The bioecology of Dacus ciliatus (Diptera: Tephritidae) in a butternut squash production system within the Western Cape of South Africa

ENGLISH ABSTRACT: The lesser pumpkin fly, Dacus ciliatus (Loew), is a major pest of butternut squash (Cucurbita moschata Duchesne, 1786 (Cucurbitaceae)) in the Western Cape Province of South Africa. Despite its economic significance, limited research on its bioecology and the absence of effective monitoring protocols have constrained the development of sustainable Integrated Pest Management (IPM). This thesis investigated the bioecology of D. ciliatus in a commercial butternut production system with the primary aims of optimising an adult monitoring protocol using commercially available tools and characterising its seasonal population dynamics and its relationship to crop damage. Field studies demonstrated that a monitoring system using McPhail traps baited with Questlure™ and placed above crop canopy height (approximately 50 cm above ground surface) was the most effective, capturing significantly more flies than traps baited with Biolure™ or those placed higher (150 cm). In the Questlure™ + 50 cm treatment, females predominated, with a male-to-female ratio of approximately 1:3. Using this optimised system, seasonal population dynamics were found to be strongly influenced by host availability and sowing dates, with fly populations at two sites peaking late summer (February) and mid-autumn (April), respectively. The most vulnerable crop stage was identified as small, immature butternuts (<200 g), which accounted for over 80% of recorded infestations. These small fruit act as the primary breeding substrate, producing approximately 20 flies per 100 g of infested tissue and fuelling population growth. Crucially, a statistically significant positive relationship was established between trap captures, measured as flies per trap per day (FTD), and the probability of fruit damage. The odds of damage increase by approximately 10.6% for each one-unit increase in FTD. This model demonstrates that as trap captures rise from 0 to 20 FTD, the probability of fruit damage increases to approximately 40%, providing a clear, quantitative early-warning signal for growers. Collectively, these findings provide a foundational understanding of the bioecology of D. ciliatus and present the first evidence-based, quantitative tool for predicting infestation risk in the region’s commercial butternut systems. This work validates a practical monitoring protocol and provides the predictive model necessary for the future development of data-driven economic thresholds and more effective IPM strategies for this pest.

Grantor dc:publisher

Stellenbosch : Stellenbosch University

Year dc:date.issued

2026

Author dc:creator

• Van Rooyen, Rigardt Johan

Advisors dc:contributor.advisor

• Addison, Pia
• Bekker, Gerard Francois Hermanus van Ginkel

Language dc:language.iso

en

Repository record dc:identifier.uri

https://scholar.sun.ac.za/handle/10019.1/135656

OAI identifier oai:identifier

oai:scholar.sun.ac.za:10019.1/135656