Field evaluation of a novaluron-based larvicide tablet (MOSQINOk^® 0.8P) in irrigated rice agroecosystems of The Gambia: a controlled quasi-experimental study

Abstract

Background

Larval source management remains a potentially important supplementary strategy for malaria vector control, particularly in irrigated agroecosystems, where mosquito breeding habitats are relatively stable and predictable. This study evaluated the operational effectiveness of a novaluron-based larvicide tablet (MOSQINOk^® 0.8P) for reducing mosquito larval and adult densities in irrigated rice-growing communities in The Gambia.

Methods

A quasi-experimental controlled field study was conducted between September and December 2025 in two irrigated rice-growing communities in the Central River Region of The Gambia. Janjangbureh was assigned as the intervention site and Sankuli Kunda as the untreated control. Larval habitats were monitored weekly, while adult mosquitoes were collected biweekly using CDC light traps. Difference-in-differences analyses using negative binomial generalized estimating equation models were used to estimate intervention effects.

Results

MOSQINOk^® application was associated with a significant reduction in total larval density per dip (IRR = 0.459, 95% CI 0.216–0.974; p = 0.042), corresponding to an estimated 54.1% reduction relative to the control site. Late instar larval densities also declined in the intervention arm (IRR = 0.485, 95% CI 0.186–1.260), although this was not statistically significant. Pupae were nearly absent in treated habitats following larvicide application. Adult Anopheles densities also declined in the intervention arm, with difference-in-differences models showing reductions in overall adult Anopheles (IRR = 0.385, 95% CI 0.121–1.225), Anopheles gambiae s.l. (IRR = 0.316, 95% CI 0.098–1.014), and Anopheles funestus (IRR = 0.443, 95% CI 0.107–1.834).

Conclusions

These findings provide preliminary evidence that MOSQINOk^® 0.8P can contribute to suppression of mosquito larval populations and may reduce adult vector densities in irrigated rice agroecosystems. Larger and longer-term studies are warranted to confirm effectiveness and scalability.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s13071-026-07501-2.

Background

Despite substantial progress in malaria control, residual malaria transmission remains a major challenge in many endemic settings, particularly where mosquitoes exploit breeding habitats that are not adequately targeted by conventional interventions, such as long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) [1–4]. In irrigated rice agroecosystems, persistent aquatic habitats can support continuous mosquito breeding and contribute to sustained malaria transmission [5–7].

Larval source management (LSM) is recommended by the World Health Organization as a supplementary vector control strategy in settings, where breeding habitats are relatively fixed, findable, and manageable [8]. Interest in LSM has increased in response to operational challenges affecting conventional interventions and increasing insecticide resistance among malaria vectors [9]. Rice cultivation environments may represent particularly suitable settings for LSM, because breeding sites are often predictable and accessible for repeated intervention [6, 8].

Novaluron is an insect growth regulator that acts by inhibiting chitin synthesis, disrupting larval development and preventing successful moulting and pupation [10–12]. Previous studies have demonstrated promising effects of insect growth regulators against mosquito larvae; however, evidence from operational field evaluations in irrigated rice ecosystems of West Africa remains limited [10–12]. In The Gambia, persistent vector breeding habitats associated with irrigated agriculture may continue to contribute to malaria transmission despite reductions in malaria burden [13–15].

This study evaluated the effectiveness of a novaluron-based larvicide tablet (MOSQINOk^® 0.8P) for reducing mosquito larval and adult densities in irrigated rice-growing communities in the Central River Region of The Gambia.

This study was conducted in two villages in the Central River Region (CRR) of The Gambia: Janjangbureh (intervention) and Sankuli Kunda (control). The communities were located approximately 8–12 km apart and had no direct hydrological connection. Both communities are characterized by irrigated rice cultivation systems and comparable rural settlement patterns, supporting their suitability for controlled comparison (Fig. 1).

Fig. 1.

Location of the intervention site (Janjangbureh Island) and control site (Sankuli Kunda, south bank) in the Central River Region (CRR), The Gambia. The River Gambia separates the two study villages, which are approximately 8–12 km apart and hydrologically independent

A controlled quasi-experimental difference-in-differences (DiD) design was used, consisting of a 1-week baseline period followed by 12 weeks of post-intervention monitoring.

