E-541 jP a n © X\ June 1941 CONTROL OF DOG FLY BREEDING IN BEACH DEPOSITS OF MARINE GRASSES By S. W. Simmons and W. E. Dove, Division of Insects Affecting Man and Animals The dog fly occurs in all temperate regions of the world as a pest of warm-blooded animals and is a severe biter of man. Most often it is considered to be a parasite of livestock and is commonly known as the stable fly (St omo xys calcitr ans (L.)), 1 but in some localities its occurence in unusually large numbers as a pest of dogs has resulted in the name "dog fly." When it occurs in small numbers among domestic animals it attracts little attention, but among persons on sea beaches even a few flies may become most annoying, Under favorable conditions large numbers of flies develop, and their bites become serious for man and domestic animals. Outbreaks of this fly may result in a considerable loss of blood among cattle, with a reduction in their weight and vitality as well as in the supply of milk. When the animals fight the attacking flies they are likely to receive mechanical injuries, and when they make efforts to escape the flies by standing in water for long periods they may experience swollen feet and joints. If they stand in water of the swamps they often become mired and thus perish. In a coastal area of northwestern Florida the dog fly often occurs in outbreak numbers during the best season for tourists to visit the beaches, and at such times it is capable of seriously affecting resorting interests and incidentally the values of real estate. In order to avoid outbreaks of this pest it is necessary to under- stand (1) how dog flies develop along the seacoast of northwestern Florida and elsewhere, (2) how natural control affects breeding in marine grass deposits, (3) how to determine the time and place for treatment, and (4) how to treat breeding places. i Bishopp. F. C. The Stable Fly. U. S. Dept. Agr. Farmers' Bulletin No. 1097. (Revised October 1939. ) How the Dog Fly Develops As with other kinds of flies, there are four stages in the life cycle of this pest — the egg, the larva, the pupa, and the adult fly. The eggs are about one twenty-fifth of an inch in length, elongate, and of a creamy- white color. They are laid on moist portions of fermenting marine grasses on bay beaches, in cured hay exposed to the weather, and to a considerable extent in mixtures of peanut hay and manure about animal feed lots. When wet by fall rains, peanut litter left in fields after threshing also proved to be an important source of dog flies throughout several counties of northwestern Florida, southern Alabama, and southwestern Georgia. 2 Within 1 to 3 days the eggs hatch and the newly emerging larvae feed upon this fermenting material. Under the most favorable conditions they may become fully developed within about 7 days, but if the food is unfavorable or the air temperatures are low, they may require 30 days or longer. When full- grown the larvae are about four-fifths of an inch in length. In fermenting media they move rapidly and can conceal themselves quickly. When fully developed the larvae become shorter and thicker, and the outer surface hardens and gradually assumes a dark-brown color. This stage is known as the pupa, or resting stage, and it may last from 5 to 20 days or longer before the fly appears. When the fly emerges from the pupal case it inflates the head so as to force its way to the surface of the material in which it pupated. Upon reaching the surface the wings become dry and the fly soon goes for its first flight. The complete development from deposition of the egg to emergence of the fly may take place in a period as short as 14 days, but usually it ranges from 20 to 25 days, with an average of 20.4 days at Panama City, Fla. A developmental period of 76 days has been observed during lower temperatures, and even longer ones undoubtedly occur during the winter months. Both males and females are capable of taking blood of animals and may be observed resting on the walls of a building or on fences while digesting the blood meal. During warm weather flies may feed a second time during the same day. If adult flies do not obtain blood, food, or water they usually die within 2 or 3 days. In laboratory cages they have been kept alive for 47 days. On an average the female flies begin to lay fertile eggs when they are 11 days old. Where Dog Flies Breed on the Seacoast of Northwestern Florida Along the seacoast of northwestern Florida there are no feed lots which would account for outbreak numbers of dog flies. Cattle, sheep, and hogs are permitted to graze on the range throughout the year, and during * Dove, W. E., and Simmons, S. W., Control of Dog Fly Breeding in Peanut Litter U. S. Dept. Agr. , Bur. Ent. and Plant Quar., E-542 (multi- graphed). June 1941. - 3 - the winter they enter swamps for food and protection. In exceptional cases hogs or cattle are given some supplemental feed about farms and dairies, but v/hen such cases are found it is usually not difficult to find some dog fly larvae developing in fermenting hay mixed with manure. However, the numbers of larvae and pupae found in such places are most certainly not sufficient to account for any outbreak or unusually great annoyance by these flies. During the principal season for dog