Agriculture: climate change, study on the invasiveness of tropical fruit flies

The climate in most of the Mediterranean, the rest of the EU and North and Central America is currently not suitable for the establishment of tropical fruit flies, although their potential geographical distribution varies significantly. These are the findings of international research[1] involving institutions from four different continents, including ENEA, which studied the effects of climate change on the invasiveness of these insects.

"Due to climate change, the climatic suitability for their establishment will increase in various ways depending on the species, and the likelihood of invasiveness is also increased by globalisation. This information was not available until now and at most was a simple hypothesis”, explains Luigi Ponti, co-author of the study and researcher at the ENEA Laboratory of Sustainability, Quality and Safety of Agri-Food Production.

Models were used to perform the research,[2] which for the first time described and simulated in detail the biology of each of four important tropical fruit fly species in the Tephritidae family[3], making it possible to forecast how these species differ in geographical distribution and invasive potential.

"For the first time, we were able to simulate the physiology and population dynamics of these four insects in relation to weather conditions and climate change scenarios[4] with a spatial resolution of 25-30 km and a daily temporal resolution never achieved before", adds the ENEA researcher.

Of the insects detected in America and the European Union, fruit flies are considered to be one of the invasive species with the greatest economic impact on agriculture. In California, for example, the damage suffered by the fruit and vegetable sector to date amounts to some 25 billion dollars, not counting the costs incurred by the quarantine or eradication programmes for these insects, which average around 30 million dollars each, but can be as much as 100 million, as was the case in this state between 1980 and 1981. In addition to the high costs, the effectiveness of these interventions is increasingly limited because they are conducted without a sound scientific basis that would allow for a better understanding of the actual and potential threat posed by these insects.

“The ability to predict the evolution of areas favourable to the development of tropical fruit flies under climate change scenarios is crucial, so much so that some of the species we have studied are already included in the European Food Safety Authority's (EFSA) recent list of the top 20 quarantine organisms for EU Member States based on the economic, social and environmental problems they can cause.[5] The species surveyed represent a very serious threat to a strategic sector such as Mediterranean fruit and vegetable cultivation, particularly for perennial fruit crops such as stone fruit (e.g. peach and plum), pome fruit (e.g. apple and pear) and citrus, but also for annual crops such as melon, cucumber and tomato", Ponti explains.

The biology of tropical fruit flies is apparently similar, but in reality – as the study showed – it varies enough to result in different responses to weather conditions, as well as to hosts. And it is precisely these differences that determine their potential for geographical distribution and abundance. Temperature is the main guiding variable, with relative humidity also affecting adult survival and reproduction. "Thanks to the mathematical models we use, we are able to precisely estimate the effects of temperature and humidity on development and mortality rates at all stages (egg, larva, pupa and adult) and on their reproduction, using the same basic conceptual model that facilitates and makes more efficient the identification and collection of biological data needed for risk assessment by invasive species", the researcher points out. This technology has been made available through a long collaboration between ENEA and the laboratory of Professor Andrew Paul Gutierrez at the University of California, Berkeley. By projecting the very biology of these insects into the future climate, this technology allows both greater reliability of results than the statistical correlation between the presence of a species and the climate observed at a given location (the most widely used method for estimating the risk from invasive species), and the possibility of also evaluating possible control and eradication strategies at a regional level.

The Mediterranean basin is one of the regions of our planet most prone to climate change, and consequently to the establishment and spread of harmful exotic insects due to the increase in temperature that makes the Mediterranean climate more similar to tropical zones. Estimating the geographic distribution and abundance of invasive species is crucial for developing policies to manage them. Thus, the need to obtain such estimates has increased significantly in these countries. With regard to the Mediterranean fruit fly and the melon fly, the international study predicts a higher prevalence in Egypt and the Nile Delta, although for the latter species most of the Euro-Mediterranean region could be unfavourable due to the expected decrease in rainfall, up to 40% in some areas of the Mediterranean, as humidity levels affect its reproduction. In addition to the Nile Delta, the Oriental fruit fly is also expected to increase in the south-west of Spain and in Israel. The Mexican fruit fly, on the other hand, has a greater potential presence in Morocco, the North African coast, southern Portugal and Spain, but also in some areas of Sicily and southern Italy and in Crete.

In general, in the Euro-Mediterranean region the study predicts a limited spread of tropical fruit flies northwards due to the cold weather and in Saharan Africa due to the warm climate, low relative humidity and lack of host plants. "Most of the temperate areas where we conducted our study are currently not climatically suitable for the establishment of these insects, but climatic suitability is expected to increase in some areas due to climate change, increasing their spread also due to globalisation. To meet this challenge, which is still ongoing, investment is needed to collect valid biological data to develop more models that assess the geographical distribution and abundance of these and other invasive species that are also dangerous to human health, and thus provide a solid scientific basis for control and eradication policies", concludes ENEA's Luigi Ponti.

Invasive species cause around ten times more economic damage each year worldwide than natural disasters, and their numbers are set to increase due to the dual effects of climate and globalisation. An example of this is the numerous harmful tropical insect species that have recently become established in the Mediterranean basin, such as the tomato moth (Tuta absoluta), a subtropical species of South American origin that arrived in Spain in 2006, already resistant to most commercial insecticides, and which in just a few years has travelled some 4,000 km, reaching every corner of the Mediterranean basin.

Luigi Ponti, ENEA - Laboratory of Sustainability, Quality and Safety of Agri-Food Production, 

Gutierrez, A.P., Ponti, L., Neteler, M., Suckling, D.M., & Cure, J.R. (2021). Invasive potential of tropical fruit flies in temperate regions under climate change. Communications Biology, 4(1), 1–14. https://www.nature.com/articles/s42003-021-02599-9

[1] The sponsors of the study are: ENEA (Italy); CASAS Global (Center for the Analysis of Sustainable Agricultural Systems), Kensington, and Division of Ecosystem Science, College of Natural Resources, University of California, Berkeley, (California, USA); mundialis GmbH & Co. KG, Bonn (Germany); The New Zealand Institute for Plant and Food Research Ltd., Christchurch, and School of Biological Sciences, The University of Auckland, Auckland (New Zealand); Facultad de Ciencias Básicas y Aplicadas, Universidad Militar Nueva Granada, Bogotá (Colombia).

[2]  The project used physiologically based demographic models (PBDM), made available by CASAS Global in the context of geographic information systems (GIS) to develop an integrated tool for a dynamic understanding of complex agroecosystems undergoing global change. The basic idea of the PBDM approach is that all living organisms can be considered as consumers, with similar problems of resource acquisition (input) and allocation (output), which allows each organism in the food chain to be described by analogy using the same mathematical and conceptual model.

[3] The Mediterranean fruit fly (Ceratitis capitata), the melon fly (Bactrocera cucurbitae), the Oriental fruit fly (Bactrocera dorsalis) and the Mexican fruit fly (Anastrepha ludens).

[4] IPCC climate scenarios RCP8.5 and A1B.

[5] https://www.efsa.europa.eu/it/news/priority-plant-pests-eu-5-things-you-need-know