MOSQINOk^® 0.8P tablets were applied directly to aquatic habitats in the intervention arm at a standardised operational dose of one tablet per approximately 200 L of estimated water volume, estimated from habitat surface area and mean depth measurements. The tablet formulation is designed to gradually release novaluron into relatively stagnant aquatic habitats, providing sustained exposure of immature mosquito stages to the active ingredient. Fixed larval monitoring sites were sampled weekly using standard dipping methods, while adult mosquitoes were collected biweekly using CDC light traps at sentinel households.

The primary outcome was total larval density per dip. Secondary outcomes included late instar (L3–L4) density per dip and adult mosquito densities per trap-night.

Difference-in-differences analyses were conducted using negative binomial generalized estimating equation (GEE) models with a log link to account for overdispersed count data and repeated observations (Table 1). Sampling effort was incorporated using log-transformed offsets based on the number of dips per site visit for larval outcomes and trap-nights for adult collections. Effect estimates are presented as incidence rate ratios (IRR) with 95% confidence intervals. Analyses were conducted in R version 4.3.2 using the geepack package.

Table 1.

Baseline characteristics and entomological indicators by study arm

Characteristic	Control	Intervention
Sampling sites (n)	20	20
Total dips at baseline	390	400
Mean larval density per dip (SD)	1.94 (2.40)	2.66 (1.82)
Trap locations (n)	5	5
Trap-nights at baseline	15	15
Mean adult Anopheles per trap-night (SD)	3.40 (3.66)	2.80 (2.43)

A total of 40 fixed larval sampling sites (20 per arm) were monitored throughout the study period. Baseline mean larval density per dip was 2.66 in the intervention arm and 1.94 in the control arm (Table 2). Adult mosquito surveillance was conducted at five sentinel households per arm using CDC light traps.

Table 2.

Difference-in-differences estimates for larval and adult mosquito outcomes following MOSQINOk^® 0.8P application

Outcome	IRR	95% CI	p value	Relative reduction (%)
Total larvae per dip	0.459	0.216–0.974	0.042	54.1
Late instars (L3–L4) per dip	0.485	0.186–1.260	0.137	51.5
Adult Anopheles per trap-night	0.385	0.121–1.225	0.106	61.5
Adult An. gambiae s.l. per trap-night	0.316	0.098–1.014	0.053	68.4
Adult An. funestus per trap-night	0.443	0.107–1.834	0.263	55.7

Following application of MOSQINOk^® 0.8P, larval density declined relative to the control arm, with sustained reductions observed in treated sites during the post-intervention period (Fig. 2A). The difference-in-differences model showed a significant reduction in total larval density (IRR 0.459; 95% CI 0.216–0.974; p = 0.042), corresponding to a 54.1% reduction relative to control trends.

Fig. 2.

Weekly larval dynamics by study arm: A total larval density per dip, B late instar (L3–L4) density per dip, and C pupal density per dip. Values represent site-week means with site-clustered 95% confidence intervals. The dashed vertical line indicates the timing of larvicide application at ISO week 40

Late instar (L3–L4) density also declined in the intervention arm following treatment (Fig. 2B), although estimates were imprecise (IRR 0.485; 95% CI 0.186–1.260; p = 0.137).

Pupal density declined markedly in treated habitats (Fig. 2C). More than 90% of post-intervention site-week observations recorded zero pupae in the intervention arm, whereas pupae continued to be detected intermittently in the control arm. Across the post-intervention period, only one pupa was recorded in treated habitats compared with 12 in the control arm.

Adult mosquito densities also declined following intervention and were broadly consistent with observed larval reductions. Overall adult Anopheles abundance declined relative to the control arm (IRR 0.385; 95% CI 0.121–1.225; p = 0.106), corresponding to a 61.5% reduction. Anopheles gambiae s.l. abundance also declined (IRR 0.316; 95% CI 0.098–1.014; p = 0.053). Anopheles funestus densities showed a similar directional trend (IRR 0.443; 95% CI 0.107–1.834; p = 0.263), although confidence intervals were wide. Additional temporal trends are presented in Supplementary Figure S1 (Additional file 1).

This study provides field evidence that application of a novaluron-based larvicide formulation was associated with reduced mosquito larval densities in irrigated rice agroecosystems of The Gambia. The significant reduction in total larval density and marked suppression of pupae are biologically consistent with the expected mode of action of insect growth regulators, which inhibit chitin synthesis and interfere with normal larval development and successful emergence [10–12].

