Abstract
An investigation into the bionomics, morphology, distribution and control of the Still Bay midge (Leptoconops demeilloni [Diptera: Ceratopogonidae])
Leptoconops demeilloni (Clastrier and Nevill 1984), also known as the Still Bay midge, is a notorious bloodsucking pest in the Fynbos biome (Western Cape Province of South Africa, 34.389423 °O 21.417447 °S). The pest is considered to be of economic importance especially to the tourism industry in the coastal town of Still Bay.
The phenology of the midge was studied by defining a midge activity index between 0 and 5 points. Peak activity occurred on 26 September and 11 October 2019, 6 October 2020, and 9 October 2021. Midges appeared in the first week of September and the last midges were recorded in the last week of November during the three-year study. Adult midge activity showed a positive correlation with temperature (R = 0,05, p = 0,003) and a negative correlation with wind speed (R = -0,45, p = 0,008). Sunshine and the time of day were not significantly correlated with activity. Activity was recorded between the temperatures 13,5 °C and 30,0 °C. Activity was recorded between the wind speeds of 4,0 km/h and 24,8 km/h.
Male and female midges fly only during the daytime and do not enter shelters such as houses, sheds and the shade of trees. Male swarms are formed between one and five metres above ground level and can be between one and four metres in diameter. Copulation takes place in these swarms. Midges overnight underneath loose detritus on the soil surface. Midges are not attracted to UV (ultraviolet) or white LED (light emitting diode) lights and live for a maximum time of 10 days when kept in captivity.
Both male and female midges were recorded to feed on the nectar of Cotula sororia (Asteraceae) during the study.
The larvae and pupae of L. demeilloni were recovered from dune sand after the detection of the breeding sites using eclosion funnel traps and an extended soil moisture meter. The method of larvae and pupae extraction by means of desiccation or water flooding of dune sand in containers is described. An average of 0,49 larvae per litre of sand were extracted (n = 169). An average of 0,48 pupae per litre of sand were extracted (n = 131). The habitat of the Still Bay midge can be defined as well-drained, deep red-grey sand derived from eroded aeolianite hills and the sand has a low clay, salt, and organic content with few calciferous stones. The dune sand has a low pedologic development. Sand grains have an average diameter of 0,33 mm (SD = 0,10 mm), the pH+ = 7,3, salt content 67 ppm., organic content 1,06% and soil moisture between 1,3 and 21,4%.
The meroistic ovaries of female midges contained between 57 and 148 follicles (avg. 113 SD = 23,98). An investigation into the ovarial development of free flying midges (n = 243) revealed that 6,2% contained blood, 68,7% had semi-developed eggs and 0,4% contained fully developed eggs. Eggs developed harmoniously and anautogenically. The dark-grey banana-shaped eggs are on average 0,42 mm in length (n = 10, SD = 0,01 mm) and 0,08 mm in diameter (SD = 0,01 mm). Eggs took on average 23,3 days to develop and are sensitive to desiccation.
The midge is univoltine. Larvae developed over a period of 10 months through four larval instars. Instar 4 male larvae measured an average of 6,65 mm in length (n = 8, SD = 0,42 mm). The ventrolateral rod plus the front part of the mandible were 0,34 mm in length (SD = 0,03 mm). The body diameter was on average 0,41 mm (SD = 0,12 mm) and the body in a coiled position 1,17 mm in diameter (SD = 0,12 mm). Instar 4 female larvae measured an average of 6,02 mm in length (n = 6, SD = 0,82 mm). The ventrolateral rod plus the front part of the mandible were 0,34 mm in length (SD = 0,01 mm). The body diameter was on average 0,35 mm (SD = 0,07 mm) and the body in a coiled position 1,15 mm in diameter (SD = 0,23 mm). A typical instar 1 larva measured 0,09 mm in length, instar 2 measured 2,7 mm, instar 3 measured 3,6 mm and instar 4 measured 6,3 mm.
Pupae appeared between the first week of September and the third week of October with a peak between weeks two and four in September. Male pupae were found to be an average of 3,27 mm in length (n = 10, SD = 0,13 mm) and 1,03 mm in diameter (SD = 0,06 mm). Males took an average of 13,53 days to develop (n = 17, SD = 4,48), with a maximum of 21 days. Female pupae were found to be an average of 3,01 mm in length (n = 10, SD = 0,11 mm) and 0,89 mm in diameter (SD = 0,04 mm). Females took an average of 15,42 days to develop (n = 12, SD = 4,18) with a maximum of 23 days.
The taxonomic characteristics of L. demeilloni larvae are described. The dorsal medial rod, ventrolateral rod, frontal pillars, mouth parts, pharyngeal skeleton, genal arch, sensillae on the head and anal segment, and pharate pupa are described.
Larvae feed indiscriminately on detritus. The rectal papillae are utilised to push the faeces out of the rectum.
The cephalo thorax and the abdomen of the male and female L. demeilloni pupae are described.
The distribution of L. demeilloni within the municipal area of Hessequa in the southern part of the Western Cape is defined. Factors that promote or contain the distribution of L. demeilloni within the Fynbos biome are discussed. Sparsely grown sand fields and dunes with reed bushes (Restionaceae) and scattered mole hills (made by Bathyergidae) characterise the habitat of the midge.
The life cycle of L. demeilloni is described. The eggs are laid over a period of 10 weeks from the middle of September to the first week of December. From the first week in October there are larvae in the soil that develop through four instars to pupae. The larvae move deeper underground if the soil dries out. Some develop faster than others until they reach the fourth instar. All larvae reach maturity in September and move closer to the soil surface. Pupae and adults were spotted at the same time in the first week of September. Control against adult flying midges is therefore only important for two and a half months of the year. The rest of the year the midges are underground at various depths. The control of the midge is discussed and a systematic priority approach to the problem is suggested.
The Leptoconops collection of the author is to be transferred to ARC Onderstepoort Veterinary Research, Private Bag X5, Onderstepoort, 0110, South Africa, after publication.
Keywords: Ceratopogonidae; demeilloni; Diptera; Leptoconops; Still Bay Midge
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