Assessment of selected larvicides for the control of Culex pipiens biotype pipiens and Culex pipiens biotype molestus under laboratory and semi‐field conditions

Abstract

BACKGROUND: The insecticidal efficacy of selected mosquito larvicides [teflubenzuron, S-methoprene, diflubenzuron, temephos, Bacillus thuringiensis var. israelensis (Bti) and polydimethylsiloxane (PDMS)], applied alone or in combination, against two Culex pipiens biotypes, Cx. p. pipiens and Cx. p. molestus, was compared under laboratory and semi-field conditions. Registered larvicides were evaluated at the label dose, whereas those not registered for mosquito control were tested at the label dose per area, i.e. as plant protection products. Teflubenzuron and temephos were also evaluated at half the label dose, alone or in combination with a non-ionic surfactant. Larval mortality and adult emergence were assessed after 1, 3, 5, 7, 9, 11, 16 and 20 days of exposure. RESULTS: In all cases, PDMS showed complete (100%) larval control after 3 days of exposure against both biotypes. With temephos, all larvae were dead after 1 day of exposure, even at half the label dose. For teflubenzuron, mortality reached 100% at the end of the bioassays and adult emergence was avoided completely, whereas for diflubenzuron, mortality was close to 100% at the end of the trials. By contrast, in S-methoprene-treated water, control was below the 90% mortality threshold for both biotypes. In semi-field trails, in all treatments with Bti, all larvae of both biotypes were dead after 1 day of exposure. CONCLUSIONS: Based on our data, and with the exception of S-methoprene, the larvicides tested remained below the threshold for effective treatment against larvae of Cx. p. pipiens and Cx. p. molestus for the entire duration of the study. Most of the larvicides tested showed high levels of efficacy against Cx. p. pipiens and Cx. p. molestus larvae in laboratory and semi-field bioassays.

Residual efficacy of selected larvicides against Culex pipiens pipiens (Diptera: Culicidae) under laboratory and semi-field conditions

“…The use of PDMS has been proposed in large areas, such as catchments, but also in the urban and suburban environment, where the use of other insecticides is not always possible. In a recent study, Rumbos and Athanassiou ( 2020 ) indicated that this formulation was effective for the control of the biotype tested here, but also for Cx. pipiens biotype molestus , even at concentrations that were lower than the label rate.…”

“…pipiens biotype molestus , even at concentrations that were lower than the label rate. In that study, the authors found that PDMS had a remarkable “speed of kill,” as all larvae were dead after 3 days of exposure in semi-field tests, while larval mortality ranged between 60 and 88% even from the first day (Rumbos and Athanassiou 2020 ). Previous laboratory and field/semi-field tests confirmed the high efficacy of this insecticide for additional species of the genera Culex , Aedes , and Anopheles (Cilek et al 2020 , Webb and Russell 2009 , 2012 ).…”

“…Spinosad that is based on metabolites of the actinomycete Saccharopolyspora spinosa has been proved very effective against different species of mosquitoes as well (Hertlein et al 2011 ). Recently, several inert materials such as the polydimethylsiloxane (PDMS) have been also assessed successfully as larvicides, due to their incontestable advantages, which combine low levels of interaction with the environment, mechanical (non-neurotoxic) mode of action, and considerable residual activity (Bukhari and Knols 2009 ; Mbare et al 2014 ; Rumbos and Athanassiou 2020 ; Nikolaidou et al 2021 ). Due to its physical mode of action PDMS (a monomolecular film, which act on mosquitoes by closing off their respiratory structures and finally leading to suffocation) and its friendly environmental profile, many countries have exempted it from formal registration as a biocide (Bukhari and Knols 2009 ; Nikolaidou et al 2021 ).…”

“…In the vast majority of the studies, the insecticidal effect of mosquito larvicides is examined under laboratory conditions, while there is still inadequate information regarding their field performance based on experimentally sound studies. Moreover, even when different larvicides are compared in the laboratory, these data are not expanded to further testing under “controlled” field conditions, i.e., the so called “semi-field” tests, despite the fact that there is strong evidence that different insecticides behave in a dissimilar way when they are applied in the field (Msangi et al 2011 ; Rumbos and Athanassiou 2020 ). In this context, the results for laboratory experiments should be coupled with field and semi-field tests, as many larvicides are heavily influenced by biotic and abiotic conditions that are not present in the laboratory.…”

Densovirus Oil Suspension Significantly Improves the Efficacy and Duration of Larvicidal Activity against Aedes albopictus

“…To reduce the present reliance on insecticide-based mosquito control, bio-control solutions aim to be long-term control agents and target a variety of mosquito species [ 12 ]. Bacillus thuringiensis ( Bti ) formulations [ 13 , 14 ], for example long-lasting Ae. albopictus larvicidal Bti -blocks, are highly effective in vector control strategies [ 15 ] and plant essential oils used for mosquito control [ 16 , 17 , 18 ].…”