Pest species identified ranged from little known pests of limited worldwide distribution, through to pests such as Prostephanus truncatus and some Trogoderma species which are of quarantine concern generally, the presence of which requires complex management procedures. As well as being pests associated with grain, all have the potential of establishing in natural habitats. Comments have been made in the data sheets (Appendix 1: Biological Assessment of Arthropod Pests Associated with Stored Maize Grain and Admixture Grain Commodities and Arthropod Pests Known to Vector Maize Diseases in North America) as to some possible adverse consequences that introduction of these pests may have to the natural environment. Once established in natural habitats, official control and eradication is likely to be difficult.
Information on the status and distribution of important insect pests of stored grain is relatively reliable both in North America and in Australia, allowing a reasonable comparison to be made between the two faunas. However, in comparison, knowledge of many mould-feeding and minor genera is limited. Insufficient information is available to say if such species known to occur in North America are present in Australia. Some mould feeders can survive for substantial periods in clean, dry grain but are unlikely to be able to feed or reproduce in it; these species were included in the analysis.
A wide range of incidental insects can also be harvested along with grain. These form a sample of the local fauna and may include many species not found in Australia. The likely species involved are impossible to predict. Most of these incidental insects are unlikely to survive for significant periods in grain in storage, especially if it is clean with minimal admixture. No attempt was also made to assess risks associated with parasites and predators that can be associated with pest species. Nevertheless, measures that effectively control arthropod pest species can be expected to control any species associated with them.
Most major economic pests of stored grain with the exception of those identified above (List 1a, b, c & d), are common to both North America and Australia. While this may be so, genotypes of a given species may be different in either continent. Strains in one place may be more resistant to pesticides and fumigants than elsewhere. Importation of such strains could cause problems with using control treatments. Currently, there is no information regarding strains of major storage insects present in the USA and Canada that are significantly more tolerant to pest control treatments than those known to occur in Australia. However, this may be due to lack of data as survey results in USA and Canada, particularly for phosphine resistance, are rudimentary. In the absence of data and because of the widespread use of phosphine fumigation in the USA it should be assumed that some degree of phosphine resistance is likely to be present, at least in common stored product pests. Dosages will need to be targeted accordingly if phosphine is chosen as a disinfestant.
Our knowledge of the Australian mite fauna, native and exotic, associated with stored products is incomplete and there are no in-depth recent surveys. It is not possible to assert that a given mite, not currently recorded here, is not present in Australia. No mite species listed by the USDA key (Smiley, 1991) and not recorded to date in Australia is known to be significantly destructive to well-stored grain. No assessment can be made as to the potential environmental impact of mites likely to be associated with stored maize, though some are likely to become established outside of grain stores, if not already present. Well-managed clean, dry grain is unlikely to contain significant numbers of mites.
No specific references were found concerning snails as an agronomic problem associated with trade in maize grain in the USA and Canada. No species are listed as storage pests by Godan (1983), although field slugs are mentioned as causing considerable damage to young maize plants. Snails may however be harvested as an incidental contaminant. As such they are likely to form a sample of the local fauna and may include species not found in Australia. Information does not appear to be available as to the ability of such species to survive in stored grain. Experience with the importation of maize grain from the USA in 1995 indicates that the risk of importation of molluscs is low.
5. PROPOSED PEST RISK MANAGEMENT OPTIONS
The final phase of the PRA process is the allocation of appropriate risk management options to address the pest risk identified within the pathway. Risk of importing these pests can be minimised by a combination of management methods, which include:
Selection of grain from areas free of pests (Area Freedom)
Prevention of infestation during transportation, storage and handling
The following comments are provided for the information of the Operational Procedures Technical Working Group and the Risk Analysis Panel.
Risk of infestation increases with decline in grain quality, measured in terms of its physical condition (eg. % brokens, immature or mouldy grains), temperature and moisture content, and extent of admixture of trash and other material. Many insect species find it much easier to become established in grain consignments containing admixture and damaged grains. Risk of importation of species identified as of quarantine concern to Australia, with the exception of Caulophilus oryzae and Prostephanus truncatus that attack whole grains, would be reduced if only high grades, grain in good condition with minimal admixture, was imported. Grain moisture content should be less than 14%, which is independent of grade. A number of species including C. oryzae and Glischrochilus spp., are adversely affected by low moisture content. Complete removal of admixture of pulses from maize reduces the risk of species from List 1c being imported to negligible levels.
Sieving and grain cleaning will remove most snails and other incidental contaminants. It may however be difficult to remove such contaminants that are of similar size and density to maize grains, such as pulses.
Lower grades of maize are notoriously difficult to fumigate as regions of bulk cargo can be very high in trash and fines – this material tends to segregate during handling and transport of the grain and forms pockets and layers through which fumigants may have difficulty passing. This results in non-uniform distribution of gas and an increased risk of fumigant survivors. These problems are compounded if fumigation is undertaken in-ship. Clean grain is much easier to fumigate properly.
