Enhancement of Aedes albopictus collections by ovitrap and sticky adult trap
© Velo et al. 2016
Received: 18 March 2016
Accepted: 7 April 2016
Published: 21 April 2016
In the last decades, Aedes albopictus has become an increasing public health threat in tropical as well as in more recently invaded temperate areas due to its capacity to transmit several human arboviruses, among which Dengue, Chikungunya and Zika. Enhancing the efficiency of currently used collection approaches, such as ovitraps and sticky traps, is desirable for optimal monitoring of the species abundance, for assessment of the risk of arbovirus transmission and for the optimisation of control activities.
Two sets of 4 × 4 Latin-square experiments were carried out in Tirana (Albania) to test whether modifications in ovitrap shape and size and in oviposition substrate would increase collections of Ae. albopictus eggs and whether hay-infusion would increase adult catches by sticky trap. Generalized Linear Mixed Models with negative binomial error distribution were carried out to analyse the data. Cylindrical ovitraps lined with germination paper yielded significantly higher egg catches than those exploiting either the (commonly used) wooden paddles or floating polystyrene blocks as oviposition substrates. No difference was observed between cylindrical and conical shaped ovitraps. Ovitraps and sticky traps baited with hay infusion yielded significantly higher egg and adult catches than un-baited ones. A significant relationship between ovitrap and sticky trap catches was observed both in the absence and in the presence of attractants, with ovitrap catches increasing more than sticky trap catches at increasing adult female densities.
This study provides grounds for optimisation of ovitraps and sticky traps as monitoring tools for Ae. albopictus by (i) supporting use of germination paper as most appropriate oviposition substrate; (ii) suggesting the possible use of stackable conical ovitraps for large scale monitoring; (iii) confirming the use of hay-infusion to increase egg catches in ovitraps, and showing that hay-infusion also significant increases adult catches by sticky traps.
Aedes albopictus is the mosquito species which has been capable of the widest geographical expansion thus becoming a public health threat in tropical as well as in more recently invaded temperate areas due to its capacity to transmit several human arboviruses, among which Dengue, Chikungunya and Zika [1, 2].
Surveillance and monitoring of the species is thus an instrumental activity to be carried out to prevent infestation of new areas, to assess the risk of arbovirus transmission and to optimize control activities. Ovitrap is the simplest and most widely used monitoring device for Ae. albopictus, as well as of other container-breeding species such as Aedes aegypti, the major vector of yellow fever, dengue and Zika . Ovitrap is a small black plastic vessel mimicking the preferred breeding site for the species, i.e., tree-holes, rock-holes and other small natural containers in its original habitat in south-east Asia, and small man-made containers in more recently colonized urban environments . The vessel is partly filled with water and either contains a wood or masonite rough paddle standing in the water or has the internal walls lined with seed germination paper for females to lay eggs. Mosquito presence and abundance is indirectly estimated by counting eggs laid on the paddle or on the germination paper. Baiting ovitrap with hay-infusion has been shown to increase Ae. albopictus egg catches [5–11]. A more direct approach to monitor the same fraction of Ae. albopictus population monitored by ovitrap (i.e. oviposting females) is represented by sticky traps, which basically are ovitraps whose internal walls, or some internal additional structures, are lined with adhesive films to which the mosquitoes approaching the traps remained stuck [12, 13].
The aims of this study were to test under field conditions whether (i) some small modifications in shape and size and oviposition substrate could increase Ae. albopictus ovitrap-catches; (ii) hay-infusion could increase sticky trap catches, as already shown for ovitraps; and (iii) ovitrap and sticky traps baited with hay-infusion maintain the correlation shown to occur in the absence of the infusion .
Ovitraps (Ov) and Sticky traps (ST ) were used to collect Ae. albopictus eggs and adults, respectively. Two shapes of black plastic ovitrap were tested: Ov-A, a cylindrical vessel, 9 cm high, 11 cm in diameter with an overflow hole at 7 cm from the base, and Ov-B, a truncated cone (12 cm high; 6 cm lower diameter; 8 cm upper diameter; overflow hole at 9 cm from the base). Three types of oviposition substrates were provided to Ov-A: heavy-weight seed germination paper lining the internal walls (Ov-A1); a floating block of white polystyrene (5 × 5 × 2.5 cm; Ov-A2), and a wooden paddle (12.5 × 2.5 cm) with one rough side (Ov-A3 and Ov-B).
