Female Caught Research Articles

Potential bias in estimates of the size of maturity of crabs derived from trap samples

Abstract The size at the onset of sexual maturity (SOM) of female crustaceans is typically estimated using logistic regression analysis of the proportions of mature females in sequential size classes. The validity of this approach depends on the composition of the samples reflecting accurately that present in the environment. However, catches obtained by traps, a passive fishing method, typically contain disproportionately greater numbers of large crabs, whereas those obtained using active fishing methods, such as seine-netting and otter trawling, will presumably represent far better the size composition of the population. Moreover, we demonstrate that samples of female Portunus pelagicus caught by trapping were predominantly mature, whereas those collected by seining and trawling contained numerous immature as well as mature females. Therefore, the samples of females collected by trap are clearly biased towards mature crabs. Consequently, for any size class, it would be predicted that the proportion of mature females in trap catches will be overestimated, so shifting the logistic curve fitted to the proportions of mature crabs in each size class to the left, and yielding an underestimate of the SOM. This conclusion is substantiated by the fact that the carapace width of female Portunus pelagicus, at which 50% of individuals reach maturity (SOM50), was estimated to be markedly greater when using the proportion of mature females obtained by seine-netting and otter trawling, i.e. 101.1 mm, than by trapping, i.e. 86.1 mm. Data on the size and maturity status of the deep-sea crabs Hypothalassia acerba and Chaceon bicolor are available only from trap catches. From the above data for P. pelagicus, it is considered likely that, through a greater vulnerability of mature females of these species to capture by traps, the respective SOM50s derived for female H. acerba and C. bicolor from trap samples (carapace lengths of 69.7 and 90.5 mm) will be considerable underestimates of the true values.

Cutaneous leishmaniasis vector control perspectives using lambdacyhalothrinresidual house spraying in El Ingenio, Miranda State, Venezuela

An indoor vector control trial was carried out between December 1996 and February 1997, with the aimof reducing the population densities of Lutzomyia ovallesi, the proven vector of cutaneous leishmaniasis (CL) in El Ingenio, Miranda State, Venezuela. Houses were matched according to their structure (‘ bahareque’ [mud and straw], concrete, and wood) and randomly assigned to a control group ( n = 19) or a treated group ( n = 20) that was sprayed with 25 mg/m 2 of lambdacyhalothrin. This dose was selected on the basis of a laboratory susceptibility test of Lu. ovallesi to pyrethroids. Sandfly abundance was measured using CDC light traps 7–79 d post-intervention, by which time the sandfly population in control houses had declined to very low levels, at what was the end of the transmission season. Catches of total females, fed females, and males were significantly lower in sprayed than in control houses immediately after treatment. Catches of total females and males in sprayed houses recovered, reaching the level of control houses after 7 and 11 weeks, respectively. Bioassays following WHO protocols using contact cones on a sandfly laboratory colony showed a short residual effect of the insecticide explaining these results. In contrast, there was no tendency for catches of fed females in the sprayed houses to recover during the course of the trial. Previous studies of adult population dynamics showed a short season of high abundance. Combined with these results, we believe that 2 indoor sprayings with lambdacyhalothrin, the first at the beginning of November and the second at the beginning of January, would considerably reduce the Lu. ovallesi population, and control CL transmission in this and other foci where this sandfly is considered to be the main vector.

Emergence of Leuctra nigra (Plecoptera) from a southern English stream

The emergence of the stonefly Leuctra nigra into pyramidal traps was related to the supply of emergent substratum locally available in a small, stony stream. Hence, in a manipulative experiment, the mean catch of adults per trap increased with the total length of the interface between water and protruding substratum (i.e. the wetted perimeter) and 53 % of the variability in the catch per trap was accounted for by this factor. An overall female biased sex ratio was found in the catch. This apparent bias was due to intrusion into the traps of mature females (individuals with mature eggs). After excluding these individuals from the catch only a slight disparity was found in the sex ratio of newly emerged adults. Mature females accounted for 31% of the female catch and 19.6% of the total catch in the pyramidal traps. Hence, estimates of total emergence will be too high, and not just the sex ratio misrepresented, if these individuals are not excluded.

