The population dynamics of a recently introduced crayfish,Cherax quadricarinatus (von Martens, 1868), in the Sanyati Basin of Lake Kariba,Zimbabwe

The redclaw crayfish Cherax quadricarinatus is a recent introduction into Lake Kariba, Zimbabwe where it is rapidly spreading. There are, however, no studies that have investigated the population dynamics and possible ecological impacts of the crayfish in the lake. This study investigated aspects of the population biology of C. quadricarinatus in the lake. These included sex distribution, population growth and mortality parameters, probability of capture, recruitment and length at age of ge at maturity. Crayfish were captured monthly between January and December 2013 from 13 sites using opera house bait traps. Aspects of the species population biology were estimated using a fish stock assessment tool, FISAT II. A total of 3 205 crayfish were captured, of which 44% were male, 49% female and 7% intersex individuals. The asymptotic length (L_∞ = 112.88 mm), curvature parameter (k = 0.72), longevity (t_max = 4.17 years), growth performance index (? = 3.96), total mortality (Z = 2.06 y^?1), fishing mortality (F = 1.07 y^?1), natural mortality (M = 0.99 y^?1), rate of exploitation (E = 0.52) and length-at-first capture (L_c = 44.48 mm) were estimated. Fecundity averaged 503 ± 229 eggs female?1 and increased with size (weight and carapace length). Gravid females were found throughout the year but the highest proportion of females with eggs (4–10%) were found in the first quarter, whereas recruitment peaked in May and June. The smallest crayfish with eggs (48.75 mm CL) was estimated to be 1.02 years and the largest (94.82 mm CL) 2.85 years. Cherax quadricarinatus exhibited an r strategy life-history pattern due to continuous reproduction and high fecundity. In Lake Kariba, C. quadricarinatus is now well into the establishment stage of the Introduction–naturalisation–invasion continuum and spreading in the lake. Management options should thus aim at preventing further spread and reducing the population size and the potential negative impacts of this species.     

Laboratory assessment of different methods of applying a commercial granular bait formulation of methomyl to control adult houseflies (Musca domestica l.) in intensive animal units

Female houseflies (Musca dornestica L.) from a susceptible and a multiinsecticide-resistant strain were used to evaluate the toxicity of a granular insecticide bait formulation of the carbamate insecticide methomyl. The bait was either applied to a polystyrene bait target, or scattered over the floor of the fly treatment cages. Targets were treated with methomyl granules dissolved in water with and without apaste carrier or the granules were applied directly to the target using an adhesive. Scattered methomyl granules proved the most effective of the various treatments assessed. Based on the KT₉₀ values, the most effective suspended bait treatments against susceptible or resistant flies were water as a carrier, and egg or ‘Lap’ wallpaper paste as adhesives; the least effective were ‘Polycell’ regular wallpaper paste as an adhesive and carboxymethylcellulose as a carrier. A proportion of flies from both the susceptible and the resistant strain recovered from knockdown; most of these lost their ability to fly (‘hoppers’) and subsequently survived the treatments. When bait targets treated with methomyl granules were fixed at right angles to the wall/floor junction of the treatment cages most of the recovered flies fed again on the bait and received a lethal dose. The mortality of flies exposed to suspended bait targets was influenced by the level of light. For both strains of fly, mortality decreased as the light intensity was lowered; this effect was more pronounced with the resistant strain. The implication of these results for the application of methomyl bait is discussed.     

The occurrence of Contracaecum sp. larvae (Nematoda: Anisakidae) in the catfish Clarias gariepinus (Burchell) from Lake Chivero, Zimbabwe

Clarias gariepinus were collected from Lake Chivero, Zimbabwe, and examined for nematode parasites from November 2000 to May 2002. Of the 202 specimens collected, 42.6% were infected with third-stage larvae of Contracaecum sp. in the body cavity. The intensity of the infection was 1-7 worms per fish (mean intensity = 2.2). Seasonal variation in the prevalence of the parasite was not obvious and there was no significant difference in the prevalence of infection between males and females (chi 2 = 2.228; P > 0.05). No significant relationship between host size and prevalence was established. There was also no significant relationship between intensity and the body condition factor (r = 0.11; P > 0.05). The low parasite prevalence may have been caused by the disruption of the infection cycle since piscivorous birds, which are the final hosts of the parasite, do not feed on C. garieplnus in Lake Chivero.     

