Biologically combat the weevil with nematodes

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Thick-eared weevils and their larvae often infest gardens for years and eat on the leaves and roots of certain cultivated and ornamental plants; in sensitive plants an infestation often leads to death. Since the feeding sites can lead to diseases and fungal infestation, countermeasures must be taken immediately. A biologically meaningful alternative to conventional chemicals are nematodes, small nematodes, which cause a deadly infection with bacteria in the larvae. In doing so, beneficial animals are spared, there is no danger to humans either.
The weevil is a small beetle that can spread quickly in ideal conditions. The dreaded pest is often noticed very late, since the animals are active only in the night hours. Black-eared weevils move on foot and can easily negotiate long distances:

  • Gray beetle with 1.0 to 1.5 cm in length
  • Nocturnal and active insects
  • More than 90 percent of the population is female
  • Lay down after a short period of maturation without mating up to 1,000 eggs
  • First larvae hatch in May, the last ones often only in the fading out of August
  • Prefer specific host plants
  • Prefer sandy and humus rich soils in the root area for oviposition
  • 2-3 weeks after oviposition the larvae hatch, they start to eat immediately
  • Larvae hibernate in the ground, pupate in the following spring
  • Spread quickly into terraced housing estates and garden colonies
  • Can also crawl up on balconies and roof terraces
Food and infestation
Although the weevil is not fussy, it still prefers certain plants. At the top of the beetle's menu are woody plants whose leaves are a little rougher. It can be seen from the characteristic bungling that the pest drives its mischief at night. The leaf damage, however, hardly damages the affected plants, which is one of the decorative annoyances in the garden. The actual damage is caused by the larvae of the weevil, which can spread explosively under ideal conditions. Even large and extremely healthy woody plants can be killed by an infestation, so these are always on an infestation to check out:
  • Appreciate boxwoods, yew trees, cherry berries, roses and rhododendrons
  • Also likes to eat strawberries and many different perennials
  • Also falls on some potted plants, for example Clematis, Angel Trumpet and Mandeville
  • The host range also includes soft fruit and grapevines
  • Leaves semicircular and bay-shaped feeding sites along the leaf margins
  • Offspring of the beetle lives as a larva in the ground and causes great damage there
  • Larvae are 7-10 mm tall and of a white-yellowish color
  • Occur very numerous, damaging for months by eating the roots and tubers
  • There is a strong loss of water and nutrients
  • Weak and stunted plants can be observed as consequences in the summer months
  • Bacteria and fungi invade via the nibbled roots
The dangerous larvae can be fought with native and specially bred nematodes, this method has been proven by many hobby gardeners. The nematodes always need enough moisture in the soil to move quickly, but waterlogging is not tolerated at all. The following aspects are crucial in the use of nematodes:
  • Nematodes are active nematodes in the soil, live parasitic
  • Only have a minimum diameter of 0.03 mm
  • Can be observed with a good magnifying glass
  • Attack the weevil larvae like torpedoes
  • Penetrate into the bloodstream via the breathing holes, leaving dedicated bacteria there
  • Due to the resulting infection, the larvae die within 2-3 days
  • Extremely high efficiency, is about 90 percent
  • Are very sensitive to strong UV light
  • Do not like cold and dry soil
Tip: There is now a special nematode mixture on offer, which works even at a floor temperature of only 8 degrees Celsius, but this must be achieved for at least a few hours. This mixture is ideal for gardens in colder regions.
The nematodes offer the gardener many advantages, their use is always preferable to chemical agents. The effect has been tested for many years, the thread worms are in the dormant stage in the specialized trade in a sealed plastic bag available. These are in a kind of powder, mixed with clay minerals. The nematodes are to be deployed as soon as possible after purchase, but they can survive in cool storage for several days in a plastic bag. The following advantages characterize the use of nematodes:
  • Nematodes leave no residue
  • Are completely biological
  • Nematode-treated areas can be entered by man and animal at any time
  • No long wait
  • Larvae do not develop any resistance
  • Simple application, no danger to humans, animals or other plants
  • Protects beneficials, such as bees and earthworms
  • Not absorbed by the infected plant
  • There are no known side effects
  • The insert has been proven in practice for many years
Tip: weevils can live up to three years, so it makes sense to repeat the treatment with nematodes in the next two years. In this way the infestation is permanently stopped.
Use and timing
The application rate of the nematodes appears relatively high at first glance, but the thread worms reach only a small size. In addition, a large amount is required for quick success. The nematodes have a very lasting effect on the weevil, as the parasites continue to multiply in the body of the dead larvae. However, the right time to use it is important so that it can also lead to success:
  • Approximately 500,000 nematodes are needed per m² of application area
  • Infects the larvae with bacteria that cause death within three days
  • Continue living in the dead larvae, creating up to 300,000 new roundworms this way
  • Ideal months of combat are April, May, August and September
  • Soil temperature must not be lower than 12° C, optimal 15-25° C
  • Before spreading, infuse the affected plants thoroughly
  • Ideally deploy only in the evening or in cloudy skies
  • Fill the contents of the bag in the watering can with stale tap or rainwater
  • Then pour root area around infested plants
  • Water regularly for optimal effects over the next 6-8 weeks
  • Soil should not dry out in the future, nematodes need moisture
  • Application in the field as well as in pots and containers possible
  • Larva dies after 24-48 hours, then turns red to brown
  • Further propagation of the nematodes until the larva is completely consumed
  • Repeat treatment in spring and autumn
Tip: The regular watering of the plants should be done on a warm day's work in the morning, so that the floor can then heat up again.
The weevil is a dangerous pest in the garden and can even attack balconies and roof terraces. An infestation is often detected very late, because the beetle is nocturnal and does not show in daylight. A first indication are feeding spots on the leaves, but the biggest damage is caused by the beetle larvae in the soil. If an infestation is found, immediate control measures should be used to prevent further spread and death of the infested plants. Nematodes are ideal for the fight, because they work fast and have no side effects on humans, animals and other plants. For the nematode use, the soil temperatures are crucial, if these are too low, the thread worms can not survive. Optimal days of use are at the end of spring and outgoing summer, when the ground has already warmed up well. The nematode mixture can be easily fed through the irrigation water. If the treatment is repeated over 2-3 years, the weevil will be effectively and sustainably controlled.

Video Board: Biological control of the black vine weevil with entomopathogenic nematodes.

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