You might have heard about Vine weevils - they are little bu****s but read on and find out how to deal with them............
Vine weevil adults and larvae (Otiorhynchus sulcatus) are serious pests of a wide variety of plants. The larvae are the most damaging, usually found in the compost of container grown plants, but can also attack the roots of plants in open ground. Outdoors they particularly relish Fuchsia, Primula, Cyclamen and Begonia and, sadly, Heucheras amd their related genera (Tiarellas etc).
Adult weevils feed at night, making holes and irregular notches in plant foliage. Although this is a nuisance, it doesn’t harm the plant and is best tolerated. In fact, Heuchera foliage is not often attacked, probably due to the hairness of it. The plump creamy vine weevil larvae are the real pest, devouring plant roots. Plants wilt as though short of water. After watering there is no improvement, and the plant declines even further. When taken out of the pot the root system will have virtually vanished and the larvae will be clearly visible in the compost. Outdoors, a gentle tug at the crown of the plant will usually pull it away from the soil surface, as there is no root system left. Some times the first clue that something is wrong is when the plant blows across the garden like a ball of tumbleweed with all roots having been munched through.
An exploratory dig around the planting area usually reveals the larvae about 2-5cm down, in what was left of the plant roots.
Description of pest
Vine weevil - adult and larvae
Vine weevil larvae are up to 1cm in length with a plump, creamy white body and a brown head. When found, they are usually curled into a ‘C’. The adults, also 1cm long, are dull matt black in colour with a pronounced ‘snout’. If seen, they tend to play dead, lying totally still, sometimes on their backs with their legs tucked in! They cannot fly ( although I find it hard to believe), but will crawl determinedly, even upside down for long distances to their favoured laying sites,
The female weevil (males are rarely seen) emerges from the soil in early spring. Over a 3-4 month period each adult can lay 1000 eggs close to suitable host plants. Not all of these will hatch. The larvae emerge after about 2 weeks, then tunnel into the soil or compost, feeding on plant roots. After approximately 3 months, as the weather cools, the larvae burrow to overwinter at a greater depth. As the temperature increases in spring the larvae pupate, and, 3 - 4 weeks later, new adults crawl from the soil. In homes and heated glasshouses consistent warmth enables weevils to be active all year round and to complete the cycle in less than a year.
It's not all bad news
Heucheras are tough; total loss of roots will not kill them if you take simple action before they die of dessication. Often, an affected plant can easily be cleaned of any odd weevils hiding in the remaining roots. It can be therapeutic to stab them with a sharpened match stick and pick them out of their little hidey-holes -or for the sqeamish just give them a good hosing down to wash out the pests.
Chickens and Bantams love the weevil grubs so give them access to soil and plants that are infected - within seconds no a grubs will remain
Once you have cleaned your rootless plant you can simply re-pot it in fresh compost - give it a drink and keep it in a shady place for a few weeks - you'll be amazed at how quickly it recovers!
The real bonus is that a large plant can be easily broken up into several pieces and they'll all grow away as long as they have a basal
Prevention and control
* Be vigilant: Any new plant, particularly those favourites mentioned earlier, should be taken out of its pot, and checked thoroughly for weevils and their larvae. Too often this pest is introduced with the soil in summer bedding plants, or house plants. Watch for adult weevils crawling up walls or furniture, or lying quietly in out of the way corners, indoors and out. Destroy when discovered.
* If a plant, outdoors or indoors, wilts suddenly, check the root system without delay. If caught in time, plants can be saved either by repotting in a new pot with fresh soil, or by replanting in a different place. All larvae found should be destroyed. Traps for adult weevils: Corrugated cardboard has been used successfully, made into a roll and left for the adults to hide in during the day. Moist sacking or planks of wood laid alongside affected plants provides a dark daytime hiding place for the adults that can then be collected during the day when they are relatively inactive.
* Barriers: Put a strip of wide PVC tape, such as brown plastic parcel tape, around individual pots and tubs, and smear this liberally with insect barrier glue which the weevils cannot cross. The tape can be easily removed at the end of the season.
* Biological control: There are two species of tiny parasitic nematodes effective against vine weevil, Steinernema kraussei or Heterorhabditis megidis. When purchased, they come in a packet of powder ready for mixing with water. The compost or soil is drenched with this mixture; the nematodes seek out the vine weevil larvae and enter through their natural body openings, quickly killing them. The nematodes then reproduce and move off in search of further prey. They are completely harmless to plants, humans and animals.
Steinernema kraussei: Requires a minimum soil temperature of 5ºC (40ºF) to be effective. Use in the autumn from September to November for best results, if damage appears in the spring then it can be applied from March.
Heterorhabditis megidis: Requires a minimum soil temperature of 12°C (54°F). Use in the autumn from August to early September for best results, if damage appears in the spring then it can be applied from April/May.
To achieve success with biological control be sure that:
1. Adequate soil temperature should be maintained for two weeks after application to ensure effective treatment.
2. Nematodes are not applied to dry compost or soil. They need moisture to move through the soil to infect the larvae.
Chemical control - Bio Provado is a new product based on thicloprid that can be used as a soil drench to control vine weevil grubs. For new container plants try Levington Plant Protection Compost which also contains the same chemical. Always read the labels before using garden chemicals.
Thanks to Garden Organic and the RHS for some of the above information.