Selection of grain from areas free of pests (Area Freedom)
Several species identified as of quarantine concern to Australia appear to have restricted distributions in the USA. Caulophilus oryzae, Prostephanus truncatus and Cathartus quadricollis appear to be restricted to southern states with P. truncatus and C. quadricollis at least being much more widely distributed in Mexico. If it is possible to guarantee the source of grain, obtaining it from more northerly areas will reduce the risk of importation of these species, although it will not completely eliminate the risk. Other species identified as of quarantine concern however, are appear to be widely distributed and it will not be possible to identify maize producing regions free of these pests. In general however, infestation pressure declines as one moves into more northerly grain growing areas. If maize is to be sourced using the principles of area freedom, this will require detection, monitoring and delimiting surveys for pests of quarantine to be carried out annually, also the dedication and monitoring of rail cars. This is unlikely to be commercially acceptable in the USA as this is not normal practice.
Prevention of infestation during transportation, storage and handling
A number of species identified of quarantine concern, notably Cryptolestes turcicus, and the Tribolium and Trogoderma species, are not host specific and can be pests infesting residues present in grain handling systems. Such species can infest maize grain when handled through contaminated facilities. Use of well managed handling and transportation systems will reduce this risk. Fumigation is a non residual treatment and will not confer protection of the grain during subsequent handling and transportation.
Ships used for the importation of maize need to be ‘fit for purpose’. Vessels can become infested with insects of quarantine concern from previous cargoes and not necessarily only those associated with maize. This could include species which are not established in North America including the khapra beetle, Trogoderma granarium. Prior to loading grain, ships must be clean and free of infestation, at least to the standard expected of vessels which handle Australian grain exports. This includes not only the hold, but all other areas of the vessel including crew quarters and engine room and related areas from which infestation could arise.
There is little or no data available on the effects of fumigants, contact insecticides or other control measures on most of the pests identified as of quarantine concern. Nonetheless, most are unlikely to be more tolerant than Tribolium castaneum with methyl bromide (Bond, 1984), Sitophilus oryzae with phosphine (National Working Party on Grain Protection, 1997) or Rhyzopetha domininca with heat (Banks & Fields, 1995), these being the most tolerant pests that the Australian dosage rates are aimed at. Exceptions are likely to be Trogoderma species, as larvae in diapause, which are exceptionally tolerant of methyl bromide (Rees & Banks, 1998) and also species in the family Bruchidae, which can be exceptionally tolerant of phosphine and many contact insecticides (National Working Party on Grain Protection, 1997). Resistance status is unknown of all these pests from North America.
We also note that ship fumigation is an uncertain process and is most unlikely to be carried out to a standard required to give kill to a level expected for quarantine purposes. It is extremely difficult to ensure good gas distribution in the hold or any other part of a ship, even if the ship is stationary. The problem is further compounded if any bulk commodity being fumigated contains a significant quantity of fines, trash and admixture; a common component even of high grades of maize.
The normal practice used by the USA for grain shipments is for grain to be treated with phosphine at US label rates as an in-ship treatment for the duration of the voyage. This methodology is not considered adequate for quarantine purposes due to difficulties in obtaining and assessing appropriate distribution of gas.
Processing maize prior to shipment can reduce risk of importing the identified pest species. Risk of importing species, eg. Caulophilus oryzae and Prostephanus truncatus that require whole grain, can be much reduced by milling and processing the grain. Other species present may be eliminated by the insecticidal nature of such processing. However, once the product has cooled, some species identified as of quarantine concern could reinfest, notably Cryptolestes turcicus, and the Triboilum and Trogoderma species. Therefore, if this option adopted, continued security from reinfestation must be assured.
Heat can be used for the processing or devitalisation of grain and may be insecticidal. Temperatures above 50C are insecticidal, and becomes rapidly more insecticidal as temperature increases above this. All storage pests are killed by a few minutes actual exposure to either wet or dry heat of 65C (Fields, 1992; Banks, in press). Time allowance needs to be made for the heat to penetrate the grain kernel to this temperature.
Banks, H.J. (in press) Prospects for heat disinfestation. Proceedings of the Australian Postharvest Technical Conference, Canberra, May 1998.
Banks, H.J. & Fields, P. (1995) Physical methods for insect control in stored-grain ecosystems, pp. 353-409 In: Jayas, D.S., White, N.D.G., Muir, W.E. (1995) Stored-Grain Ecosystems. Marcel Dekker: New York
Bond, E., J. (1984) Manual of fumigation for insect control. FAO Plant Production and Protection Paper No. 54. FAO: Rome
FAO (1995). Reference Standard, Principles of Plant Quarantine as Related to International Trade. ISPM Pub. No. 1, Rome.
FAO (1996). Part 1 - Import Regulations, Guidelines for Pest Risk Analysis. ISPM Pub. No. 2, Rome.
Fields, P. G. (1992) The control of stored-product insects and mites with extreme temperatures. Journal of Stored Product Research 28: 89-118
Godan, D. (1983) Pest Slugs and Snails, Biology and Control. Springer-Verlag : Berlin, 445 pp.
National Working Party on Grain Protection (1997) Appendix 1. Draft guidelines for treatment of stored grain 1997/98, residual grain protectants for grain within or which may enter the central handling system. Brisbane: Australia
Rees, D.P. & Banks, H.J. (1998) The khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae), a quarantine pest of stored products: Review of biology, distribution, monitoring and control. A report written for AQIS, Canberra, Australia.
Smiley, R. L. (1991) Mites (Acari) in: Gorham, J. R. Ed (1991) Insects and Mite Pests in Food, an Illustrated Key. USDA Agriculture handbook No. 655, Washington DC, USA, Vol. 1, 3 - 44.