Comparison of hand netting and pan trapping methods for estimating hover fly (Diptera: Syrphidae) diversity in the tropical agroecosystems of East Africa

Monitoring hover fly populations is essential for establishing baseline
data on community dynamics and their potential links to ecosystem service
provision. While pan traps and hand netting are widely used in monitoring
insects in natural habitats, their comparative performance in estimating
hover fly (Diptera: Syrphidae) diversity in Afrotropical agroecosystems
remains understudied. This study assessed the effectiveness of yellow pan
traps and hand netting over two consecutive years (2022–2023) in a
cucurbit agroecosystem in Morogoro, Tanzania, across two contrasting
landscapes (plateau and mountainous zones). Weekly sampling was conducted
for eight consecutive weeks during both the rainy (April–June) and dry
(September–November) seasons across 20 cucurbit plots. In each plot, nine
yellow pan traps were deployed for 48 hours, and hand netting was
conducted along three 15-m transects for 45 minutes per session. Hand
netting consistently recorded higher hover fly diversity than pan traps
across all Hill numbers (q = 0, 1, 2). Species accumulation curves reached
asymptotes more rapidly with hand netting, indicating greater sampling
efficiency and community coverage. Although more labour-intensive, hand
netting provided a more comprehensive estimation of hoverfly diversity in
both agroecological zones. In contrast, pan traps captured only a few
additional low-abundance species overlooked by netting, which are unlikely
to contribute significantly to ecosystem service provision. Therefore,
hand netting proved to be the most robust and cost-effective method for
monitoring hover flies in agricultural landscapes. These findings provided
evidence to support the development of standardized monitoring protocols
for hover flies in tropical agricultural landscapes.