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Study: Impact of climate change on the potential distribution of Tuta absoluta

17/11/2023 - François-Xavier Branthôme
Globally, the introduction of invasive species in new environments outside their native range is increasing at unprecedented rates. This could be further exacerbated by climate change. Based on model prediction, under future climatic scenarios (the years 2050 and 2070), the establishment risk index is expected to increase across Europe, North America, Northern Asia, South Australia, and South and East Africa.

 Phthorimaea absoluta (Meyrick) (= Tuta absoluta) (Lepidoptera: Gelechiidae), is the most damaging insect pest threatening the production of tomato and other solanaceous vegetables in many countries. In this study, researchers predicted the risk of establishment and number of generations for P. absoluta in the current and future climatic conditions under two Shared Socioeconomic Pathways (SSP2-4.5 and SSP5-8.5) of the years 2050 and 2070 using insect life cycle modelling (ILCYM) software. They used a temperature-dependent phenology model to project three risk indices viz., establishment risk index (ERI), generation index (GI), and activity index (AI) based on temperature data. 
The model projected large suitable areas for P. absoluta establishment in the Southern hemisphere under current and future climatic scenarios, compared to the Northern part. However, the risk of P. absoluta is expected to increase in Europe, USA, Southern Africa, and some parts of Asia in the future. Under current conditions, P. absoluta can complete between 6 and 16 generations per year in suitable areas. However, an increase in generation index between 1 and 3 per year is projected for most parts of the world in the future, with an increase in activity index between 1 and 4. Results provide information on the risk of establishment of P. absoluta which could guide decision-makers to develop control strategies adapted for specific agro-ecological zones.

Globally, the introduction of invasive species in new environments outside their native range is increasing at unprecedented rates. This could be further exacerbated by climate change. Once invasive species arrive and establish themselves in a new environment, they can pose serious threats to crop production if they are not effectively managed. This was the case with the invasion by the South American tomato pinworm, Phthorimaea absoluta (Meyrick) (= Tuta absoluta) (Lepidoptera: Gelechiidae). Since its first report in Spain in 2006, P. absoluta has rapidly spread and become established in many countries across the Afro-Eurasian Supercontinent, causing devastating yield losses, especially for solanaceous vegetables. Over seventeen years after its first detection, P. absoluta continues to expand its geographical range with the latest invasion reported in Togo in West Africa and China in Asia, the largest global producer of tomatoes. The success of invasion and the subsequent establishment of P. absoluta is attributed to its innate dispersal ability, high reproductive rate, short life cycle, wide thermal tolerance, and high phenotypic plasticity.

Although tomato is the primary host of P. absoluta, the pest also attacks and completes its life cycle on other wild and cultivated plants belonging to the family Solanaceae such as the black nightshade, Solanum nigrum L., potato, Solanum tuberosum L. and eggplant, Solanum melongena L. The damage is caused by larval feeding on the leaves, flowers, stems, and fruits, causing tunnels that disrupt sap flow and photosynthesis. The infestation of P. absoluta on tomato fields is highly detrimental, and it can cause a yield loss of up to 100% if it is left uncontrolled, leading to severe economic impacts, especially for small-scale farmers. Although several attempts have been made to manage the pest using eco-friendly strategies, the application of synthetic insecticides remains the main practice for controlling P. absoluta. Therefore, a better understanding of the ecological traits and distribution of P. absoluta could help in developing effective control measures adapted to specific agroecological zones.

Over the last few decades, modelling the responses of insect pests to climatic factors, and predicting their geographical distribution, abundance, and risk of invasion have gained much interest. Temperature is known to be the key abiotic factor that affects the distribution, abundance, and population dynamics of insects. In the context of climate change, the increase in temperature from 1.5 to 5.8 °C by the end of the twenty-first century is likely to have either direct or indirect effects on the population of many insect pests. The temperature increase could directly affect insects' life histories, physiology, and behavior. The indirect effect is expected to be through the interactions between the pests and their host plants and natural enemies, which in turn might influence the distribution and population dynamics of insect pests. Under favorable climatic conditions and in the absence of natural enemies, rapid multiplication of the pests occurs, and this might result in huge crop losses. In this regard, predicting the responses of insect pests to climate change has become more relevant for assessing the risk of invasion and developing ecologically sound management strategies.