DUNGENESS CRAB, CANCER MAGISTER, DO NOT EXTRUDE EGGS ANNUALLY IN SOUTHEASTERN ALASKA: ANIN SITUSTUDY

The reproductive biology of female Dungeness crabs was studied with crab-pot and dive-transect sampling in five bays within or near Glacier Bay National Park and Preserve, southeastern Alaska, in April and September yearly from 1992 to 1998. A large percentage of nonovigerous, mature females was found in April, a time when females were expected to be brooding eggs that hatch in May and June. Our study examined differences between ovigerous and nonovigerous females collected in April and September samples to corroborate our previous laboratory study in which we found nonannual egg extrusion among Dungeness crabs. Seasonal differences in the catches of ovigerous and nonovigerous females, crab sizes, shell condition, and appendage injury were examined. Additionally, all crabs collected from two bays were tagged beginning in the fall of 1995; tagging was conducted twice annually. Our pot and dive data indicate that females, particularly larger ones, do not extrude eggs annually. Larger females have lower molting probabilities, which limits mating potential and increases reliance on stored sperm. The tagging study confirmed that at least some females do not extrude eggs in one year and then extrude eggs at a later time without molting, thus skipping at least one reproductive season. A reproductive cycle of Dungeness crabs in Alaska is introduced which includes earlier egg extrusion by larger females and nonannual egg extrusion.

Attractiveness of p-anisaldehyde to the varied carpet beetle, Anthrenus verbasci (L.)(Coleoptera: Dermestidae).

p-Anisaldehyde, a common flower volatile component, was found to be a potent attractant to adults of the varied carpet beetle, Anthrenus verbasci, irrespective of sex. The male catch with p-anisaldehyde (1 g) was almost the same as that with the female sex pheromone (1 mg), but the female catch was significantly larger. The catch with the combined attractant (p-anisaldehyde+the sex pheromone) was almost the same as the sum of the catches with each single attractant. Among 22 related compounds, two positional isomers and 20 functional-group-substituted compounds, p-anisaldehyde dimethylacetal, p-methoxyphenylacetone and p-ethoxybenzaldehyde were almost as attractive as p-anisaldehyde.

Results of a fishery-independent survey for pelagic sharks in the western North Atlantic, 1977–1994

Catch and effort data from a fishery-independent longline survey in the western North Atlantic were used to investigate the biology, distribution and abundance of pelagic sharks. Data were restricted to the period from 1977 to 1994, and for the months of June–August. Catches were dominated by the blue shark ( Prionace glauca), with smaller numbers of shortfin mako ( Isurus oxyrinchus), porbeagle ( Lamna nasus), thresher ( Alopias spp.) and silky ( Carcharhinus falciformis) sharks. Detailed analysis was restricted to P. glauca due to small sample sizes for the other species. Male blue sharks were caught more often than females. The catches included juveniles and adults of both sexes but very few adult females. Annual sex ratios varied, but normally favored males. Monthly sex ratios were even in June, but favored males in July and August. Due to differences in occurrence between males and females, distribution and abundance data were analyzed separately for each sex. Male catch per unit effort (CPUE) was highest between 15 and 20 °C, while female CPUE peaked at approximately 15 °C and decreased with increasing water temperatures. CPUE for males and females were standardized using a generalized linear model with years, months and day-night as factors. Males showed an approximately 80% decline between the mid-1980s and the early 1990s, while a significant change in female catch rates could not be demonstrated. The results of the analyses are discussed in relation to fishery-dependent studies in the western North Atlantic, along with previous work on the biology and distribution of pelagic sharks.

Electroantennogram and behavioural responses of the malaria vector Anopheles gambiae to human-specific sweat components.

Afrotropical malaria vectors of the Anopheles gambiae complex (Diptera: Culicidae), particularly An. gambiae sensu stricto, are attracted mainly to human hosts. A major source of human volatile emissions is sweat, from which key human-specific components are the carboxylic acids (E)- and (Z)-3-methyl-2-hexenoic acid and 7-octenoic acid. Electrophysiological studies on the antennae of An. gambiae s.s. showed selective sensitivity to these compounds, with a threshold at 10(-6) g comparable to that of known olfactory stimulants 1-octen-3-ol, p-cresol, isovaleric acid, and lower than threshold sensitivity to L-lactic acid and the synthetic mosquito repellent N,N-diethyltoluamide (DEET). A combination of the acids released at concentrations > 10(-5) g in wind tunnel bioassays significantly reduced the response to CO2, the major attractant released by human hosts, for strains of An. gambiae s.s. originating from East and West Africa. Field trials with odour-baited entry traps (OBETs) in Burkina Faso showed that 7-octenoic acid significantly increased (by 1.7-fold) the catch of females of An. gambiae sensu lato (comprising two sibling species: An. arabiensis Patton and An. gambiae s.s.) in OBETs baited with CO2, whereas combinations of the acids significantly reduced the catch in CO2-baited traps (by 2.1-fold) and in whole human odour-baited traps (by 1.5-fold). The pure (E) and (Z) geometric isomers of 3-methyl-2-hexenoic acid gave comparable results to the (EIZ) isomer mixture. These results provide the first experimental evidence that human-specific compounds affect the behaviour of highly anthropophilic An. gambiae s.l. mosquitoes. The compounds appear to inhibit the upwind flight' response to known long-range attractants, and may serve either to mask' the attractants present or, more probably, to 'arrest' upwind flight when mosquitoes arrive at a host under natural conditions. In the final approach to hosts, vectors are known to reduce their flight speed and increase their turning rate, to avoid overshooting the source. In our experimental apparatus, these changes in flight behaviour would reduce the number of mosquitoes entering the ports of the collection devices.

