Oriental fruit fly threatens corn and wheat in Brazil

The oriental caterpillar Mythimna separates, a quarantine pest absent in Brazil, has found climatically favorable areas in the country and could pose an economic risk to wheat, corn, sugarcane, rice, and sorghum crops. A study mapped the global threat posed by the species and estimated the risk to Brazilian municipalities based on climatic suitability and the economic importance of these crops.

The greatest risk falls on wheat. Among the producing municipalities, 45,8 percent fell into the moderate risk class and 48,9 percent into the high risk class in case of invasion. In sugarcane, 47,1 percent of the municipalities presented moderate risk and 17,0 percent high risk. In rice, the rates reached 28,7 percent and 8,5 percent. In corn, 29,2 percent of the municipalities were at moderate risk and 5,5 percent at high risk. In sorghum, 15,2 percent presented moderate risk and 0,3 percent high risk.

Researchers Gabriel Dorotel da Silva Ferreira and Cesar Augusto Marchioro, from the Federal University of Santa Catarina, used ecological niche modeling with the MaxEnt algorithm. The model combined records of pest occurrence, climatic variables, altitude, and agricultural production data. The analysis also incorporated a risk matrix with five classes of probability of occurrence and five classes of municipal economic importance.

Risk areas

The study identified areas of risk outside the pest's native distribution. These regions include North America, Central America, South America, Europe, southeastern Australia, and New Zealand. In Brazil, the most susceptible areas are concentrated in the South and Southeast. In these regions, there is an overlap between favorable climate and significant agricultural production.

The variable with the greatest influence on the model was the average annual temperature, accounting for 41,64 percent of the total. Temperature seasonality accounted for 20,96 percent. Altitude accounted for 19,31 percent. Together, these variables explained most of the potential distribution of Mythimna separates. Suitability increased up to about 15 degrees Celsius and decreased at higher temperatures. The model indicated greater suitability at altitudes below 500 meters.

Crop damage

The species causes damage to widely distributed crops such as corn, rice, sorghum, sugarcane, and wheat. According to researchers, its migratory capacity increases the risk of dispersal after an eventual entry. Radar studies cited by the researchers show movements at altitudes of 50 to 500 meters, with speeds of 4 to 12 meters per second and flights of about ten hours per night. This pattern allows for movements of up to 144 kilometers per night.

Global analysis showed a large exposure of agricultural areas. In rice-growing areas, 35 percent are located in moderately to highly suitable regions. For corn, the rate reaches 39 percent. For wheat, it reaches 46,9 percent. In Brazil, 90,7 percent of rice-growing areas and 36,2 percent of sugarcane-growing areas are located within climatically suitable zones for the pest.

Researchers calculated the Normalized Concentration Index to measure the economic relevance of each crop in the municipalities. For wheat, 36,6 percent of the 1.016 producing municipalities showed moderate to very high dependence. For sugarcane, 22,9 percent of the 3.169 producing municipalities fell into these categories. For corn, 10,0 percent of the 5.110 municipalities showed moderate to high dependence. For rice, the index reached 7,1 percent of the 1.665 municipalities. For sorghum, it reached 6,5 percent of the 644 municipalities.

Priority for surveillance

Economic risk maps indicate priority for phytosanitary surveillance at entry points and municipalities with greater dependence on host crops. The study cites targeted inspection of agricultural products and residues, trap networks in priority municipalities, standardized notification procedures, and contingency plans at the municipal level.

The research compares the situation with invasions already recorded in Brazil. Helicoverpa armigera entered the country and already had a wide distribution when it was first registered in 2013. Eradication became unfeasible. In another case, Cydia pomonella was detected early and underwent a coordinated eradication program, with traps, host removal, and local management.

According to the researchers, municipal-scale maps can guide the allocation of surveillance resources. Integrating climate risk, likely routes of introduction, and the economic value of crops allows for the anticipation of priority areas for prevention against Mythimna separates.

Further information at doi.org/10.1002/ps.70954