Climate-based simulation models are important tools for forecasting pests’ risk in agricultural ecosystems under different climatic scenarios. Two types of pest simulation models, namely inductive and deductive are used to predict the distribution, abundance, and ecological niche of pests based on climatic data. The inductive method relies on the environment of the localities in which the insect species is found. This method correlates the occurrence of the species to environmental data of each location to generate the probability of occurrence under different climatic scenarios. On the other hand, the deductive method is based on the thermal requirements for pest development (thermal thresholds for development), which are obtained by fitting linear and nonlinear functions to insect development, survival, and fecundity. In addition, the thermal requirements of the pest are incorporated with climatic data into a geographic information system (GIS) to simulate the impact of climate change on the protentional distribution of the pest in a given location.

FIG. 1 The establishment risk index (ERI) of Phthorimaea absoluta  projected worldwide using temperature-based phenology model under (A) current climatic conditions, (B) future scenarios for 2050 (SSP2-4.5), (C) future scenarios for 2050 (SSP5-8.5), (D) future scenarios for 2070 (SSP2-4.5) and (E) future scenarios for 2070 (SSP5-8.5). The areas with ERI > 0.6 indicated that Phthorimaea absoluta could permanently establish.

The potential distribution of P. absoluta under a warming climate was projected by four different CLIMEX models using information from literature, specifically on the pest occurrence data and its thermal requirements. Among these, two models projected the potential distribution of the pest worldwide, while others focused the projection on a regional level. Although CLIMEX combines both inductive and deductive approaches to simulate pest distribution, the thermal requirements data for P. absoluta used in these studies to adjust models’ parameters were not validated under field conditions.
More recently a study by Fand et al. predicts the invasion risk of P. absoluta in India using the ecological niche Maxent model under the current and future climatic scenarios. However, none of the above studies projected the future distribution of P. absoluta under different Shared Socioeconomic Pathways (SSPs). In addition, different modelling approaches for P. absoluta are required to enrich the existing literature for a better understanding of the distribution of the pest under changing climate. Therefore, this study aimed to predict the impact of climate change on the future distribution of P. absoluta under two SSPs (SSP2-4.5 and SSP5-8.5) for the years 2050 and 2070 using Insect Life Cycle Modeling (ILCYM) software.

The establishment risk index (ERI) of Phthorimaea absoluta 
Researchers classified the establishment risk index (ERI) into five classes viz., unsuitable areas (ERI = 0.0–0.2), marginally suitable areas (ERI = 0.2–0.4), suitable areas (ERI = 0.4–0.6), highly suitable areas (ERI = 0.6–0.8) and optimal areas for survival (ERI ≤ 0.8). Overall, most parts of the Southern hemisphere of the world are projected to be highly suitable for P. absoluta establishment compared to Northern hemisphere under both current and future climatic scenarios (Fig. 1). Under the current climatic conditions, the pest could permanently establish (ERI ≤ 0.6) in most parts of sub-Saharan Africa except for the Sahara Desert which is projected to be unsuitable. Most countries in South America as well as some countries in Asia (i.e., Thailand, Myanmar, Bangladesh, Indonesia, and Malaysia), showed high or optimal suitability for P. absoluta establishment. Globally, under the SSP2-4.5 scenario of the year 2050, the suitable areas under current conditions will remain favorable for the pest's establishment but an increase in suitability is expected in some parts of Africa (e.g. South Africa, Botswana, and Namibia) and the Southern part of Australia. Although the optimal areas will dramatically decrease across the globe under the SSP2-4.5 scenario of the year 2050, they will remain highly suitable for P. absoluta to thrive. The same trend is projected for the year 2070 under both SSP2-4.5 and SSP5-8.5 scenarios, where ERI will be changing from optimal to suitable or highly suitable in most parts of Africa, South America, and Asia.

Generation Index (GI) of Phthorimaea absoluta  projected worldwide using temperature-based phenology model under (A) current climatic condition, (B) future scenarios for 2050 (SSP2-4.5), (C) future scenarios for 2050 (SSP5-8.5), (D) future scenarios for 2070 (SSP2-4.5) and (E) future scenarios for 2070 (SSP5-8.5).

Some complementary data
Abdelmutalab G. A. Azrag, Francis Obala, Henri E. Z. Tonnang, Brian N. Hogg, Shepard Ndlela & Samira A. Mohamed, « Predicting the impact of climate change on the potential distribution of the invasive tomato pinworm Phthorimaea absoluta (Meyrick) (Lepidoptera: Gelechiidae)”. Sci Rep 13, 16477 (2023).

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