The biology of the ommastrephid squid, Todarodes sagittatus, in the north-east Atlantic

Todarodes sagittatus (N=1131) were opportunistically sampled from commercial and research trawling in Irish and Scottish waters between 1993 and 1998. The results suggest that the species is common in deep waters (>200 m) to the west of Ireland and Scotland, particularly in late summer and autumn. The size of squid caught was related to depth, with larger squid caught deeper, and is indicative of an ontogenetic, bathymetric migration. Females were more common (sex ratio 1·00:0·46), and attained a larger maximum size (520 mm mantle length (ML)) than males (426 mm ML). Mature females (360–520 mm ML) were caught in deep water (>500 m), between March and November, with a large catch of mature females taken off the west coast of Ireland in August 1996. Mature males (300–426 mm) were found from August to November. Potential fecundity was estimated to range from 205,000–523,500 eggs female−1. Putative daily increments in statoliths indicated a life cycle of slightly over a year, with rapid growth of approximately 1·8 mm d−1 during subadult and adult life. Fish were the most important prey of T. sagittatus and 17 fish prey taxa were identified, of which pelagic species were the most important.

Fecundity and Egg Size of Scyllarides Squammosus (Decapoda: Scyllaridae) at Maro Reef, Northwestern Hawaiian Islands

Abstract The body size-specific fecundity of Scyllarides squammosus is described based on the numbers of eggs carried externally on the pleopods of “berried” (ovigerous) females collected during June 1999, at Maro Reef, located mid-chain in the Northwestern Hawaiian Islands (NWHI). Fecundity was positively and nonlinearly related to “tail” (abdomen) width (TW); fecundity ranged from 53,807 to 227,489 eggs in 43 females spanning 52–77-mm TW. The smallest berried female encountered at Maro Reef in 1999 was larger than the smallest berried female observed during most previous years' (1986–97) lobster research cruises. Fecundity of the median-sized (60-mm TW) female caught at Maro Reef in 1999 by the NWHI commercial lobster trap fishery was an estimated 89,660 ± 3,980 (SE) eggs. Brooded eggs averaged 0.67 ± 0.006-mm diameter (range: 0.61–0.77 mm), equivalent to 0.17 and 0.13–0.24 mg, respectively. Egg size was unrelated to female body size. Fecundity and egg size estimates and their body size relationships ar...

Reproductive behavior and morphology of Paraleptophlebia spinosa (Ephemeroptera: Leptophlebiidae): implications of variation in copula duration

The reproductive behavior and genital morphology of a leptophlebiid mayfly Paraleptophlebia spinosa Ueno were studied in a population inhabiting a mountain stream in Kyoto, central Japan. Males formed swarms along gravel shores, caught females in the air, and mated on the ground. The distribution of the oviposition sites coincided with that of the swarming sites along the stream shore. The lobes of the male penis were characterized by a shallow pocket opening on the ventral side. The female vestibule formed a square-shaped chamber into which eggs were released from the oviducts. The number of eggs carried by females caught during copulation in the field showed a greater variation than the number of eggs of virgin females. Copula duration was strongly correlated with the number of eggs carried by the female. Significantly short copula duration with spent females (egg loading less than 20% of the estimated fecundity) may indicate that males could identify spent females. Copula duration with partially loaded females (egg loading between 20% and 80% of fecundity), however, was not shorter than that with fully loaded females. Male mating tactics with respect to copula duration and the possibility of kinematic sperm displacement are discussed with reference to the behavioral and morphological characteristics of the species.

