By Mary Hearty

Integrated pest management approach needs to focus on using interventions that lower maximum concentration of pesticide residues, especially by applying them according to label directions and the appropriate withdrawal period.

This is according to a new study, by lead author Dr Tertia Grové of the Agricultural Research Council Institute for Tropical and Subtropical Crops published in the CABI Agriculture and Bioscience journal that has reviewed progress made towards an eco-friendly insect pest management approach in subtropical agro-ecosystems in South Africa.

The research, highlights how an integrated pest management strategy should aim to reduce Maximum Residue Levels (MRLs) caused by pesticide use to tackle pests on avocados, litchi and mangoes in South Africa.

Dr Grové suggests that if insecticide application is needed, it is important to take the characteristics, applications and costs of insecticide into consideration, adding that insecticides’ application can cause the development of resistance by pests and cause problems with residues on fruits and in the environment.

Dr Grové said, “To minimize the reliance on insecticides is the best solution for a healthy environment. The lowering of MRLs in certain markets will increasingly place pressure on growers. An IPM strategy should therefore aim to use interventions that lower MRLs.”

She suggests that more eco-friendly options to tackle fruit pests can include bait techniques against fruit flies, natural chemical sulphur for the suppression of thrips and mealybugs can be dealt with by the release of parasitoids and other natural predators.

Dr Grove notes that if a residue level exceeds the set Maximum Residue Level (MRL) the product is not permitted to be sold or exported. She defines MRL as the maximum concentration of a pesticide residue to be legally permitted in or on food commodities and animal feeds

She said, “The paper provides details on the important insect pests of subtropical crops and the current management strategies use for controlling these pests.

“An integrated pest management strategy should aim to use interventions that lower maximum residue levels. A challenge that still remains is the sucking bug complex on avocado and more environmentally friendly strategies used for suppression need to be developed.”

She adds that an effective trapping system to monitor adult sucking bugs coming into the orchards will be important for effective management and the development and testing of mating disruption products, attract and kill products, and biological control products for litchi moth is important.

“Biological control products also need to be tested against citrus thrips on mango. The mango industry needs to find more environmentally safe suppression methods that can be used with sanitation to manage mango seed weevil as it is a pest of phytosanitary concern,” Dr Grové added.

South Africa is a known exporter of subtropical fruits – exporting to markets in the European Union and the United Kingdom – but fruit production in the country is susceptible to various insect pests that can significantly contribute to yield losses.

These include litchi moth, Cryptophlebia peltastica (Meyrick) (Lepidoptera: Tortricidae) on litchi, and the citrus thrips, Scirtothrips aurantii Faure (Thysanoptera: Thripidae) and the mango seed weevil, Sternochetus mangiferae (Fabricius) (Coleoptera: Curculionidae) on mango.

In terms of avocado pests, for example, in 2019 the percentage loss of avocado fruit due to insect pests was determined at 12.58% according to the South African Subtropical Growers’ Association.

Furthermore, with respect to the mango seed weevil, Dr Grové highlights that this is an important pest and the economic impact is primarily based on the fact that it is a major phytosanitary pest- a pest of potential economic importance to an area where it is not yet present- restricting access to new foreign markets and contributing to substantial rejections of fruit destined for existing export countries.

She adds that a study conducted over two seasons in Limpopo indicated that 39.86% and 23.64% of ‘Tommy Atkins’ fruit were infested with mango weevil eggs if uncontrolled.

Twenty years ago, mainly broad-spectrum insecticides (organophosphates and pyrethroids) had been registered for control of pests and progress was made in adopting eco-friendlier management approaches.

However, the lowering of MRLs for pesticides on food products by importing countries provides new challenges for growers.

“The set of economic thresholds for the important pests also needs some attention. The use of botanical pesticides has not yet been tested on a large scale and could possibly contribute to the control of pests in the future,” she said.