Controlling Invasive Lake Weeds

Plant Science Day 2000 Short Talk

Gregory J. Bugbee
Department of Soil and Water
The Connecticut Agricultural Experiment Station

Dr. Jason C. White
Department of Soil and Water
The Connecticut Agricultural Experiment Station 

Lakes and ponds are one of Connecticut’s most important natural resources. There are over 3000 fresh water lakes and ponds in the state. These bodies of water provide recreation, wildlife habitat, drinking water, irrigation supplies, scenic vistas, and tranquil escapes. Connecticut’s lakes and ponds are relatively young. Most were created during a retreat of the glacier that covered the area about 10,000 years ago. When first created they were clear and the bottoms were nutrient poor. Aquatic plants have difficulty growing under these conditions. Over time nutrients from natural and manmade sources buildup in water and sediment. This process is called eutrophication and it leads to the growth of aquatic plants. Aquatic plants are not necessarily bad because they often provide habitat for wildlife. However, excessive aquatic vegetation becomes unsightly, interferes with recreation, and gradually turns the lake into a swamp.

In 1995 I was asked to survey the aquatic vegetation in Bashan Lake, East Haddam CT and comment on controlling an invasive weed called Milfoil (Myriophyllum heterophyllum). Milfoil formed dense patches in about 10 acres of the 276-acre lake and was interfering with recreation. It was being chopped and spread by boats. Past control measures have included: hydroraking, handpulling and application of the herbicide Diquat. These measures slowed the spread of the milfoil, but did not adequately reduce its presence.

In 1999 the Connecticut Department of Environmental Protection (CTDEP) through the Town of East Haddam gave the Connecticut Agricultural Experiment Station a two-year grant to research alternative methods for controlling the milfoil. After a town meeting where the various alternatives and their potential for success were discussed, it was decided to study controlling the milfoil by spot treating it with an aquatic herbicide called 2,4-D. This product had the advantage of having a track record of success for spot treating milfoil as well as the disadvantage of government restrictions on use where lake water is used in the home or for irrigating plants.

The form of 2,4-D with the least restrictions is called Aquacide. It is a sodium salt form of 2,4-D that cannot be used where water is used for drinking. In order to proceed the Town of East Haddam notified each lakefront homeowner that water from Bashan Lake is not fit for drinking. The CTDEP then granted a permit with the proviso that we test lake water and nearby wells for 2,4-D. This testing would determine the dissipation rate of the chemical, provide an exact timeframe for resumption of irrigation and give information of the potential for 2,4-D to move to peoples drinking water wells. Dr. Jason White will speak on this testing shortly.

In early July 1999, Aquacide was applied to the milfoil patches at a rate of 100 lbs per acre. Milfoil control was not complete and only about half the weeds were controlled. In May 2000, an ester formulation of 2,4-D, called Navigate was used at a rate of 200 lbs. per acre. This product proved very effective and all the milfoil in the treated areas was controlled. Unfortunately, some milfoil in areas not treated was discovered in early July and small spot treatments made in mid-July. The effectiveness of these small spot treatments is still being evaluated.

I’d like to give a bit of background on the specific chemical we used in this project. It was an issue of concern for the CTDEP upon issuing the permit for application and also for the residents of the lake. The chemical is 2,4-dichlorophenoxyacetic acid or 2,4-D. It is a member of a class of compounds called chlorophenoxy herbicides. 2,4-D is a systemic broadleaf herbicide that it is specific to dicot plants. This means you can put it on your lawn to kill the broadleaf weeds but not the grass. Since it is systemic, it is absorbed through the leaves and then translocated from the tips of the roots to the tips of the shoots. It acts as a synthetic plant hormone and over a short period of time, causes the plants to essentially grow themselves to death.

2,4-D is a widely used compound; there are about 1500 products that have it registered as the active ingredient. The vast majority of those are intended for terrestrial use. It is commonly used on the grazing lands in the western U.S. and in the sugarcane industry of Hawaii. In terms of physical characteristics, its half-life in water is 5-50 days. That is the amount of time required for half the applied material to be degraded. The wide range has to due with the importance of the physical characteristics of the specific body of water and on the exact formulation used. We’ll talk about formulations shortly. EPA lists a MCL or maximum contaminant level of 70 parts per billion in drinking water. The short-term effects to exposure of levels much higher than the MCL include dizziness, stiffness of joints, and headache. In terms of chronic effects from long-term exposure to very high levels, one can experience liver and kidney problems and more serious neurotoxic effects. 2,4-D is not a human carcinogen. The EPA lists a reference dose of 0.1 mg/kg/day. That means that a 180 pound person can be exposed to 6 mg of 2,4-D per day for his/her entire life and expect no adverse effects. This is a far more 2,4-D than could ever be contacted by exposure to a treated lake. The safety of the herbicide has to do with the fact that this is a synthetic plant hormone that has no real mode of toxicity in animals except at extremely high levels.

In terms of environmental concerns, the half-life of 2,4-D in soil is 3-10 days and a range of common soil bacteria rapidly degrades it. In addition, exposure to ultraviolet light will cause the breakdown of the molecule. The problem comes when considering groundwater or well water. Here, oxygen is limited so the numbers of bacteria capable of degrading the compound are very low. In addition, there is no light and temperatures are often very low. In short, once 2,4-D gets into ground- or well water, it is there for a very long time and hard to remove. As Greg mentioned with the drinking water wells, this was a concern for this project.

I briefly wanted to discuss the chemical structure of 2,4-D. The first year, we were only permitted to use the sodium salt formulation due to label restrictions relative to domestic water intakes (people taking lake-water directly into their houses). Due to the monitoring results of the first year, we were able to apply the ester formulation the second year. The ester has a long carbon chain attached to the central ring. The presence of this carbon chain gives the compound a range of characteristics different from the salt formulation including decreased water solubility, increased sorption to plants, and longer half-life. All of these will lead to better weed control.

The monitoring portion of the study was in 2 parts. First, we needed to monitor 2,4-D levels in the lake due to a temporary ban on use of lake-water for irrigating plants. The areas of the lake monitored were the 3 main coves where application occurred. In addition, we took samples about 100 feet outside an application area and also at the center of the lake (700 yards from the nearest application site). At all lake sites we sampled the surface and at a depth of about 9-10 feet. We could lift the ban once levels in the lake were shown to be below 100 ppb. Secondly, we monitored the drinking water of 5 residents of the lake. Here, we took water samples from the kitchen sink and analyzed for 2,4-D.

We observed drastically different levels in the lake depending on the formulation used. In 1999, we used the salt formulation. Here, only once in the first few days did we record a level above the 100-ppb benchmark. In addition, within 3 days we were detecting 2,4-D at the center of the lake. Like any salt, this formulation rapidly dissolved into the entire lake and was rapidly diluted to very low levels. Consequently, weed control was only fair; about 50%.

This year we applied the ester formulation and 2,4-D in levels in all application areas were about 1000-2000 ppb. This is the level desired for complete weed control under laboratory conditions. Those levels were maintained for several days. After about 1 week, we were able to detect 2,4-D at the center of the lake; indicating dissolution at a slower rate than with the salt formulation. As a result of the prolonged higher levels, weed control appears to be 100%. Importantly, at no point during the 2-year study did we detect 2,4-D in any drinking water wells.

In conclusion, 2,4-D is an appropriate chemical for the control of invasive milfoil. Interestingly, the formulation used will have significant influence on the extent of weed control. The salt formulation is more expensive and more difficult to use and gives fairly poor weed control. The ester formulation is cheaper, easier to apply, and much more effective.