flies in the summer of 1940 five trained entomologists scouted intensively along the coasts for breeding places of the pest. When they encountered abnormally large numbers of flies they attempted to correlate the abundance with that of some nearby breeding places. Since this was seldom possible, detailed examinations were made to determine if breeding occurred in any depressions that received seepages of salt water. It was thought that fermentation of fresh-water vegetation in such places might afford favorable breeding places for the flies. About the edges of land-locked pools which contained water with a small percentage of salt, examinations of samples of decayed vegetation were repeatedly made, and soil obtained from such places was sifted and carefully examined. These procedures at different locations along the coast did not reveal the presence of a single dog fly larva or pupa. At the same time that examinations were made for breeding places, inquiries were made among fishermen and others regarding abundance of flies at different places. These men repeatedly stated that dog flies were troublesome to men in boats as far as 12 or more miles from shore, and in some instances they indicated that the flights were several times this distance. All of them agreed that the flies invariably appeared soon after breezes began blowing from the land. From these reports and from observa- tions it appeared that light winds and breezes brought the flies from inland breeding places, and as a result of this influx of flies it could be expected that any extensive coastal breeding place would produce a large number of flies that would drift with the winds and bite man and domestic animals at different locations along the coast. During the latter part of August the appearance of large numbers of unfed flies along the beaches of inner bays and sounds followed very closely the occurrence of windrows of two species of bay grasses on nearby beaches. These grasses are known as Ha lodule w rightii , or shoal grass, a narrow- leafed grass slightly thicker than pine needles, and T halassi a testudinum , or turtle grass, the leaves of which are about the width of carpet grass. They were not found on any of the beaches of the Gulf but were found growing on the bottom of shallow bays and sounds, where individual blades were washed to nearby shores by tidal action. Because the blades of the grasses were fragile, they broke off easily with the tides and were found deposited in huge windrows on the beaches. When high tides were accompanied by wind, the windrows were formed high above the normal tide lines. In the absence STATE jgSrBO** - 4 - of high or storm tides the grasses were deposited along the water's edge of the daily high tide. For long distances along certain shores of East Bay and Santa Rosa Sound, just above the normal watermarks of the daily high tides, and on both of the beaches systematic examinations revealed extensive infestations of dog fly larvae and pupae. Full-grown larvae and pupae were found in deposits of grass 1 to 3 weeks old and beginning to turn brown, the greater numbers of pupae being present at the higher elevations which were not being submerged by any of the tides. In the old or brown portions eggs and newly hatched larvae could not be found, but they were readily observed in large numbers in green grasses recently deposited by the tides. Thousands of the eggs contained in some samples of these green grasses were removed from the tidal deposits and brought to the laboratory. Most of them hatched during the night, and the young larvae demonstrated that they were capable of maintaining them- selves on small quantities of the grasses without the addition of any moisture. When representative lots of these young larvae were reared to maturity the adults proved to be dog flies. Following these experiences it was possible to select favorable breeding locations in different deposits of grasses and to find dog fly larvae and pupae with ease. From observations made on the beaches, and on caged lots of infested grasses collected from numerous locations along the beaches, it was evident that the fermenting deposits of marine grasses were extremely favorable for the development of dog flies. To prove this beyond any doubt, more than 45 lots of fresh, uninfested grass were collected from the bottom of shallow bays and exposed in nature. Each lot, consisting of about 2 bushels of grass, was placed near animals in a pasture, where it became infested. After exposure some lots were placed in individual cages, and any immature stages of insects contained in them were allowed to develop. These experi- ments demonstrated that the green bay grass readily supported and quickly developed a dense population of dog fly larvae. From the experiments with Th alassia testudinum there was an average emergence of 82.4 flies per cubic foot of grass, and from H alodul e wrightii an average of 38.7 flies per cubic foot. One 9-cubic-foot pile of turtle grass kept moist with fresh water was of unusual interest. It was rinsed slowly with 25 gallons of fresh water, after which, on September 6, it was exposed for deposition of eggs. Three days later the material was wet down with 5 gallons of fresh water from a sprinkler can. Four days after the