BAR TODAY || Official Quarterly Publication of the Bureau of Agricultural Research

Grain borers are becoming resistant to pesticides
by Junelyn dela Rosa

January-March 2002
Volume 4 No. 1

A lesser kind of grain borer (Rhyzopertha dominica Fabricius) is becoming resistant to phosphine - a type of fumigant or substance that is used to disinfect stored grains and protect them from pests.
Phosphine has been popularly used since the 1980s because it is easy to apply and does not leave any harmful residues in the grain.

Rhyzopertha dominica Fabricus

The first signs of resistance were recorded in the early 70s but the resistance levels then were low to pose a serious problem. Today, however, there are widespread reports of higher phosphine resistance in some countries, which resulted to more researches on grain borers and phosphine resistance.

A survey in 1995 indicated that R. dominica strains that are phosphine-resistant are present in the Philippines. The survey used the resistance method recommended by the Food and Agriculture Organization (FAO). It involves exposing adult grain borers to a phosphine dose of 0.08 mg/ liter phosphine for 20 hours at 25 degrees centigrade and 70 percent relative humidity.

To check whether these results have changed recently, a research team composed of Ms. Miriam Acda from the Bureau of Postharvest and Research Extension (BPRE) and Messrs. Mervyn Bengston and Gregory Daglish of the Queensland Department of Primary Industries (QDPI) tested three strains of grain borers for phosphine resistance.

The team found that two out of the three R. dominica strains are resistant to phosphine. These strains were the ones taken from Cebu and South Cotabato. On a positive note, the strain from Manila was found susceptible to phosphine. All the adult grain borers were killed at a phosphine rate of 0.03 mg/liter.

The eggs and pupae of this grain borer were found more resistant than the adults. This implies that the eggs and pupae could survive the current phosphine dosage used in warehouses and storage rooms.

The researchers said that the reason why grain borers from Cebu have become highly resistant to phosphine could be attributed to substandard or faulty fumigation procedures that are being done in private and government-owned warehouses.

At present, Cebu is the trading zone for grains in the Visayas and Mindanao hence, most of the grains that are transported and stored in Cebu are fumigated there. However, there are no gas-tight storage facilities where fumigation could be effectively carried out. Lack of proper facilities, faulty fumigation practices and no resistance monitoring could be the reason for the grain borers to have developed resistance to phosphine over time.

To sustainably manage these pests, scientists recommended more studies on the nature of phosphine resistance in borers, and the development of susceptible strains through genetic analysis.

Source: Response to phosphine of susceptible and resistant strains of Rhyzopertha dominica (Fabricius) (Coleoptera:Bostrichidae) from the Philippines by Miriam Acda (Bureau of Postharvest and Research Extension), Mervyn Bengston and Gregory Daglish of the Queensland Department of Primary Industries (QDPI), Queensland.