Midges

In the late 1990’s midges had become a big nuisance in and around Straits Pond, and a leading indicator of the poor health of the water body. Through the dedication and perseverance of the SWPA, and the completion of the tidal gate at West Corner, the midge population at Straits Pond has been effectively eradicated.

The following data is from a 1994 Straits Pond Study

Data collected over a three year period determined that there are three generations of midges during the spring through fall seasons.

  • The first generation is when the over-wintering larvae mature and leave the pond in mid-April as adult flies.
  • An absence of larvae was recorded from mid-April through May.
  • The second generation was recorded by larvae peaks in June followed by a decline in the early part of July.
  • The third generation is recorded by larvae population peaks in August and declines in September when the larvae have matured and emerge from the pond as adult flies.

Salinity and midges:

"If raising salinity decreases midge infestations, why has it not been done?" The short answer is because it has taken time, scientific inquiry, and significant site-specific data to demonstrate this fact in the scientific literature. Midge control for Straits Pond was treated as a yearly, isolated event without considering longer-term solutions such as increasing salinity.

Background:

New Data about salinity above 18-22 parts per thousand (ppt): A very thorough and scientifically rigorous study was performed regarding the salinity of Chironomidae midges as part of the East Harbor/Pilgrim Pond restoration project.

Tim Smith, Coastal Zone Management, described this project during the Straits Pond Forum Series. This study supports a similar conclusion stated by Linda Beres, et. al., in the 1990s Straits Pond studies that salinity maintained above approximately 18-22 parts per thousand (ppt) inhibits midges from pupating, or becoming free-flying insects.

It is important to note that midge larvae can persist for a considerable period of time at high salinities so it is necessary to maintain tidal flow and high salinity levels throughout the season.

New data about Salinity above and below the tide gate:

Although midges occur down stream from the West Corner Culvert, their existence appears to be insignificant. A significant variable between the Weir River and Straits Pond is salinity. During the spring the salinity in the Weir River is typically above the 20 ppt range, and the salinity in Straits Pond is in or near single digits ppt. The low spring salinity in Straits Pond allows the midges to pupate.

New data about Communication (exchange of waters):

It was previously thought that it took two tidal cycles to clear the Weir River water from the West Corner Culvert area. Recent data indicates that only one tidal cycle is necessary. Significant data has been collected during the past year indicating that there is good communication between Straits Pond and the Weir River.

With the tide gates in a full-open position the outgoing tidal flow sufficiently transports water from the Pond through the estuary into Hull Bay. The incoming flood tide water is significantly different from the previous ebb water as proven by higher salinity readings. Springtime salinity of the flood tide water on the Weir River side of the culvert is similar with respect to salinities observed in Hull Bay (near the pier). There can be significant salinity benefit to Straits Pond by improving exchange between Hull Bay and the pond with each tidal cycle.

New data about Stratification: 

Although previously it was thought that there was stratification in Straits Pond, data from recent water quality sampling indicates that there is little if any stratification and it is likely to be short lived due to the shallow depth of the pond and mixing via wind. With the equal disbursement of the salinity, if the ppt were to be 20+ ppt, this would significantly impair the ability of the midges to pupate.

New data about Flushing:

We now know that with the current culvert and tidal gates the maximum tidal exchange of water in Straits Pond during one tidal cycle is approximately 1', depending on the tidal range in the estuary. Greater flushing would have a positive effect on all of the sources of the pond's problems: low salinity, high water temperature, low dissolved oxygen, and high nutrient overload. It is important to stress that salinity is only one indicator of Strait Pond's water quality impairment.

Questions:

Will 1' tidal exchange be a sufficient volume to maintain salinity at a rating of 20+ ppt during the spring?

How often will the maximum tidal exchange of 1' be achieved due to the wide range of tidal levels throughout the year?

Will a tidal exchange of less than 1' effectively transport water from the pond out through the estuary?

Will a tidal exchange of less than 1' increase the salinity of the pond?

How does the effective volume for flushing change as a result of variations in rainfall?

Midges: Biology, Ecology & Problem Populationdownloadable Power Point Presentation courtesy of the Massachusetts Office of Coastal Zone Management (CZM)

Unpublished Material: Dr. Joel Margolith, Harvard University, Dept. of Tropical Health, notes from presentation on May 27, 1992, Hull, MA at Lab on George Washington BLVD. [Ed. Note: Dr. Margolith is an entomologist and acknowledged as an expert in the study of mosquitoes. At the time of his presentation he was a visiting professor at Harvard University and was working with the Massachusetts Department of Public Health to reduce the threat of Eastern Equine Encephalitis, which was on the rise in the Bristol County and Plymouth area.]

1980 IEP Study on Strait’s Pond for Town of Hull, Massachusetts.

Report on Midge Mitigation Study: A Study of the Water Quality & Midge Population Dynamics of Straits Pond in the Towns of Hull, Cohasset, and Hingham, Mass., Hull Public Schools For Town of Hull, Massachusetts, by Linda Beres and Faith L. Burbank, 1992.

Straits Pond Report 1994, Hull Environment & Service Corps, by Linda Beres, 1994.

Midges

There are 5,000 recorded species of midges. 2,500 species of Chironomidae are found in North America. Most are non-biting and live in water; some have adapted to a low oxygen environment. Of the 40-50 species of midges that may be inhabiting Straits Pond, a small percentage (4-5%) are non-biting aquatic filter-feeders. Filter feeders form swarms in the adult stage and are a nuisance. Midges have a 4-stage life cycle: egg to larvae to pupae to adult. They inhabit the pond during their larval and pupae stages, primarily existing in the shallow edges of the pond. The first generation historically emerge from the pond as adult midges in late April.

Midge Species in Straits Pond

Past studies identified the species of midge found in Straits Pond as C. Decorus (1953 Mass. Dept. of Public Health Study) or C. Riparius (IEP 1980 Study). During the 1992 Straits Pond study, attempts were made to positively identify the species of Chironomid midge found in Straits Pond by sending specimens of midge larvae to the Museum of Natural History in Washington DC. Communication with Dr. Ashad Ali of the Univ. of Florida and Dr. Joel Margolit of Harvard University tentatively indicate the species may be C. chironomus. The 1994 Straits Pond Study refers to the “species of midge fly known as Chironomous Riparius”, so it appears that final identification of the specific species was made.