Oviposition rates in Ov-A1, Ov-A2, Ov-A3 and Ov-B were compared in 4 × 4 Latin-square experiments carried for 20 weeks from June to October 2011 in nine suburban sites (located at > 500 m from each other) within a 2.25 ha area in Tirana in Albania, the first country in a Europe to be infested . In each experimental site, the four ovitrap types were located in shaded sites at the corners of a 50 m-square area and rotated clockwise on a weekly basis, so that each trap was in the same position every four weeks. Egg counting was carried out under a stereomicroscope in the lab.
The collection capacity of Ov-A1 and ST either baited or not with hay-infusion (60 g hay in 10 l of water fermented in open buckets at room temperature for one week) was assessed by Latin-square experiments carried out in July-August 2011 in 20 suburban sites in Tirana (located at > 500 m from the each other). In each site, the four traps (Ov-A1 and ST with clean water and Ov-A1 and ST with hay-infusion) were located in shaded positions at the corners of a 50 m-square area and rotated clockwise on a weekly basis for four weeks. Eggs were counted as in Experiment 1. Adults collected by sticky traps were counted and morphologically identified .
Generalized Linear Mixed Models (GLMM) with negative binomial error distribution were carried out to analyse the data. For Experiment 1, the model response variable were egg catches and the explanatory variable was ovitrap type. For Experiment 2, the model response variables were egg and adult female catches for Ov-A1 and ST, respectively, while the explanatory variable was presence/absence of hay-infusion (treatment). In both GLMMs julian day of collection and site were included as random crossed effects. Since likelihood ratio test (LRT) carried out to compare models either including or not the random effects showed that the role of crossed random effects was highly significant (P-value of LRT < 10−6), these were included in the best models for both experiments.
Correlations between Ov-A1 and ST weekly catches and between averaged catches over the four week-long Experiment 2 were assessed by Kendall’s rank test. Moreover, after checking for the normality of error distribution (Shapiro-Wilkoxon normality tests, Ov-A1: W = 0.962, P = 0.201; ST: W = 0.988, P = 0.949), a standardized major axis regression (SMA) was used as in  to assess the relationship between means of log-transformed catches of Ov-A1 and ST in each site over the four weeks and to test whether this relationship was affected by hay-infusion. Specifically, SMA was preferred to a classical linear regression since sampling errors were expected to occur both for Ov-A1 and ST trap catches . Analyses were carried out using R version 3.2.0, with “glmmADMB” and “smatr” packages.
Results and discussion
Mean number ± standard deviation of Aedes albopictus eggs/ovitrap/day and adult females/sticky trap/day during the 4-week sampling carried out in Tirana (Albania) in 2011
No. of eggs per ovitrap
18.0 ± 32.2
51.4 ± 20.6
59.7 ± 66.9
28.7 ± 34.4
H2O + Hinf
62.3 ± 74.3
72.0 ± 68.0
98.3 ± 98.3
103.4 ± 107.2
No. of females per sticky trap
2.1 ± 2.2
2.1 ± 2.1
2.3 ± 2.3
2.8 ± 2.6
H2O + Hinf
2.4 ± 2.5
2.9 ± 3.6
3.5 ± 2.7
2.7 ± 2.4
Generalized Linear Mixed Model of Aedes albopictus egg catches by different types of ovitraps
GLMMs for numbers of Aedes albopictus eggs (response variable for ovitrap model) and adult females (response variable for sticky trap model) collected with or without hay-infusion
Sticky trap model
Overall, our results provide grounds for optimisation of ovitraps and sticky traps as monitoring tools for Ae. albopictus, by (i) supporting the use of germination paper as most appropriate oviposition substrate; (ii) suggesting the possible use of stackable conical ovitraps; and (iii) confirming the use of hay-infusion to increase egg-catches in ovitraps, and showing that this attractiveness also significant increases adult catches by sticky traps.
The authors are grateful to Francis Schaffner, who provides the cylindrical vessel of black plastic container to perform the experiments and to Erion Muhaxhiri for technical assistance.
This work has been funded by Institute of Public Health, Tirana and the EU grant FP7-261504 EDENext, and is catalogued by the EDENext Steering Committee as EDENext343. The contents of this publication are the sole responsibility of the authors and do not necessarily reflect the views of the European Commission. RR was partially funded by the Autonomous Province of Trento (Italy), Research funds for Grandi Progetti, Project LExEM (Laboratory of excellence for epidemiology and modelling; http://www.lexem.eu).
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