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Trends in numbers of aquatic invertebrates in a large canadian river during 4 years of black fly larviciding with methoxychlor diptera simuliidae

, : Trends in numbers of aquatic invertebrates in a large canadian river during 4 years of black fly larviciding with methoxychlor diptera simuliidae. Quaestiones Entomologicae 19(1-2): 53-92

Methoxychlor was injected and maintained for 15 min .apprx. 0.3 ppm, into Saskatchewan River System in Saskatchewan once in 1976, 6 times in 1977, 7 in 1978, 19 in 1979 and 5 times in 1980. Severe outbreaks of S. luggeri originated from various portions of the river up to 200 km long during the first 3 yr, but not in 1979 or 1980. With suprageneric taxa serving as units, trends in numbers of non-simuliids were measured 1977-1980. Average densities of combined non-simuliid invertebrate populations attaching weekly to artificial substrates in mid-river sites in all 3 branches of the Saskatchewan River peaked in 1979 (P < 0.01) the year of maximum larvicide use, but in 1980 returned to just above the 1977 level. Average numbers of invertebrates in benthic samples from river margins also generally peaked in 1979 or 1980 in all 3 river branches. In 1 or more of the 6 locations sampled, numbers of certain families of Ephemeroptera (baetids, heptageniids, caenids, leptophlebiids and polymitarcyids), of Trichoptera (hydroptilids, leptocerids and brachycentrids), and of Diptera (simuliids, tanypodines, orthocladiines, tanytarsines and empids) declined after 1977 or 1978; in other locations many of these taxa peaked in 1979 or 1980. Some larvae dislodged by methoxychlor treatments apparently reattached in downstream sites. In 25-73% of 23 tests there were increases rather than decreases in numbers of various non-simuliid and simuliid taxa attaching to rope-piece substrates, 25-92 km downstream during the week following an injection. Significant upward trends in average annual densities of suprageneric taxa indicated that effects of methoxychlor treatments essentially were neutral when compared with effects of unidentified extrinsic ecological processes. Check lists of benthic species collected from all 3 rivers at the conclusion of tests in 1980 proved the survival of a varied fauna representing apparently complete ranges in feeding habits, activity patterns and life cycles. The relatively intensive series of methoxychlor larvicide treatments required to prevent damaging outbreaks of S. luggeri from the Saskatchewan River was not permanently harmful to non-simuliid taxa in the river, at least at the suprageneric level.

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