Efficacy of natural and synthetic insect growth regulators for the control of sand fleas (Tunga penetrans) in laboratory bioassays

Abstract text

Introduction:Tunga penetrans, (sand flea) is the cause of a tungiasis, a severely neglected tropical parasitic skin disease. Even if the disease is treated, re-infection occurs almost immediately due to off-host stages. It is, therefore, vital to identify targets for parasite control as treatment and prevention tools. Insect growth regulators such as pryriproxyfen have the ability to mimic a natural hormone in insects and disrupt their growth and is used widely for parasite control in pets and livestock, and is available in low-income countries for plant pest control. Neem oil, a natural product available in many tropical regions, has similar properties on a range of insects.Objective:To implement dose-response bioassays under controlled laboratory conditions to assess the efficacy of both products against Tunga off-host stages and to establish the optimum concentration for field evaluations.Material and Methods:Dose-response tests with four increasing exposure amounts of neem and pyriproxyfen were implemented for their impact on off-host stage development and adult emergence. These experiments were replicated 10 times on different dates containing 10 larvae each from different collection batches.Results:Pyriproxyfen was confirmed as a powerful insect growth regulator. Whilst in the unexposed controls 100% of all larvae successfully developed into adults, having pupated at day 10 after collection and taking a median of 9.5 (IQR 4) days in the cocoon until adult emergence on day 30 (IQR 4) after collection. Pyriproxyfen prevented 100% of the larvae to pupate in as low concentrations as 0.0075 parts per million (ppm) active ingredients. Neem oil was only effective at relatively high dosages. The mode of action for the neem oil, was consistently different from pyriproxyfen since larvae died in the process of trying to pupate around 10-12 days after collection and out of the larvae that managed to pupate, pupation took place by day 10 as in the controls but only 30% managed to emerge as adults. However, the effective dose of 15 ppm for LD50 would translate in an application of 60 ml of neem oil per square metre of floor area in the field.ConclusionsPyriproxyfen-based products for spray application on infested household floors should be tested for potential disease control. The impact of neem oil was only achieved at extremely high application rates and hence would not be a feasible product to take forward.