These are herbivorous insects that will happily munch their way through an entire garden of prized plants. The adult damage, although unsightly, is not usually a problem. In late summer they lay eggs into the soil hatching into small white grubs that over winter within the roots of the plant. These are a much bigger problem as they will feed extensively on the root system, often killing the plant in the process. Container growing is particularly hazardous when faced with vine weevil larvae.
The nematode enters through the natural orifices and excretes bacteria that poisons the blood, killing the grub. The soil needs to be at least 5 degrees C for it to work efficiently, and the nematode is applied in autumn. For extreme cases, they need to be reapplied in spring and again the following autumn.
There are two main pests of lawns that can be controlled by nematode use, and it is important to recognise which one you have as the controls are different. Leatherjackets are the larvae of craneflies, and the eggs hatch a few weeks after they were laid in August-October. A mild winter will allow them to eat the roots of lawns soon after hatching. The lawn will start to go yellow and die off. A colder winter just pushes the damage back a few months. Chafer grubs similarly eat the roots of grasses over winter, showing yellowing, dead patches. The grubs are bigger and fatter than leatherjackets and are very attractive to hungry birds who will start to dig up the lawn, which is the main difference between identifying the two.
The soil needs to be warmer for the nematodes, 10-12 degrees C is a minimum, but they will need applying in autumn, and a second application in spring is occasionally necessary.