What does Azolla look like?
The plant is fairly easy to identify, each frond consists of a main stem growing at the surface of the water which has alternate leaves and adventitious roots at regular intervals along the stem. Secondary stems develop at the axil of certain leaves. Azolla fronds are triangular or polygonal in shape, and float on the water surface individually or in mats (see image on right).
How does Azolla reproduce?
Azolla reproduces both vegetativley (pieces of the fronds break off and form new plants) and by the production of spores. The spores function like the seeds of terrestrial plants and can over winter at the bottom of the pond. When conditions are right these spores will float to the surface again, where they initiate sexual reproduction and so produce a new generation of plants.
How can one tiny weevil control the weed?
Individually each tiny weevil will not have much effect, however en mass the weevils can be devastating. Weevils are capable of rapid reproduction and within a matter of weeks weevil populations can reach several million. All mobile life-stages of the weevil (adult and larvae) feed voraciously on the plant, which make this insect a force to be reckoned with.
How can it be that they only attack one plant?
Many insects are monophagous, that is to say they will only attack one species of plant. This is not as surprising as it seems since many endangered insects are under threat because the host plant or its habitat has become rare. These organisms have spent hundreds and thousands of years evolving to overcome the defences of their food plant and during this time they have been forced to become restricted to this plant to complete their lifecycle.
What happens to the weevils once all the weed has been eaten?
Larvae of the weevil cling on to the dying Azolla and will drown when it sinks. The adults can only feed on Azolla species, therefore once their food source has become exhausted they will disperse in the search of more Azolla. The weevils can fly, although there has been little research into the distances that they can travel. The weevils will migrate into other neighbouring pools as their population expands. The speed at which the weevils control the Azolla infestation is related to the size of the initial release (number of adults and larvae) and the prevailing weather conditions. Under ideal conditions, control can be rapid.
What happens to the weevils in the winter?
The weevils are native to west coast of North America. Insufficient research has been conducted in the UK to say exactly what happens to the weevils during the winter. Our best guess is that the majority die, however a few adults find sheltered spots where they go into diapause (a period of biological dormancy with decreased metabolism). These weevils sit out the winter and become active the following spring when the Azolla starts to regrow. Regrowth is either from spores (seeds) shed during the summer, or vegetatively from plants that survived the winter. If the winter is mild and your ponds are sheltered then some weevils may survive to the following year.
Does biological control work?
There have been many notable successes in biological weed control not least the example of the weevil Cyrtobagous salviniae against the floating water weed Salvinia in Sri Lanka. The plant was introduced into the country during the second world war to prevent the enemy aircraft from identifying waterways and it did the job so well that almost all water bodies in the country were affected. The weevil was released in 1986 and within 4 years it had destroyed around 80% of weed infestations. Since its discovery Cyrtobagous has been a successful control agent against this weed in more than 10 tropical countries around the world.
When is the best time to apply the weevils?
The weevils can be released throughout the growing season of Azolla. The best time to release is early in the season before the Azolla has become out of hand. Azolla soon takes advantage of the increased daylength and warmth of late spring/early summer and can grow extremely rapidly doubling its area every 4-5 days. Usually one release of the weevils is sufficient to exert control, however if you have a large infestation or require more rapid control more releases may be necessary.