Irradiation can fight Mango Weevil infestation

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RADIATION can fight the mango pulp weevil (MPW) or Sternochetus frigidus in mango, which can help the Philippines pass the quality standards for the United States market for tropical fruits, according to government researchers.

The Philippine Nuclear Research Institute (PNRI) of the Department of Science and Technology and the Department of Agriculture-Regional Field Office 4B (DA-RFO-4B, Mimaropa) jointly conducted the study on using irradiation to combat the MPW.
Researchers Glenda Obra and Sotero Resilva of PNRI and Louella Rowena Lorenzana of DA-RFO 4B undertook the tests and study.

The current treatment for mangoes for the export market is vapor heat treatment (VHT) that is not effective against MPW. The adults of the pest can live for 1.5 years while the females can lay almost 800 eggs during their lifetime.

“It has been a long-term goal for Philippine mango exporters to send mangoes to the US because they pay a premium price for fresh carabao mangoes. The MPW was first reported in the Philippines in 1987 in the southernmost city of Bataraza in Palawan province,” the researchers said in their paper.


“Since 1897, the Palawan Island group has been placed under quarantine to prevent the spread of the pest. A Special Quarantine Administrative Order specifies that the movement, transfer or carrying of mango plants, fruits or parts from Palawan is prohibited,” they added.

Citing government statistics, the researchers said the area planted to carabao mangoes in 2013 was 146,425.04 hectares and production was 671,861.93 metric tons valued at P19.2 billion.

They added MPW is native to places like northeast India, Bangladesh, Myanmar, Thailand, Malaysia, Singapore, and Indonesia, including Irian Jaya.

Although the mangoes were from Brooke’s Point, they were treated at the PNRI’s Multipurpose Irradiation Facility in Diliman, Quezon City. Samples of the MPW, in various stages of growth, were also brought to the facility.

Because of the absence of an artificial diet for the mango pulp weevil, mass-rearing of the insect was done under field conditions in Palawan using developing mango fruits on mango trees as substrate.

The researchers explained a suggested dose-response of 100 Gy (or Gray, a unit of ionizing radiation) would be sufficient to prevent reproduction in adult S. frigidus. Also, confirmatory tests using 100 Gy showed that 95 eggs laid by one adult MPW failed to hatch.

“Since the efficacy of treatments was estimated based on prevention of oviposition.
Therefore, the confirmatory dose was increased to 150 Gy for the remaining three trials.

The total number of insects treated with a target dose of 150 Gy was 4,549 adults whereas 440 adults served as the untreated control. No eggs were laid by any irradiated females in the three replications or trials, indicating complete sterility,” the researchers added.

Confirmatory testing also showed 164.1 Gy was found sufficient to cause sterility on MPW.

For the control, the mean number of eggs laid per female was 510 eggs with percentage with a 95.9-percent hatchability.

The researchers said it was in 2006 that low-dose generic radiation treatments were approved for the first time by the United States Department of Agriculture-Animal and Plant Health Inspection Service. The agency approved a generic dose of 150 Gy for tephritid fruit flies and 400 Gy for all insects except pupa and adult Lepidoptera.

“Even 400 Gy generic dose is used to treat mangoes from India and Pakistan, several tropical fruits from Thailand, guava from Mexico and dragon fruit from Vietnam for export to the US,” they added.

With their findings, the researchers recommended a maximum dose of 165 Gy for mangoes exported to the United States. That level of dose, as compared to 400 Gy, will minimize quality problems associated with fruits being exposed to irradiation treatment.

The 165 Cy dose is also costs lower and requires lesser time to administer on mangoes.
Also, when compared to chemical treatments for fruits bound for the export market, irradiation leaves no toxic residues.

“Irradiation treatment does not leave any toxic residues to the commodity and is therefore safe to the consumers. Furthermore, unlike vapor heat treatment [VHT] which can only be used for treatment of fruit flies but not of MPW, irradiation can be used to control both insect pests,” the researchers said.

When it comes to cost effec-tiveness, irradiation treatment amounts to P4.20 per kilogram while using VHT costs P17.30 per kilogram.

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