second application a third appli- cation was made of 3 gallons of fresh water. The repeated additions of fresh water simulated the occurrence of rain, and it is significant that numbers of full-size larvae were present in this grass 6 days after eggs were laid on the material. Full-grown larvae were found 7 days after egg deposition, and some fully developed flies emerged 6 days later, giving a minimum period of 13 days for development from egg to adult. This is a - 5 - shorter period than that reported for development of dog flies in any other breeding medium and is identical with a minimum period obtained in turtle grass kept in the laboratory. The flies continued to emerge from this lot of grass until November 1, or for 38 days, and a total of 742 adults were produced. A limited amount of fly breeding was encountered among other kinds of fermenting vegetation on the beaches. Normal breeding was found in . Vallisneria american a, a broad-leaved grass growing in bays with openings into the Mississippi Sound, and 162 normal-sized flies were reared per cubic foot of this grass which had been exposed to dog flies in nature. From an alga, Enteromorpha pl umosa , which dried into thin sheets on a beach of Santa Rosa Sound, Fla., normal dog flies were reared. Since this alga ap- pears only in small quantities and is limited to certain of the beaches, it could not be an important producer of flies in this area. In 1936 Kings found that during an outbreak of dog flies an alga known as Saxgassum became infested on the gulf beaches. This species was not present in quantities in this area in 1940, and therefore it was of minor importance as a producer of flies that year. Undoubtedly other kinds of marine vegetation are capable of producing flies if washed ashore and allowed to ferment at the time adult flies are present to deposit eggs. Since enormous quantities of turtle grass and shoal grass were found infested with all stages of the dog fly on the bay and sound beaches during 1940, since no such extensive infestations were found in any other material on the coast, and since about 45 lots of the 2 kinds of grasses removed from the beaches developed large numbers of normal flies, some with a minimum period for development, it can be concluded that these 2 species of marine grasses represented the principal breeding media on the beaches of the north- west coast of Florida under conditions prevailing in 1940. How Natural Control Affects Breeding in Marine Grass Deposits By the first week of September grass deposits extending for many miles along Santa Rosa Sound and East Bay and on portions of other bays were infested with dog fly larvae and pupae. These immature stages could be found with ease, especially in locations where 'rains had fallen upon the grass soon after it was deposited on the shores. About the time dog flies began to emerge in noticeable numbers from these places the equinoctial tides became high enough to inundate most of the infested portions of the grass deposits. By the middle of September most of the infestations were eliminated through submergence by tides, leaving only a few at unusually 3 King, W. V., and Lenert, Louva G. Outbreaks of S tomo xys calcitran s L. (Dog Flies) along Florida's Northwest Coast. The Florida Entomologist 19: 33-39. 1936. high elevations on the beaches. The higher equinoctial tides which followed destroyed even these infestations. Tests made to determine how long last-stage larvae could withstand submergence showed that about 50 percent of the larvae failed to recover when inundated for 6 hours, and that none recovered from 15 hours of such exposure. Observations in the field demonstrated clearly that the equinoctial tides controlled the infestations in all grasses that were submerged. Also, they suggest that such a natural control may result from equinoctial tides each year if the windrows of grass are within reach of these tides. On the other hand, if the windrows are deposited by storm tides, normal equinoc- tial tides would not be expected to submerge all the deposits, and in con- sequence the production of flies in outbreak proportions would be expected. According to available reports a serious dog fly problem does not exist along the coasts of Mississippi and Alabama and about the littoral of the southern half of Florida. Examinations revealed that marine grasses grow in shallow water in the bays and sounds along these coasts, and that the broken blades of marine grass were prevented from washing ashore by the presence of bulkheads, or a sea wall near the water's edge, but more often by the presence of dense growths of tall reeds, Juncus roemerianu s and others In southern Florida root growth of the red mangrove trees extended into the water and served as a barrier against grass deposits on the beaches. Where the marine grasses were not washed ashore there was simply no oppor- tunity for the breeding of the flies In a few unprotected situations along the coasts of Alabama and Mississippi places were occasionally found which did not have such barriers along the beaches. In such places small quantities of marine grass occurred on the beaches and were heavily infested with dog fly larvae and pupae. How to Determine the Time and Place for Treatment Because submergence of windrows