The milfoil weevil is a tiny, specialized aquatic insect native to North America that might have some very beneficial effects for humans and local ecosystems. This stems from the fact that it only eats milfoil (and harmless to humans).
There are two types of milfoil to be found in the United States. There is a native one and an invasive species of Eurasian milfoil. The native species is not a problem but the Eurasian one is a major environmental threat. It is this one that is making the milfoil weevil so important.
Eurasian milfoil (from this point forward all milfoil will be considered Eurasian milfoil unless otherwise noted) was most likely introduced to the United States sometime between the late 1800’s to 1940’s as either a stowaway on a ships ballast or discarded shipping material. Because of its ability to travel on boat anchors, bottoms, and propellers it spread widely, bringing with it drastic ecological shifts and additional problems for humans. Luckily this can spread the milfoil weevil as well
It spreads quickly and destroys ecosystems by choking out the native plant life which reduces food for water fowl, reduces habitat for fry, and reducing fishing by animals. The large mats it forms reduce the oxygenation of water by wind that leads to stressed fish and algae blooms.
For humans, it reduces the recreational uses of the water by bathers, boaters, and fisherman. For communities, the dense mats can clog water intakes or overflows, causing water shortages in some places and flooding in others. In dams, they clog and break generators and reduce power production.
The tiny milfoil weevil seems to be the panacea to this plague. The fact that it is native prefers Eurasian milfoil over native milfoil, and kills the colonies gradually, giving native species time to recover, make this the ideal. Add to that the number of generations of milfoil weevils per year (at least 3) and you have a natural pest remover with none of the problems associated with other methods. When looking at how the milfoil spreads it becomes clear and why milfoil weevils control it so well.
It spreads when little pieces break off and sink to the bottom, there they take root. Using large aquatic harvesters are not the solution as the break up the milfoil too much and it grows right back. Vacuum dredging is a little better in that it gets the little pieces, but cause a great deal of water disruption and leaves the bottom bare, so it will need replanting with native species.
The milfoil weevil though prefers Eurasian to native milfoil so it eats that first, slowly weeding it out by burrowing into the stems and eating it from the inside out. With a short life span(milfoil weevils live about 30 days) at least 3 generations will live and die till they head to the shore to winter. Although they have wings they have rarely been know to fly so no one know if milfoil weevils fly to shore or swim. Regardless, once there they survive even the harshest Minnesota winters.
Milfoil weevils are a wonderful example of using an existing animal (or plant) to control a non-native invasive plant (or animal). It is good wildlife management as it brings ecosystems back into balance
