Factors affecting density‐independent survival of an island population of tsetse flies in Zimbabwe

Abstract

Analysis is presented of the factors affecting survival probability in populations of tsetse flies Glossina morsitans morsitans Westwood and G. pallidipes Austen (Diptera: Glossinidae) on Antelope Island, Lake Kariba, Zimbabwe. For mature male and female adult G. m. morsitans mean temperature (T bar ) accounted for 70 and 50%, respectively, of the variance in mark-recapture estimates of survival when the flies were not subjected to trapping. Saturation deficit (SD) only accounted for 36 and 33%, respectively. Maximum temperature (T max ) and SD accounted for 36-42% of the variance in male and female G. pallidipes. For the corresponding Moran curve estimates of the survival over all developmental stages, SD lagged by three weeks accounted for 61 and 41% of the variance for male and female G. m. morsitans, respectively, and 64 and 56% for G. pallidipes. The corresponding figures for plots against T max were 44, 23, 23, and 21%, respectively. The same patterns were seen in the whole data set once allowance was made for the effect of trapping on survival and for an effect of season, correlated with an index of photosynthetic activity. For male G. m. morsitans there was a significant effect of saturation deficit, but not temperature, on immature survival. Decreased adult survival at high temperatures results from the need to feed more frequently and hence to take more risks per unit time. High saturation deficits result directly in reduced emergence of healthy flies from pupae.

Responses of Anopheles gambiae, Anopheles stephensi, Aedes aegypti, and Culex pipiens mosquitoes (Diptera: Culicidae) to cool and humid refugium conditions

“…Partially dehydrated nymphs of the European tick, Ixodes ricinus, will walk along a humidity gradient towards the wet end (Crooks and Randolph 2006). Furthermore, temperature and saturation deficit are both critical in determining the survival of tsetse flies, as has been reported for Glossina morsitans morsitans and Glossina pallidipes, where these environmental factors have been shown to differentially affect flies of various ages (Hargrove 2001). Moreover, according to the latter author, the effect of an increase in these two microclimatic parameters is accentuated in tsetse with low fat levels.…”

A dynamic species distribution model of Glossina subgenus Morsitans: The identification of tsetse reservoirs and refugia

“…More recent attempts to map tsetse distributions have implemented spatial modeling techniques, which have succeeded in producing maps with both higher spatial resolutions than the fly belt maps, and estimates on the probability of tsetse presence (e.g., Rogers and Williams 1994, Gilbert et al 2001, Wint 2001. Although an improvement over the fly belt maps, the modeled predictions thus far have provided little information about intra-and inter-annual fluctuations in tsetse distributions, despite numerous studies that discuss such phenomenon (e.g., Austen and Hegh 1922, Nash 1933, Bursell 1956, Brightwell et al 1992, Hargrove 2001, Odulaja et al 2001, Bett et al 2008.…”

“…With regards to climate, tsetse populations are generally found in regions with mean annual temperatures between 19-308C (Pollock 1982b). As temperatures increase, the rates at which tsetse consume fat and water also increase, requiring the fly to either seek out a host on which to feed or risk dying of starvation or desiccation (Leak 1999, Hargrove 2001. The probability of survival drops to 50% when tsetse are exposed to temperatures greater than ;368C for three hours (Terblanche et al 2008), and temperatures greater than 408C are considered lethal (Knight 1971, Torr andHargrove 1999).…”

“…A significant negative correlation has been reported between fly populations and saturation deficits (Nash 1933, Rogers 1979, Hargrove 2001. However, the question of whether or not tsetse in dry conditions die from starvation or desiccation has been debated for some time (see Nash 1937, Buxton 1955, Bursell 1961, 1963, Rogers 1979, 1990, Hargrove 1980, Rogers and Randolph 1986, 1991.…”

“…Fly belts, although useful in delineating where tsetse were historically found, do not account for seasonal variation in tsetse distributions. All other tsetse models predict tsetse distributions at a particular moment in time despite numerous studies that discuss the seasonal expansion and contraction of tsetse populations (e.g., Austen and Hegh 1922, Nash 1933, Bursell 1956, Brightwell et al 1992, Hargrove 2001, Odulaja et al 2001, Bett et al 2008. Since tsetse distributions fluctuate over space and time, a dynamic realized niche/species distribution model that predicts when and where fly reservoirs occur will be useful to control operations for many reasons including: 1) reductions in cost by focusing on smaller areas identified as reservoirs and refugia; 2) improved ability of control efforts to target flies; and 3) operating in small reservoirs/refugia minimizes the environmental impacts associated with control campaigns.…”