This is the time of year you will find out if you have any soil borne problems with your bonsai trees. The time for repotting is from around October through to March for broadleaves and into April for evergreens. Tropical bonsai are generally repotted when it is slightly warmer so later in the Spring but again it does depend on how you are keeping your tropicals.
However, the many people in the UK tend to repot in February and March but it does depend on your location, tree species and frequency of repotting and the root development of each bonsai tree.
It is worth checking the roots of the trees annually if possible to check for such pests like Vine weevil larvae or Leather Jackets as these can be very detrimental to your bonsai roots if left un checked.
What do Vine Weevil larvae look like?
In the larvae stage they are plump, white, legless larvae with pale brown heads.
What do the adult Vine Weevil (Otiorhynchus sulcatus)?
They are around 9mm in length and have long pear shaped bodies that are dull black with dirty yellow marks on the wing cases with antennae that are bent at an angle about half way along their length. They affect most species and are common in mid spring up to autumn.
Adult beetles eat the leaves of trees and plants and take out irregular notches from the leaf margins. They are slow moving beetles that tend to emerge at night and do not fly. The female beetle can produce hundreds of eggs between April to September and these eggs are small around 1mm in diameter and brown.
What damage do they cause?
They feed on roots, corns and tubers, particularly on plants grown in containers.
What is the impact on your bonsai?
Loss of vigour, eventual root loss, potential wilt is left unchecked.
How to control them in the larvae stage?
Repot your bonsai and remove any larvae found within the soil. Also consider your bonsai mix as the larvae tend to prefer moist compost rich soils as opposed to more free draining gritty mixes. The most effective method for control is that if you suspect your bonsai tree has vine weevil larvae within it’s roots is picking out any larvae during the re-potting of your bonsai trees to avoid the need for chemical application.
There are a range of insecticides readily available from garden centers but repeat applications may be required to achieve the end result to kill off the larvae stages within the soil. Examples include Provado and Scotts Bug Clear Ultra Vine Weevil killer, which contain Thiacloprid and come in the form of a liquid drench.
The liquid drench is the most effective form of control for the soil borne larvae and should be used in mid to late summer as a preventative control. However be careful using chemicals and ensure you adjust the dosage for the use on bonsai trees and avoid using on flowering trees due to the impact on pollinating insects like bees.
A biological control can also be used in the form of the microscopic pathogenic nematodes like Steinernema kraussei or Heterorhabditis megidis which are available from suppliers of biological controls.
How to control the adult beetles?
Trap adults with sticky barriers placed on legs of greenhouse staging or benches. Systemic application of insecticides specific for vine weevil can be effective on the adult beetle but again repeat applications may be required.
In Spring and Summer pick off adults in evening via torchlight as they come out at night. Natural predators include birds, frogs, hedgehogs and predatory ground beetles.