Evaluating the Potential of Pyriproxyfen Dissemination using Mosquito Home System against Aedes albopictus at a Dengue Hotspot Area

Aedes mosquitoes were found to lay their eggs in the cryptic breeding sites. Eliminating cryptic and open breeding sites is essential in reducing dengue virus transmission. However, it is often challenging for health officers to assess these breeding sites which are usually missed during larval surveillance. The autodissemination approach may produce a better outcome by manipulating female mosquitoes to disperse insecticide to other Aedes spp. mosquito habitats. Thus, the present study aims to evaluate the effectiveness of the pyriproxyfen autodissemination technique using Mosquito Home System against the population of mosquitoes. This study was conducted in Bandar Baru Bangi, Selangor, Malaysia. The Mosquito Home System was deployed to control Aedes spp. populations at treatment sites using before-after-control-impact (BACI) design. The presence of pyriproxyfen distribution was confirmed using the WHO larval bioassay which resulted in 10-35% larvae mortalities. Autodissemination of pyriproxyfen significantly reduced the population size of mosquito eggs (p<0.05), larvae (p<0.05), and ovitrap index (p<0.05) at the treatment areas compared to the control areas. Moreover, rainfall was correlated positively against ovitrap index (r = 0.247), larvae (r = 0.420), and eggs (r = 0.422). The study provides promising results for controlling Aedes spp. populations and also highlights the potentials of this technique as an alternative in vector control programmes. However, further studies on larger scale field trials are warranted.

Anther-Feeding-Induced RNAi in Brassicogethes aeneus Larvae

The biosafety aspect of applying double-stranded RNA (dsRNA) in crop pest management is rooted in dsRNA's mode of action, which displays nucleotide sequence-specificity to a particular region of a messenger RNA (mRNA), against which the insecticidal dsRNA is designed. This prominent and promising class of insecticides therefore has the potential to target a single pest species while conferring negligible effect on non-target organisms. Recent studies examining the effect of target-specific dsRNA in adults of the pollen beetle Brassicogethes aeneus, a major pest of oilseed rape (Brassica napus) crops in Europe, suggest the potential for developing a gene-silencing approach within integrated B. aeneus management. The present study examines the efficacy of target-specific dsRNA on target-mRNA silencing, and subsequent gene-silencing-induced mortality, in B. aeneus larvae, as this life stage represents a critical target for achieving optimal integrated B. aeneus control. Treatment applications occurred via feeding on dsRNA-treated anthers for 3 d. We observed variable gene-silencing efficacy, all target treatments having a significant or marginally significant effect after 3 d of dsRNA feeding, with greater variability at 6 d. These results further validated significant gene-silencing-induced mortality observed for one of the target treatments. Moreover, gene-silencing-induced mortality occurred at a quicker rate in B. aeneus larvae compared to what has been previously observed in B. aeneus adults. Finally, we consider refinements that must be made to B. aeneus larval bioassay setups to promote and strengthen future larval studies regarding this important crop pest species.

Susceptibility of South Texas Aedes aegypti to Pyriproxyfen

An integral part to integrated mosquito management is to ensure chemical products used for area-wide control are effective against a susceptible population of mosquitoes. Prior to conducting an intervention trial using an insect growth regulator, pyriproxyfen, in South Texas to control Aedes aegypti, we conducted a larval bioassay to evaluate baseline levels of susceptibility. We used seven serially-diluted doses ranging from 2.5 ppb to 6.3 × 10−4 ppb. We observed 100% inhibition emergence (IE) at even the lowest dose of 6.3 × 10−4 ppb in our susceptible reference colony of Ae. aegypti Liverpool. In our field strain of Ae. aegypti (F5 colonized from South Texas) we observed 79.8% IE at 6.3 × 10−4 ppb, 17.7% IE at 1.25 × 10−3 ppb, 98.7% IE at 1.25 × 10−2 ppb, and 100% emergence inhibition for the remainder of the doses. Given that commercial pyriproxyfen products are labeled for doses ranging to 50 ppb, we conclude that the field population sampled by this study are susceptible to this insect growth regulator.

The effect of coexistence between larvae of Anopheles gambiae and Culex quinquefasciatus on larvicidal efficacy of Bacillus thuringiensis var. israelensis

Background: The efficacy of Bacillus thuringiensis var. israelensis (Bti) is affected by several factors including the species of the mosquito. Mosquito larvae of different species are found to coexist in larval breeding habitats. This study evaluated whether the coexistence between Anopheles gambiae and Culex quinquefasciatus affect the larvicidal activity of Bti. Methods: Two parallel larval bioassay experiments were conducted to test A. gambiae sensu stricto (s.s) and C. quinquefasciatus larvae susceptibility to Bti. They were followed by three parallel bioassays in which A. gambiae s.s and C. quinquefasciatus larvae were mixed in different proportions such that the earlier species contributed three quarters, half and a quarter of the larvae in each testing cup respectively. In each bioassay, six Bti concentrations were tested in four replicates and repeated on three different days. Larvae mortality was scored 24 hours after application of Bti and subjected to Probit analysis. Results: C. quinquefasciatus was significantly more susceptible to Bti than A. gambiae s.s at both lethal concentration values (LC50 and LC95). In coexisting scenario, LC50 of Bti was significantly lower when the proportion of C. quinquefasciatus exceeded 50%. No significant variation in susceptibility to Bti was observed at LC95 in any proportion of coexistence between the two species. Conclusion: The findings show that larvae of C. quinquefasciatus were significantly more susceptible to Bti than those of A. gambiae s.s. Moreover, when larvae of the two species coexisted, there was a general trend of increase in sensitivity to Bti with higher proportion of C. quinquefasciatus. Although this increase in sensitivity of coexisting larvae to Bti is worth noting, our findings suggest that it will not impact larval control where A. gambiae s.s and C. quinquefasciatus coexist.

Larvicididal Activity of Natural Repellents Against the Dengue Vector, Aedes aegypti

ABSTRACT The present research aimed to evaluate the larvicidal activity of several recently discovered natural repellents formulated in lotions against larvae of Aedes aegypti. We used a modified larval bioassay method by the World Health Organization standards in evaluating larval mortality at 24-, 48-, and 72-h exposure. Among the test repellents, 2-undecanone showed 100% mortality of Ae. aegypti larvae, followed by catnip oil, capric acid, coconut oil fatty acids, methyl caprate, methyl laurate, and coconut oil methyl esters. The repellent, 2-undecanone showed median lethal concentration (LC50) values of 73.07, 26.45, and 15.68 ppm at 24-, 48-, and 72-h exposure, respectively. Larvicidal activity varied among the other repellents tested.