Domesticity of Lutzomyia whitmani (Diptera: Psychodidae) populations: field experiments indicate behavioural differences

The sandfly Lutzomyia whitmani (Antunes & Coutinho) is an important vector for cutaneous leishmaniasis throughout much of Brazil, and has recently been shown to consist of more than one mitochondrial lineage. It has frequently been asserted that the degree of adaptation of L. whitmani to human environments varies across its range. As a standardized test of indoor feeding for three geographically distant populations of L. whitmani, catches inside experimental chicken sheds of varying degrees of wall closure (0%, 33%, 67% and 98%) were compared. Each increment in shed closure reduced catches of females (relative to the most open shed) by a similar degree for each population: geometric mean catches dropped by 11-40% with 33% closure, by 41-62% with 67% closure, and by 69-100% with 98% closure. Geometric mean catches of males from the two more northerly populations also decreased with increasing shed closure, by 18% and 22% for 33% closure, 58% and 69% for 67% closure, 91% and 93% for 98% closure. Males from the most southerly population showed significantly different behaviour, with 33% closure causing a 54% increase in geometric mean catch, 67% closure causing a 6% increase, and 98% closure causing a 32% reduction. For this southerly population, sex ratios became more male biased with increasing density in more closed sheds, suggesting aggregation driven by intra-specific communication. Lutzomyia intermedia (Lutz & Neiva) was relatively more likely than L. whitmani to approach baits in the three more closed sheds, rather than the most open shed, offering a behavioural explanation for observed differences in indoor biting rates between the species.

Notes on the Ecology of a Natural Glossina austeni (Diptera: Glossinidae) Population in the Jozani Forest, Unguja Island of Zanzibar

Studies were made of Glossina austeni captured with sticky panels during the dry and wet season in the northern and central part of the Jozani forest on the island of Unguja, Zanzibar. The density of the fly population was significantly higher in the northern part of the forest than in the central. Female and male daily catches remained stable in time in the northern part of the forest but the density of the male fly population declined significantly in the central part of the forest at the end of the dry season. The magnitude of the catch and the sex ratio of the samples was highly affected by the trap site in both locations. Only samples in the central part of the forest were biased towards one of the sexes depending on the seasons. The age composition of the male flies was independent of locality and season but the one of female flies was influenced by the seasons in the central part of the forest. The breeding and abortion rate of flies were similar in the two locations both during the wet and dry seasons. The stability of the fly population in terms of density and composition is probably a reflection of the optimal environmental conditions in this primary forest habitat.

Intercrops, Cicadulina spp., and maize streak virus disease

SUMMARYIntercrops of bean and finger millet were tested as a possible means of controlling maize streak virus disease (MSVD) in maize by disrupting the mating behaviour of the insect vectors of the maize streak virus, Cicadulina mbila and C. storeyi. A series of three trials were done. In the first, MSVD incidence 2 months after sowing was reduced to 14.9% and 17.4% in millet and bean intercrops compared to 29.5% in the pure maize stand. The number of male Cicadulina spp. caught on sticky pole traps was also significantly reduced relative to the control, but there was little effect on the catch of females. There was no significant yield penalty for the millet intercrop but maize yield was 49% lower in the bean intercrop treatment than in the pure stand. In the second trial, there were two millet and two bean intercrop treatments and a maize only control. Fewer male Cicadulina spp. were caught in the intercrop treatments relative to the control but MSVD incidence was reduced in one millet intercrop treatment only for which the associated maize yield penalty was 89%. In the final trial the bean intercrop was again tested but it had no effect on MSVD incidence. These experiments demonstrated that intercropping maize with bean or millet decreased vector activity and/or vector numbers. Vector catches were predominantly male, and catches of males but not females were reduced in the intercrop treatments compared with pure stands. However the lower vector catch was not consistently associated with a significant reduction in MSVD incidence, and when it was there was often an associated yield penalty in the maize due to the intercrop.

Nutritional levels of female tsetse Glossina pallidipes from artificial refuges.

