil is unusual for an insect in being flightless. Another, strange characteristic is that the population is all female, there are no males and the females reproduce without the bother of sex. In Northern Ireland, egg laying is fairly extended, from late May through to early October, with peaks in June, July or August depending on weather (Figure 2). Each female is capable of laying upwards of 500 eggs at the base of plants, so the numbers of weevils can build up rapidly over the years. However, egg survival and hatching times are heavily influenced by weather, warm moist conditions being best for the weevil. The subsequent grubs (or larvae) feed on the roots and continue to do so right through the winter, to pupate in April/May with new generation adults emerging in May/June. In addition to these new adults, a proportion of older generation adults survives the winter, increasingly so if the weather is mild, as it often is in Northern Ireland. These old generation weevils then produce the first batches of eggs in May/June with the new generation contributing their eggs later in the season, after a period of feeding.
Control measures against vine weevil can be directed at the adult or larval stages, although mostly against the latter. For chemical control on strawberries only chlorpyrifos (e.g. SuScon Green®, Dursban®) is currently approved. This is incorporated into the soil as slow release granules or poured on as drenches. In non-edible plants in containers, fipronil and imidacloprid are also allowed. Similar to chlorpyrifos, these chemicals are either incorporated into the compost as slow-release granules (e.g. Vi-Nil®, Intercept®) or used as a soil drench (Provado®). As with any pesticides, please refer to the manufacturer's instructions for the use of these products.
An alternative to chemical insecticides is to use nematodes. Nematodes, also called eelworms, are small (mostly microscopic) unsegmented worms. Naturally extremely numerous in the soil, the vast majority of species are detritus feeders although some are plant pests and a few are parasites of insects. These insect-attacking (entomopathogenic) nematodes have been developed as a means of controlling vine weevil. The nematodes attack a grub by burrowing into its skin or wriggling inside its mouth, anus or breathing pores. Once inside the grub's body, the nematode releases a biological weapon - bacteria. The bacteria poison the grub, causing its death within 2 days. The nematodes feed upon the remains of the insect, reproduce and the cycle is completed when the young nematodes leave to search out another host.
Entomopathogenic nematodes are sold under various trade names such as Nemasys H® (Becker Underwood) or Larvanem® (Koppert). Essentially, what you get is a clay matrix containing infective-stage nematodes. This is mixed with water and spread onto the soil, whereupon the nematodes seek out the vine weevil grubs and kill them. One of the difficulties though, is that the nematodes are dependent on soil temperatures above 12°C, which should not be a problem in glasshouses but does limit their effectiveness outdoors. We found that in trials on outdoor strawberries, nematodes did not provide good control of weevils. However, other work in Maynooth has been more successful. Moreover, a cold-hardy species of nematode from Scotland has just come on the market (as Nemasys L®), which reportedly remains active and effective at temperatures as low as 5°C. This should hopefully provide another alternative to chemical control but may still remain expensive for widespread use.
Another strategy is to target the adult weevils. As a proponent of integrated pest management, i.e. controlling pests by a combination of compatible methods, I would encourage taking steps against both adult and larval stages. One of the most straightforward means of control is to prevent the weevils from reaching the plants. As they cannot fly, this can be achieved by growing plants on raised benches and then banding the legs with a treacle-like glue (available from commercial outlets). One word of warning though, if you already have weevil damage, then the compost may retain a residual weevil population, so this strategy will not work the following year. It would also be worthwhile to actively trap adult weevils, both for control and for monitoring. As they are nocturnal and hide during the day, this can be done by laying a damp piece of old carpet or such-like in the vicinity of the crop. Additionally, cardboard tubes (used toilet/kitchen rolls) can be filled with shredded newspaper and placed among the plants.
These should be left for a couple of days and then emptied into a bucket to show any hiding weevils.
Chemical control of adults would be another possibility. Sprays containing chlorpyrifos can be targeted at weevils during the night, when they are most active. However, one of the dangers of such an approach is that it may be harmful to natural predators of the weevil and end up being counter-productive. Work in Germany and England has demonstrated that ground beetles and rove beetles exert considerable mortality on weevil populations. Getting these natural predators to work for you can be difficult but it is surprising the amount of weevil eggs and grubs that they eat. Unfortunately, often such predation is only appreciated when it is absent. It is therefore important that any insecticides applied against weevil adults are carefully targeted. An old-fashioned way of doing this was to use poison baits, unfortunately, the recipe for doing this dates from 1954 and the toxic ingredients used have long since been banned. However, there has been some experimental work on baits for use in raspberry plantations in the US, so this technique may hold potential for the future.
Another potential but controversial technique for controlling vine weevil is the use of genetically modified (GM) plants. This technology has been used to produce strawberry plants that have demonstrated resistance to vine weevil attack. In these plants, genetic material from cowpea was inserted into the strawberry plant's genetic code. This makes the strawberry plant unpalatable to weevil attack by preventing the action of certain of the weevil's digestive enzymes. Such technology is at the experimental stage and is not commercially-available. Whether or not this will happen for GM crops as a whole is currently the subject of much debate.
Mention of products in this report is for illustration and does not constitute an endorsement. Please follow instructions carefully when using insecticides.
Written by Dr Archie K Murchie, Applied Plant Science Division, DARD, Newforge Lane, Belfast BT9 5PX.