On cestode and digenean parasites of Clarias gariepinus (Burchell, 1822) from the Rietvlei Dam, South Africa

Sharptooth catfish, Clarias gariepinus, from the Rietvlei Dam near Pretoria, South Africa were examined for internal platyhelminth parasites. Two adult cestodes, Polyonchobothrium clarias (stomach) (prevalence 71%, mean intensity = 5, n = 7) and Proteocephalus glanduliger (anterior intestine) (prevalence 14%, mean intensity = 2, n = 7), were found in the gut while metacercariae of one larval digenean, Ornithodiplostomum sp. (prevalence 14%, mean intensity = 140, n = 7), were found encysted in the muscles. The morphology of these species, based on light and scanning electron microscopy as well as histological analysis, and how they differ from previously described specimens, are discussed. Ornithodiplostomum is a new record in southern Africa. Infection levels of the host fish were mild compared to records from previous surveys.     

Nematode parasites of Clarias gariepinus (Burchell, 1822) from the Rietvlei Dam, South Africa

Catfish, Clarias gariepinus, from the Rietvlei Dam near Pretoria, South Africa were examined for nematode parasites. Two species, Procamallanus laeviconchus in the stomach and Contracaecum spp. larvae in the abdominal cavity, were found. The morphology of these species, based on light and scanning electron microscopy, and how they compare with previously described specimens are discussed. Infection rates were mild compared to previous surveys although Contracaecum spp. had a high prevalence of 86%.     

The effects of temperature and humidity on the toxicity of three organophosphorus insecticides to adult Oryzaephilus surinamensis (L.)

The toxicities, to a laboratory susceptible strain and to a resistant strain of Oryzaephilus surinamensis (L.), of water-dispersible powder formulations of pirimiphos-methyl, fenitrothion or chlorpyrifos-methyl under constant conditions of 25°C and 70% r. h. were compared to the toxicities when the insects were exposed to a diurnal cycle of 12.5–20–12.5°C and 70–50–70% r. h. to simulate grain store conditions in the UK during spring and autumn. All the insecticides were more effective at 25°C and 70% r. h. The LD₅₀ values for the susceptible strain were low, being 4.4 and 1.4 mg m^?2 at 12.5-20°C and 25°C, respectively, for chlorpyrifos-methyl, 18.3 and 4.1 mg m^?2, respectively, for pirimiphos-methyl, and 4.0 and < 1.O mg m^?2, respectively, for fenitrothion. The LD₅₀ values obtained from the two sets of environmental conditions for a resistant strain (484) differed by factors of 1.8 for chlorpyrifos-methyl, 4.8 for pirimiphos-methyl, and 7.3 for fenitrothion. Toxicity studies were also made with chlorpyrifos-methyl under various constant conditions of temperature and humidity from 5–30°C (5°C intervals) and 30, 50, 70 and 90% r. h., and also at O°C and 60% r. h. Chlorpyrifos-methyl was very effective and there was little or no cross resistance to chlorpyrifos-methyl in the resistant strain. From 15 to 30°C, mortality was high, and differences in mortality at the LD₅₀ level for the various humidities were slight, but there was a decrease in mortality with decreasing humidity at any one temperature, in particular, at 5°C, 50 and 70% r. h., and 10°C and 50% r. h. Chlorpyrifosmethyl was more toxic to both strains at the highest humidity (90%) throughout the whole temperature range. The LD₅₀ values for each strain decreased at each temperature as the water vapour concentration was increased. At O°C and 60% r. h., all the insects from both strains died but the cause of death was not clear.     

First record of Contracaecum spp. (Nematoda: Anisakidae) in fish-eating birds from Zimbabwe

Endoparasites of fish-eating birds, Phalacrocorax africanus, P. carbo, Anhinga melanogaster and Ardea cinerea collected from Lake Chivero near Harare, Zimbabwe, were investigated. Adult Contracaecum spp. were found in the gastrointestinal tract (prevalence 100 % in P. africanus, P. carbo and A. melanogaster; 25 % in A. cinerea). Parasite intensity was 11-24 (mean 19) in P. africanus, 4-10 (mean 7) in P. carbo, 4-56 (mean 30) in A. melanogaster and 2 (mean 0.5) in A. cinerea. The cormorants fed mainly on cichlid fishes and carp; the darters and the grey herons on cichlids. All these fishes are intermediate hosts of Contracaecum spp. Scanning electron microscopy revealed that Contracaecum rudolphii infected both cormorant species and darters; C. carlislei infected only the cormorants while C. tricuspis and C. microcephalum infected only the darters. Parasites from the grey heron were not identified to species because they were still developing larvae. These parasites are recorded in Zimbabwe for the first time.