Female tsetse flies, Glossina pallidipes Austen, caught in artificial refuges were subjected to ovarian dissection and analysed for levels of fat, residual dry weight (RDW) and haematin. There were rather small proportions of flies in ovarian categories 0 and 1, in part due to large losses in the immature and teneral stages at the hottest time of year. The distribution of the female catch among pregnancy days was close to uniform. The wet and dry weights (WW and DW) and RDW of eggs, larvae and pupae increased by 0.821, 0.303 and 0.204 mg respectively, with each mm3 increase in volume. Water accounted for 71.7% of the fat-free WW, and fat for 32.7% of the DW. Between birth and ovulation, fat increased from 2 to 4 mg and RDW from 7 to 11 mg; thoracic RDW increased by 2.5mg and changed little thereafter. Fat levels increased 3.5mg by day 6 of pregnancy, but only 0.5 mg thereafter. Over the same periods RDW corrected to zero haematin (CRDW) increased by 1 and 8 mg respectively. Full-term fat and CRDW levels were 8.2 and 19.4 mg respectively. Cumulative haematin frequencies formed a smooth curve with a slope that increased continuously. The raw data were well fitted by a model where feeding rates increased exponentially and capture probability was independent of haematin content. The mean feeding interval was 60 h; feeding probabilities of >0.9/day were only found in flies that had failed to feed for>72 h. In early pregnancy, fat levels declined with haematin for flies that had fed>36 h previously; by days 5-7 fat levels were maintained at a constant high level for 60 h post-feeding. Fat-haematin graphs for female tsetse cannot be used to estimate rates of fat utilization. Traps sample tsetse with below-average fat and RDW in early and late pregnancy respectively. Refuge samples are less biased than those from traps; they give a better picture of the dynamics of pregnancy in normal flies and facilitate the explanation of existing anomalies.

An evaluation of insecticide-impregnated screens against Glossina austeni (Diptera: Glossinidae) on Unguja Island of Zanzibar

A tsetse-control trial was carried out in a primary forest habitat on Unguja Island of Zanzibar to assess the effect of insecticide-impregnated blue cotton screens on a population of Glossinn austeni Newstead. In November 1991, screens were deployed at densities of 45–70 per km2 and the fly population was monitored monthly using sticky panels. Both female and male daily catches were not affected during the initial five months after screen deployment, but the physiological age distribution of female flies shifted significantly towards younger groups. Females were more influenced by the screens than males. The fly population was very low after the long rainy season, but increased to pre-control levels three months after the flooding of the forest floor. Thereafter, male and female apparent densities decreased, and 78–91% and 98–99% suppression was obtained for males and females respectively at the end of the 18-month intervention period. While effective control of G. austeni using blue cotton screens was shown to be feasible, it required a high deployment density and a long intervention period.

Activity patterns in Glossina longipennis: a field study using different sampling methods.

Studies n the daily activity of Glossina longipennis at Galana Ranch using a black odour-baited electrocuting target confirmed its crepuscular activity profile. Activity started at 05.00-05.30 hours and peaked at 06.00-06.30 hours, stopped by 09.00 hours, then started again at 17.00-17.30 hours with a peak at 18.30-19.00 hours, ceasing by 19.30 hours. Females made up 60% of the overall catch, and tended to arrive later than males. Other stationary sampling methods (trap, stationary ox) gave similar results. With the stationary methods, very few flies were caught outside the periods of peak activity (only 1.5% of the total between 09.00 and 17.00 hours); the ox was the only stationary bait to catch any flies between 10.00 and 16.00 hours. More flies were caught throughout the day at mobile baits (8.3% of the male and 2.3% of the female catch was taken between 09.00 and 17.00 hours). Mobile baits caught considerably more males than females (females were 17% of the catch). These males had on average higher fat and haematin reserves. Similar nutritional differences were not observed for females. There were fewer older females (ovarian category 3 or more) in mobile compared to stationary baits, and a lower proportion of the youngest males (wing fray category 1) at natural compared with artificial baits.

Evaluation of three (Z)-9-tricosene formulations for control of Musca domestica (Diptera: Muscidae) in caged-layer poultry units.

A field trial comparing the effectiveness of toxic targets impregnated with different formulations of the Musca domestica L. female sex pheromone (Z)-9-tricosene was conducted in a caged-layer, deep-pit poultry unit in southern England. Targets baited with 5 g of technical grade (Z)-9-tricosene, or 5 g of a 40% polymer bead formulation, caught significantly greater numbers of M. domestica than control targets. This increase in attractiveness of the pheromone-impregnated targets persisted for at least 24 wk. However, mean daily catch rates of M. domestica at targets baited with 5 g of a 2% wettable powder formulation did not significantly differ from control levels. Technical grade and bead formulations of the pheromone attracted significantly more males than females. However, the catches of female M. domestica at these pheromone-impregnated targets were significantly greater than female catches at control targets. Monitoring with sticky cards indicated that the introduction of toxic targets successfully suppressed adult M. domestica population density for up to 13 wk. Possible hypotheses explaining the effect of (Z)-9-tricosene on female attraction are discussed.

