The native milfoil weevil (
Euhrychiopsis lecontei) is a milfoil specialist, and will feed primarily on Eurasian watermilfoil (
Myriophyllum spicatum) if it is available. This characteristic makes it a useful biological control agent for Eurasian watermilfoil, an invasive, non-native aquatic weed. The Department of Ecology aquatic weed monitoring group collected, raised and released milfoil weevils during 2002 and 2003. The following method summarizes our experiences raising milfoil weevils in aquariums for subsequent release.
Aquarium Preparation:
Several types of containers can be used successfully to rear milfoil weevils. We have used both commercial aquariums designed for fish, and food grade plastic containers that are transparent enough to allow light through the lid and sides.
The aquariums should be thoroughly scrubbed and rinsed before and between uses, using a mild biodegradable soap if needed. Set the tanks under suspended full-spectrum fluorescent lights (lights specifically designed for plant growth – generally available at aquarium or hardware stores). Put the lights on a timer to maintain a 16-hour day length. The optimal water temperature range for the weevils is 24 ± 3°C (75 ± 5.5°F). If high air temperatures force the water temperature to rise, partial water changes can be used to cool the aquarium water.
Untreated water should be used to fill the tanks; chlorinated water supplied by most city water systems should not be used. If untreated water is not available, water treatment products are available from aquarium suppliers to rid the water of chlorine.
Aerators should be inserted into each aquarium and run constantly, bubbling gently through an air-stone or strip. Ensure that the tanks are completely covered. Adult weevils have been observed to attempt to escape from the aquariums and will crawl from the corners if the top is left ajar.
Weevil Life Cycle
The female weevil generally lays her eggs on the growing tips of milfoil stems. The oval, creamy yellow eggs are laid individually and are about 0.5 mm in length. After 3-6 days, the larvae emerge from the eggs. The larvae feed on the milfoil growing tips for 3-5 days after hatching and then move to the stem where they continue to feed. They are most often found on the upper portions of the plant, and can be seen curling around or tunneling through the stem. Larvae are typically yellow in color and are elongate with a dark, round head. Larval development takes 8-15 days. Older larvae reach a length of about 4.5 mm., at which time they craft a pupal chamber in the thicker portions of the stem below the larval feeding damage. The pupae develop in their casing for 9-12 days until they emerge as adults. Adult weevils, usually with lengths of 2-3 mm., can be typically detected on the upper 1 m (3.3 ft.) portion of the plant and have been reported to live as long as 162 days. The complete life cycle from egg to adult takes 23-27 days at 20-25
°C (68-77
°F). Research indicates that weevil productions in the lab seem to be the greatest if the weevils are kept in the aquaria for 7-14 days. See the
milfoil biocontrol website for additional information (http://fwcb.cfans.umn.edu/research/milfoil/milfoilbc.html).
Milfoil Weevil Collection:
In lakes where weevils are present, the adults can be found by snorkeling on the water surface and searching the upper sections of the milfoil beds. If the day is sunny, they can be spotted by the colorings on their back and belly flashing with their movements. Occasionally more than one weevil will be found on the same milfoil segment. They cling tightly to the milfoil and should be collected with the milfoil to avoid direct handling. Pinch the milfoil stem several inches below where the weevil is and gently place it in a sealable one gallon plastic bag. Several milfoil stems with weevils can be placed in the bag. Be sure to keep bags sealed, and watch closely as new stems are added as weevils may try to swim out. Keep the bags cool while transporting them to the aquariums.
Eurasian milfoil will also need to be collected for the weevils to eat in the aquariums (preferably from the lake where the weevils will be placed). It should not be flowering, have any visible insect injury or other damage and should have full and healthy top ends, free of excess algae.
Establishing Weevils in the Lab:
For the weevil’s food and habitat, gather about 10 milfoil plants in a bundle. Include a stainless steel bolt or galvanized nail for weight and secure it with a zip-tie. Place 5 or 6 bunches into each aquarium (as space allows). Allot approximately one weevil per gallon of water in each tank, and place the weevils as soon as possible after collection. Avoid handling the weevils directly; rather, place them in the tanks on the milfoil they were collected with.
Precise records of weevil numbers going into the tanks should be kept if they are being reared for research purposes. If lake groups are simply trying to control their milfoil problem, it is at least recommended to keep track of the approximate weevil numbers to estimate developmental rate and progress.
Throughout the rearing period, supply healthy milfoil plants to the tanks as needed, and ensure that the water levels and temperatures are maintained. Damage to the milfoil can typically be observed within a few days, evidenced by darkened and tunneled milfoil stems and top ends.
Weevil Release:
After a rearing period of one to two weeks, the weevils should be sufficient in number for introduction to the augmentation site. If weevil productivity is to be counted, remove milfoil bunches from the aquariums one at a time and place the bunch in a tray of water until ready for inspection. Examine each milfoil fragment individually under good lighting, preferably with a dissecting microscope or magnifying glass. Count and record weevil numbers in each life stage. After this analysis, gather the milfoil fragments into large zip-lock bags filled with water and keep cool during transport. Weevil introduction to the test site should occur the same day as their removal from the aquaria if possible. If this is not possible, the weevils should be kept in well aerated water until they can be transported.
The milfoil fragments with the weevils should be gently intertwined with existing dense milfoil growth near the water surface. This can be accomplished by snorkeling or from a boat, allowing minimal disturbance to the plants. The objective is to keep the fragments affixed to the milfoil tops where they have best chance of survival rather than sinking to the lake bottom. Sometimes it is necessary to tie the milfoil fragments with weevils on to rooted milfoil in the lake.
Keep precise records of the release sites and dates. This process of rearing and release can be continued throughout the summer, but should be completed by the end of August, as it is believed weevils begin their migration to the shoreline for over-wintering in September.
References:
Milfoil Weevil Rearing . Figure 1. Adult milfoil weevil on a milfoil bud (Photo by RMN Lab U of MN) The native milfoil weevil (Euhrychiopsis lecontei) is a milfoil specialist ...
Hanson, T., Eliopoulos, C., and A. Walker. 1995. Field Collection, Laboratory Rearing and In-lake Introductions of the Herbivorous Aquatic Weevil, Euhrychiopsis lecontei, in Vermont. Department of Environmental Conservation: 5-9.
Confrancesco, A. and H. Crossen. 1999. Euhrychiopsis lecontei (Dietz) as a Potential Biocontrol Agent of Eurasian Watermilfoil (Myriophyllum spicatum L.) Aquatic Plant Control Research Program: 2-4.
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