Biopesticides and Biocontrols

Azolla weevil: 100 years of Azolla biocontrol in the UK

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19 May 2023, UK: Azolla filiculoides is also known as floating water fern or fairy fern. It is a small, aquatic, free-floating fern native to the warm temperate, and tropical regions of the Americas. The species has been introduced around the globe: to Australia, New Zealand, the Caribbean, Hawaii, North and sub-Saharan Africa and Europe. It was first suspected to be introduced to the UK in the 1880s as an ornamental garden pond plant. It has since naturalised, spreading predominantly throughout the lowland and central regions of the country in slow-moving freshwater habitats like ponds, ditches, lakes, and canals.

Azolla, a prolific invasive species

The species forms thick mats on the water’s surface, which can double in size in a few days depending on the conditions. Azolla also spreads prolifically due to its reproductive methods. The most common being asexual, with fronds fragmenting to produce more plants via vegetative propagation. These can be transmitted to other areas attached to the feet of waterfowl or aquatic equipment. Alternatively, the species can also reproduce sexually. It produces millions of tiny spores which can over-winter at the bottom of the water body, and float up to the surface when conditions are right. Coupling these factors together allows for its rapid spread; negatively impacting the aquatic ecosystem, blocking out light and reducing oxygen availability, damaging submerged flora and fauna, as well as impeding water-based recreation and increasing the risk of flooding.

Azolla’s natural enemy

There are no native-specific and damaging natural enemies to control Azolla populations in the UK. However, research found that a tiny weevil from North America is highly specialised to Azolla filiculoides. Measuring in at a tiny 2mm, Stenopelmus rufinasus (also referred to as the Azolla weevil) was vigorously tested and subsequently released as a biocontrol agent in South Africa during the late 1990s, showing huge success. The weevil is known to feed extensively on this species of Azolla. It does not harm native plants but can cause considerable damage to large swathes of the water fern, leading to local eradication.

By chance, however, this weevil was already present in the UK. It was discovered in 1921 – most likely as a stowaway on introduced Azolla. Hence, due to its long residency on the British Isles, the weevil is considered an ordinary resident by DEFRA (Department for Environment Food and Rural Affairs) and Natural Resources Wales, with no licensing restrictions. In Scotland, the weevil is released under licence. Thus, being a widely naturalised species, the weevil was an excellent prospect as a biocontrol agent. Releasing suitable quantities of weevils onto an Azolla infestation can bring it under control in a matter of weeks, without the use of potentially harmful chemicals, or risking further spread during to mechanical removal.

How the weevil controls Azolla

The process by which the Azolla is brought under effective control proceeds as follows. The female Azolla weevil chews a hole into the tip of one of the fronds, laying a single, yellow-orange egg. This egg then hatches into a similarly coloured larva, which feeds voraciously on the fronds, mining down into the plant as it grows. Finally, once full to the brim, the larva pupates, making a chamber in the leaf, eventually emerging as an adult to repeat the cycle. This whole process takes between 16 and 23 days in warm conditions. What’s more, with thousands of weevils performing it, this results in the complete destruction of significant volumes of Azolla.

The naturalised populations of the S. rufinasus weevil in the UK, in contrast to those in South Africa, do not offer the level of control desired to entirely mitigate the outbreaks of the Azolla. This is likely due to the challenge of living in the British climate, restricting development rates, length of the active season, overwintering survival and dispersal abilities. Thus, since the mid-2000s, CABI has undertaken the mass production of S. rufinasus weevils to conduct augmentative releases to tackle outbreaks of Azolla as they occur.

Azolla weevil farming

The weevil farming is undertaken by the invasive species management team at CABI Egham. Each summer tens of thousands of weevils are reared and collected. This entails filling many large rearing ponds with Azolla, some being kept as food plants and others for nurturing the weevils, which are overwintered on site. All the ponds need to be covered with a fine mesh to ensure the weevils stay on their desired Azolla. This ensures any strays cannot get into the foodplants and eat up all the reserves!

Once the weevils are well established in each pond and the orders are placed, the team collects weevils with an electric pooter, sucking them up one by one while counting. They are then added to the shipment boxes containing a moist substrate for the weevils to cling onto during their transit. These are then shipped out across the UK. Each box contains thousands of tiny Azolla weevils. The recipients are water managers from a variety of environmental organisations as well as councils, businesses and private individuals. In some years, upward of 70,000 weevils are reared, shipped and released.

Weevil’s economic value

Since these efforts began, this management approach has proven highly effective and more environmentally friendly than the alternatives. Furthermore, a recent open access study reviewed the economic value of S. rufinasus biocontrol. It estimated the average annual costs of Azolla management in the absence of the weevil to be in the range of £8.4 – 16.9 million. Naturalised weevil populations reduce these costs to £0.8 to 1.6 million. In addition, the paper shows the substantial economic benefit of the augmented releases, saving a further £0.77 to 1.6 million per year in management costs and having a benefit to cost ratio in the range of 44 – 88: 1. What a large feat for such a tiny insect!

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