Diurnal Flight Activity of Diadegma insulare (Hymenoptera: Ichneumonidae), a Parasitoid of the Diamondback Moth (Lepidoptera: Plutellidae), in the Field

We studied the diurnal flight activity of Diadeg>ma insulare (Cresson) at the Collins Road Entomology Research Farm Michigan State University during the summers of 1992 and 1993. Foraging activity was measured using sticky traps placed within the broccoli canopy and by direct visual observation. Flight activity of D. insulare males was positively correlated with light intensity, whereas activity of females was positively correlated with light intensity, temperature, and wind speed. Relative humidity, percentage cloud cover, and time of day did not influence D. insulare catch. There was no significant difference between male and female catch. The patterns of males' and females' flight activity throughout the day were significantly different from a uniform distribution except on 14 and 22 August 1993 for males and 14 August for females. Activity generally began between 0800 and 1000 hours, peaked between 1100 to 1300 hours, and stopped by 2100 hours. There was no significant correlation between the numbers of males and females caught on the same trap, suggesting that an increase in numbers of females does not attract more males. Males were caught more frequently than females in September of both years, suggesting that males were more abundant or active at the end of the season. The total numbers caught (both sexes) at different times of the day in sticky traps were generally proportional to the numbers of parasitoid visually observed. The numbers of D. insulare caught were positively correlated with the numbers of diamondback moth larvae per plant. This information could be useful for developing a model that can predict the peak diurnal activity of D. insulare in the field which would help with decisions on whether pesticides should be sprayed.

Field Bioassays of Synthetic Pheromones and Host Monoterpenes for Conophthorus coniperda (Coleoptera: Scolytidae)

Four major monoterpenes, (±)- α -pinene, l( S )-(–)- β -pinene, ( R )-(+)-limonene, and myrcene are found in the cones of eastern white pines, Pinus strobus L. Mixtures of these, as well as, α -pinene or β -pinene alone, increased catches of male white pine cone beetles, Conophthorus coniperda (Schwartz), in traps baited with the female sex pheromone, (±)- trans -pityol. The monoterpenes by themselves as mixtures or individually ( α -pinene, β -pinene) were not attractants for males or females. Traps baited with (±)- trans -pityol and α -pinene caught as many, or significantly more beetles than those baited with pityol and a four monoterpene mixture (1:1:1:1) used in seed orchards in North Carolina, Ohio, and Virginia. Three beetle-produced compounds, conophthorin, trans-pinocarveol, and myrtenol did not enhance catches of males or females in (±)- trans -pityol-baited traps. Racemic E -(±)- conophthorin, E -(−)- conophthorin, and E -(+)- conophthorin significantly reduced catches of males in traps baited with (±)- trans -pityol alone. Female C. coniperda were not attracted to any of the host- or beetle-produced compounds tested. The study demonstrated that traps with baits releasing (±)- trans -pityol at about 1mg/wk with (±)- α -pinene (98% pure) are potentially valuable tools for C. coniperda pest management. Baited traps can be used to monitor C. coniperda populations or possibly to reduce seed losses in a beetle trap-out control strategy.

Effects of fly abundance on catch index of traps for Glossina fuscipes fuscipes (Diptera: Glossinidae).

The effect of fly abundance on the catch index of traps and that of rain as a source of variation in fly abundance were investigated for Glossina fuscipes fuscipes Newstead around Lake Victoria, western Kenya, using odor-baited and color-improved traps. There was a significant inverse relationship between the catch index of experimental traps and abundance of flies; the catch index being the ratio of catch in the experimental trap per catch in a reference trap. At low tsetse abundance (< 10 flies per trap per day) there was a 3-fold increase of the catch of females in the experimental trap compared with the control. Rainfall alone explained 22-87% of the total variation of fly abundance. It is suggested that fly abundance should be considered in evaluating baits for G. f. fuscipes or when using traps for monitoring. The relative depression of the catch index at high abundance may be related to avoidance of conspecifics. Flies entered standard traps in an inverse proportion to the number observed at the trap. Females approached traps in greater numbers when fewer decoys (dead flies) were placed on traps.