Oviposition behaviour and parity rates of Aedes aegypti collected in sticky traps in Trinidad, West Indies

Abstract

The oviposition behaviour of Aedes aegypti was studied using sticky traps (ST), double sticky traps (DST) and standard ovitrap traps in urban St. Augustine and rural Tamana, Trinidad, West Indies. In St. Augustine three traps were deployed in 10 houses for 10 weeks while in Tamana traps were similarly deployed (10 houses for 10 weeks). At each house one ovitrap, one ST and one DST were placed using the criteria established for ovitrap placement. The results showed large numbers of adults collected, 3602 collected in DSTs and 1,670 adults collected in STs. In addition, >9000 immatures were collected in the DST vs >7000 in the STs. Over the 10 weeks 517 Ae. aegypti eggs were collected from ovitraps from Tamana and 3252 eggs from St. Augustine. Most of the females collected were parous (99%) with many older females collected e.g. 7 pars collected in both Tamana and St. Augustine. A major finding of the study was the observation of the “death stress oviposition” behaviour displayed among Ae. aegypti females captures in the sticky traps. This is the first report of this behaviour in the field and may well explain the collection of large numbers of immatures found in the ST and DSTs. The results of this study are discussed in the context of developing surveillance and control strategies, especially for reducing man–vector contact.

Introduction

Dengue Fever and its haemorrhagic manifestations have re-emerged as a major public health problem, with high morbidity and mortality rates, because of the failure of conventional vector control programs (Gubler and Kuno, 1997) and the failure to develop appropriate vaccines which can target all four dengue serotypes (Halstead, 2008). Consequently 2.5 billion people live at risk of contracting dengue with an estimated 50–100 million cases occurring annually (Gubler, 1998, Farrar et al., 2007).

In response to the global challenge of dengue transmission many have re-examined old approaches and have refined or refashioned them. Numerous workers have examined oviposition attractants of Aedes aegypti L. (Buxton and Hopkins, 1927, Manefield, 1951, O’Gower, 1963) with Gjullin et al. (1965) reporting that grass infusion and log pond water enhancing oviposition by Ae. aegypti and Culex pipiens quinquefasciatus Say. However, Hazard et al. (1967) showed Ae. aegypti were not attracted to the odour of hay infusion in an olfactomoter.

With the emergence of insecticide resistance among the various disease vectors, a concerted effort was made to re-visit the use of botanicals and their integration into conventional ovitraps made from black jars containing tap water which were found to be attractive to gravid female mosquitoes (Fay and Eliason, 1966). The combined effect/attraction of black ovitraps and hay infusion was reported to remove large sectors of the mosquito population (Reiter et al., 1991, Trexler et al., 1998) but Chadee et al. (1993) reported the failure of this combination to elicit increased oviposition and this was later supported by other studies (Santanna et al., 2006, Gama et al., 2007). The failure to observe an increase in oviposition was subsequently found to result from changes in bacterial activity especially fermentation (Santanna et al., 2006).

The incorporation of various attractants and sticky glue components has further modified ovitraps to “lure and kill” devices. For example, ovitraps containing plain tap water were attractive to gravid females (Fay and Eliason, 1966) but now collect mosquitoes when they alight on the sticky substance during, before or after oviposition (Muir and Kay, 1998, Ordonez et al., 1997). Modifications have emerged with the addition of mouse odour (McCall et al., 1996), infusion filled ovitraps with either velour paper (Zeichner and Perich, 1999) or flannel cloth (Williams et al., 2007) ovistrips treated with synthetic pyrethroids, filled with hay infusion, using sticky traps with synthetic oviposition attractants (Santanna et al., 2006) and the use of lucerne to attract gravid females to standard ovitraps (Ritchie, 2001) and sticky ovitraps (Ritchie et al., 2003, Ritchie et al., 2004).

Recently, Chadee and Ritchie (in press) described the new double sticky trap and compared the efficiency of sticky, double sticky and conventional ovitraps. The results indicated that the double sticky traps were more efficient in collecting adult Ae. aegypti than standard sticky traps (Ritchie et al., 2003, Ritchie et al., 2004). The collection of large numbers of adults from sticky traps was far superior to human bait collections in Tanzania (Corbet and Smith, 1974), in Arizona, USA (Hoeck et al., 2003) and in Trinidad (Chadee and Martinez, 2000) and provided an opportunity to study urban and rural populations of Ae. aegypti in Trinidad (Chadee and Ritchie, in press).

In Tanzania, Corbet and Smith (1974) dissections provided information on the age structure of host-seeking Ae. aegypti females and showed 28.4% of the population was parous, while in Tucson, Arizona the host-seeking females parous rates were 44.0% (Hoeck et al., 2003). To date no studies have been conducted on the physiological age of Ae. aegypti mosquitoes collected at oviposition sites.

Several methods have been used to determine the physiological age of mosquitoes including the use of dilatations on the ovarioles (Detinova, 1962), the use of oil injections to ovarioles (Hoc and Charlwood, 1990) and by gas chromatography analysis of whole-body cuticular hydrocarbons (Desena et al., 1999). In the Caribbean region the physiology of the Ae. aegypti mosquitoes, their population dynamics and various risk factors for dengue transmission have never been conducted but rather most studies have been confined to surveillance, based primarily on house-to-house larval inspections (PAHO, 1994), on the use of modified ovitraps (Fay and Eliason, 1966), population molecular genetics (Apostal et al., 1994, Colton et al., 2003) and more recently, on the pupal surveys (Focks and Chadee, 1997, Chadee, 2004). The large number of adults collected during the Chadee and Ritchie (in press) study afforded the opportunity to age grade the females collected and to determine the age structure of populations seeking oviposition sites.

The objectives of this study were to determine the age structure of Ae. aegypti populations seeking oviposition sites and to determine the suitability of sticky traps for monitoring adult mosquito populations in the urban centre St. Augustine and in the rural village of Tamana, Trinidad, West Indies.