Ground Cover Supplement : GC Supplement - Biosecurity
9 BIOSECURITY GROUND COVER ON-FARM BIOSECURITY HIGH LEVELS OF phosphine resistance in stored grain pests are not new and are not declining. As early as 1992, high levels of resistance to phosphine were reported in India and Bangladesh. In addition, some level of resistance had already been detected in insects in Australia. At that time the level of resistance was not regarded as a threat to control. Nearly 20 years on, with multiple species of grain pests found to have strong resistance, the threat of losing control of stored grain insects through fumigation with phosphine is real. Resistance levels and frequencies The GRDC has been supporting a phosphine-resistance monitoring program since 1992-93 (Figure 1, page 10), most recently through the Cooperative Research Centre for National Plant Biosecurity's Post Harvest Integrity program. At this time the frequency of weak resistance in the GRDC's northern region (northern NSW and Queensland) was already high, with about 50 per cent of samples tested containing resistant insects (across five species being tested). Briefly, the method used has been to collect samples of insects from farms, central storages and grain merchant premises. These insects are then subjected to a 'discriminating dose' of phosphine that separates resistant from susceptible insects, followed by a second dose that separates known moderate or 'weak' resistance from higher levels of resistance. During the 1990s, the frequency of resistance steadily increased in all species as the grains industry relied more and more heavily on phosphine. The most rapid increase in frequency was in the lesser grain borer and in 1997 a new level of resistance never seen before in Australia was detected in this species. Then in 2000, this stronger resistance was also detected in two additional species: the rust-red flour beetle and the sawtoothed grain beetle. In 2007, strong resistance was detected in the flat grain beetle and earlier this year in the rice weevil. Resistance in the flat grain beetle is at an extremely strong level and we are concerned that this will threaten control with phosphine. Two-gene resistance Detailed research on resistance in the lesser grain borer indicated that two major genes control phosphine Resistance goes from strength to strength Resistance monitoring is showing that if phosphine is to remain effective, growers must only fumigate in sealable silos By Pat Collins resistance and that both genes need to be present to express the 'strong' resistance. Individually, each gene produces a relatively weak resistance, but when found together in the same insect they work jointly to produce a very high resistance. We also found that the resistance is close to recessive. This means that insects with one copy of the resistance (heterozygote or 'carriers') carry the gene but do not express the resistance strongly. So for resistance to be fully expressed, an individual must have both copies. For strong resistance to be expressed an insect must be homozygous for both genes, that is, it must have both copies of both genes. How is phosphine resistance selected? When resistance genes first appear in a population they are rare and 'resistant' individuals carry only one copy. These insects have only a low level of resistance but this is enough to enable them to survive low doses of phosphine. Low concentrations occur when phosphine is applied in unsealed silos. The gas concentration is probably high enough to kill many susceptible insects but it allows heterozygotes to survive. These heterozygotes then become relatively more common and eventually they breed with each other and produce homozygous resistant insects. Further fumigations with low concentrations continue to purify the insect population Fumigation of grain in unsealed silos is not reliable and is leading to an increased number of storage pests with phosphine resistance. IF STRONGLY RESISTANT FLAT GRAIN BEETLE BECOMES COMMON ON FARMS, PHOSPHINE FUMIGATIONS WILL INCREASE TO AT LEAST 20 DAYS.
GC Supplement - Capacity building
GC Supplement - Grain storage