of infested grass by high tides destroys the immature stages of the dog fly, and because barriers of tall grass growing at the edge of the water prevent broken blades of marine grasses from washing ashore, thereby preventing dog fly infestations, it is desirable to take advantage of these factors of natural control and to treat only the breeding places that are not affected by them. In antici- pating the exact locations to be treated, the time treatment should be applied so as to prevent economically the development of larvae and pupae, and the height of equinoctial and other high tides which may afford natural control or indicate the need for immediate treatment, it is soon discovered that the most important factor is rare judgment. The man responsible for such work should be one who can evaluate natural-control factors and quickly determine the time, place, and quantities of materials needed to supplement natural control, and who can supervise the timely application of treatments for different places likely to produce flies on the extensive beaches. In such an undertaking the following factors tend to delimit the areas needing treatment and to add encouragement to the undertaking; (1) Control of breeding during July. August, and September would take care of dog fly annoyance during the most important season for beach activities. (2) The principal breeding of dog flies is confined to the shores of inner bays and sounds where two known species of marine grasses are deposited by tides. (3) Only portions of these shores receive grass deposits. These depend upon the extent of growth of grasses in nearby bays. (4) Of the shore-line deposits, only those that are not submerged by subsequent tides need be considered as potential breeders of flies. (5) If the first windrows of grasses are deposited at low eleva- tions, they serve as a barrier which tends to keep fresh deposits of green grasses at the water's edge, where the latter will be submerged by later tides, thus making artificial treatment unnecessary. (6) Each man in charge of control work in an area should (a) keep in mind the fact that dog flies can develop within 2 weeks from the time eggs are laid, (b) anticipate needs for treatment of the principal breeding places in advance of the season and have about 25 percent of the materials for treatment delivered to readily accessible places along the beaches, (c) be able to determine quickly the elevations of grass deposits from a small boat, (d) be familiar with variations in the height of high daily tides, (e) have tide elevations marked on visible boards on different beaches where breeding occurs, to guide him in determining the need for treatment, (f) be able quickly to inspect grass for infestations, and (g) set up. for heavily infested windrows of grass which will need treatment, suitable mark- ers which would readily be recognized by the treating crew, these to be removed by the crew as soon as treatments have been made, How to Treat Breeding Places On account of the short period of 14 days required for development from the egg to the adult dog fly, and the occurrence of widely scattered breeding spots on bay and sound beeches which need to be located and treated within this short period, it is advisable to employ a trfatment that will immediately destroy infestations in the grass and will control breeding of dog flies in the treated portions of grasses throughout the season. In tests of sprays made during 1940 the most promising materials were those UNIVERSITY OF FLORIDA Nil II III 8 - 3 1262 09230 3915 that killed by contact, did not dissolve in water, and did not evaporate quickly. In a limited number of tests with various materials creosote oil was used in 19 instances. It furnished the most promising results, which varied with the degree of penetration of the infested grasses rather than with the different dilutions made with fuel oil. By diluting creosote oil with 3 parts of fuel oil, good results were obtained and the costs were reduced. In order to obtain penetration of the infested piles of grass, speed in application, and economy of treatment, high-pressure sprayers are desirable. Because of the quantities of materials that may be needed for treating long windrows of infested grasses and the difficulty in handling such quantities from boats in shallow water, it is desirable to test on an extensive scale large power sprayers mounted on trucks and to use about one- half mile of hose. It is anticipated that it will not be possible to treat all the infestations with a truck sprayer, owing to the inaccessibility of many breeding places, and that it will be necessary to use a boat for treatments on island beaches and similar places. After attempts have been made to treat all infested grasses that are not controlled by tidal waters, it is further anticipated that some loca- tions will be found which will contain abnormally large numbers of pupae and that dog flies may emerge from them in a very short time. For such places it is believed that a smaller sprayer mounted on a boat could readily apply a surface application of fuel oil containing 10 percent of concentrated pyrethrum extract. Such treatments kill the newly emerged flies as they force themselves upward to the surface of the infested material.