The near absence of pupae following intervention is particularly important, because insect growth regulators generally do not produce rapid knockdown of early larval stages but instead interrupt developmental progression to adulthood [10, 11]. Continued detection of some early instars during the post-intervention period may, therefore, reflect ongoing oviposition rather than treatment failure. Similar patterns have been observed in previous evaluations of insect growth regulators and larval source management interventions [10, 11]. The magnitude of larval reduction observed in the present study was also comparable to reductions reported in previous field evaluations of novaluron and other insect growth regulator formulations under operational conditions [10, 11].

Although reductions in adult mosquito abundance were associated with wide confidence intervals and limited statistical precision, the observed trends were broadly consistent with reductions in immature mosquito stages. Previous studies have similarly reported that reductions in larval habitat productivity may contribute to subsequent reductions in adult vector populations [6, 16, 17]. The reduction observed for Anopheles gambiae s.l. is noteworthy, because this species complex remains an important malaria vector in The Gambia and across much of sub-Saharan Africa [4, 15].

Irrigated rice ecosystems may represent suitable environments for larval source management, because aquatic habitats are relatively stable and identifiable compared with more dispersed temporary breeding sites [6–8]. WHO recommendations indicate that larval interventions may be particularly useful, where breeding habitats can be reliably located and repeatedly treated [8]. The operational use of tablet formulations may also offer practical advantages for routine implementation in agricultural settings.

This study has limitations. The quasi-experimental design involving a single intervention and control community limits causal inference and may introduce village-level confounding. The short baseline period and limited adult sampling intensity may also have reduced statistical power for adult mosquito outcomes. Furthermore, mosquito movement between surrounding areas may have influenced observed adult densities. Environmental factors such as rainfall variation and habitat turnover were not explicitly modelled and may also have influenced mosquito population dynamics. Larger multi-site evaluations with longer follow-up periods are needed to confirm effectiveness and operational scalability across different ecological settings. Overall, these findings provide preliminary evidence supporting the potential role of novaluron-based larval source management as a complementary vector control strategy in agricultural settings.

Abbreviations

CDC

Centers for disease control and prevention

CI

Confidence interval

CRR

Central river region

DiD

Difference-in-differences

GEE

Generalized estimating equations

IGR

Insect growth regulator

IRR

Incidence rate ratio

IRS

Indoor residual spraying

LLINs

Long-lasting insecticidal nets

LSM

Larval source management

MRCG

Medical Research Council Unit The Gambia at LSHTM

NMCP

National Malaria Control Programme

SD

Standard deviation

WHO

World Health Organization

Author contributions

EJ conceived and co-designed the study, coordinated field implementation, supervised entomological surveillance, conducted statistical analyses, and drafted the manuscript. MTN provided senior institutional oversight, public health policy guidance, and ensured alignment with national malaria control priorities and Ministry of Health frameworks. SS contributed to research governance oversight, facilitated institutional approvals, and supported interpretation of findings within the national health research and policy context. WS, BG, and SSG (NMCP) contributed through programme stewardship, implementation authority, operational oversight, and interpretation of results for malaria control decision-making. HMS co-designed the study, provided entomological and methodological oversight, and contributed to data analysis and interpretation. AIA contributed technical input on larvicide application and interpretation of entomological outcomes. SAZ provided technical input on vector control strategy, larvicide product characteristics, and operational considerations relevant to potential large-scale deployment. All authors reviewed the manuscript, provided intellectual input, and approved the final version for submission.

Funding

Field implementation activities were supported by the Blue Sands Foundation. The larvicide product (MOSQINOk^® 0.8P) was provided as in-kind support by RUSSELL IPM. Funders had no role in the decision to publish.

Data availability

Data supporting the main conclusions of this study are included in the manuscript.

Declarations

Ethics approval and consent to participate

CDC light trap collections were conducted at sentinel households with prior verbal informed consent from household heads in accordance with national surveillance guidelines. Community entry and surveillance activities were conducted in coordination with village leaders and the National Malaria Control Programme as part of routine vector surveillance and operational evaluation activities.

Consent for publication

Not applicable.

Competing interests

E.A.J., A.I.A., and S.A.Z. are employees of RUSSELL IPM Ltd, the manufacturer of MOSQINOk^® 0.8P. The remaining authors declare no competing interests.