Evaluating the nature and mode of action of deterrents of false codling moth, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae)

Abstract

Research on the use of semiochemicals as repellents against agricultural insect pests is still limited. As a result, Dambuza (2022) conducted a study evaluating potential repellents for false codling moth (FCM), Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), a key phytosanitary pest of citrus in South Africa. He identified eight deterrents of FCM including two essential oils (EOs) (lavender oil and peppermint oil), two plant crude extracts (marigold and garlic extracts), one fruit (mango) and three chemicals (Delegate, Coragen and Warlock). This study assessed if these compounds were true repellents of FCM. Choice and no-choice repellent trials were conducted in complete darkness in a controlled environment room. In choice trials, gravid FCM females were presented with a choice between untreated (control) and treated (with deterrent solutions or suspensions) artificial oranges in a Y-olfactometer setup. Oviposition and choice behaviours were recorded after 24 hours. In no-choice trials artificial oranges treated with deterrent solutions/suspension were placed into a cage with gravid FCM females for four hours, with oviposition being recorded every hour. Of all the eight compounds tested, only three were true repellents (P < 0.05) of FCM, two essential oils (lavender and peppermint) and one chemical (Delegate®). We also identified three odour maskants, two crude plant extracts (garlic and marigold) and one chemical (Warlock®). All the initial tested compounds were also evaluated in the presence of host volatile organic compounds (VOCs) using real fruit. The plant crude extracts were the best performers in the presence of VOCs preventing FCM oviposition the most. Lavender oil and Coragen failed to perform with similar oviposition numbers on real treated and artificial treated fruit. Various essential oils (castor oil, clove bud oil, cypress oil, and sage oil) were also evaluated as potential attractants for female FCM in choice trials. None of the tested essential oils significantly attracted FCM females (P > 0.05), castor oil and sage oil showed slight attraction. The efficacy of the true repellents and odour maskants can be further tested in semi-field trials, as they have the potential to be used as standalone treatments or in conjunction with attractants in push-pull systems. Even though there was no significant attraction of FCM females by the tested EOs castor oil and sage oil may hold some promise and require further testing. This study provides a foundation for more botanicals to be evaluated as potential attractants for FCM and development of push-pull systems.