© 2004 Plant Management Network.
Methyl Bromide Provides Phytosanitary Security: A Review and Case Study for Senegalese Asparagus
Erica E. Davis, Office of International Agricultural Programs, and Robert C. Venette, Department of Entomology, University of Minnesota, St. Paul 55108
Davis, E. E., and Venette, R. C. 2004. Methyl bromide provides phytosanitary security: a review and case study for Senegalese asparagus. Online. Plant Health Progress doi:10.1094/PHP-2004-1122-01-RV.
After the United States passed the Africa Growth and Opportunity Act in May 2000, Senegal sought to initiate exports of horticultural commodities, including asparagus (Asparagus officinalis L.), to US markets. Asparagus potentially can host many pests that are not known to occur in the US, thereby potentially necessitating quarantine treatment (3). In Senegal and neighboring West African countries, fresh asparagus may be attacked by 50 different insects and diseases. Seven of the insect species are of quarantine significance (Table 1). For all pests of concern, eggs and larvae are likely to be found on the surface of the asparagus spear.
Table 1. Seven insect species of quarantine significance to the United States found in fresh asparagus from Senegal and neighboring West African countries (8).
The risks posed by the quarantine pests associated with Senegalese asparagus depend upon the likelihood of those insects becoming established and the subsequent severity of the impacts they may have (26). Agrotis segetum, H. armigera, and S. littoralis are high-risk pests that are highly likely to become established and to have substantial adverse effects on economically important crops and environmentally sensitive plants if the pests are introduced into the US (8). Icerya aegyptiaca, M. hirsutus, N. viridis, and S. aurantii are medium-risk pests that are highly to moderately likely to establish in the US, but are predicted to have only moderate impacts if introduced (8). Previous documents have addressed the risks posed by several of these pests (7,19,23,24,25,29,30).
The US Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) regulates by law all fresh produce imported into the US. The first step of the regulatory process is a Pest Risk Assessment (PRA) conducted by the USDA-APHIS to evaluate whether a commodity can enter the US with or without quarantine treatment, or must be denied entry until quarantine treatment is available. A PRA documents all known pests associated with a commodity in the country of origin and evaluates the potential for, and consequences of, establishment of those pests in the US. Overall, a PRA provides the basis for determining whether a commodity can be safely imported, or under what conditions it can be imported to eliminate the potential for introducing exotic pests. The USDA provides specific guidelines to evaluate risks of plant pests associated with the importation of various commodities (26).
The second step in the regulatory process is a proposed rule change to amend quarantine requirements, including PRA and available quarantine treatment data, which is published in the Congressional Federal Register (CFR) for a (usually three-month) comment period. If no adverse actions develop and all comments are adequately explained or rebutted, the comment period is followed by a final rule, also published in the CFR, that outlines all regulatory requirements for importing the commodity, including any quarantine treatment schedules. The regulatory requirements are, effectively, laws, and they cannot be changed in any way without going through the regulatory process again.
The purpose of this paper is to: (i) briefly review the use of methyl bromide as a quarantine treatment, (ii) describe how the USDA-APHIS currently uses methyl bromide for imported asparagus, and (iii) explore whether methyl bromide treatment schedules might be adequate to control pests of asparagus from Senegal without adversely affecting the quality of the produce.
Use of Methyl Bromide as a Quarantine Treatment
Methyl bromide has been considered an effective, broad-spectrum, fast-acting fumigant to control many arthropods and pathogens for more than 60 years (9), and has been used to control pests of soil, stored products, and structures, and for quarantine treatments. Methyl bromide is highly toxic to vertebrates and is an ozone-depleting gas. The "Montreal Protocol on Substances that Deplete the Ozone Layer" was passed by the United Nations in 1992 and requires developed nations to reduce most uses of methyl bromide by 2005 and eliminate most uses by 2015 (9,22). However, restricted use of methyl bromide for quarantine purposes is exempted from the Montreal Protocol for the present. Alternatives to methyl bromide are being sought because the uses of methyl bromide are being or have been phased out, and the availability of methyl bromide for other uses, such as quarantine treatment, may become limited or too expensive (9,22). Nevertheless, the use of methyl bromide for quarantine treatments likely will continue into the foreseeable future.
Methyl bromide has been used as a treatment against numerous quarantine pests found in the Diaspididae (1,21,31), Noctuidae (5,16,17), Pseudococcidae (4,6,10,14,15,20,28), Tortricidae (12,19,32,33,34), and Thripidae (6,13) on a variety of commodities from cut flowers and propagative materials to fresh fruits and vegetables. Historically, methyl bromide has been applied to a commodity as a gas, primarily because treatments with gas are effective and expeditious (9,18).
Imported Asparagus and Quarantine Regulations
The Plant Protection and Quarantine (PPQ) division of the USDA-APHIS regulates and supervises methyl bromide quarantine treatments in the US and overseas. Asparagus currently can be imported into the US from more than 50 countries worldwide (Fig. 1) (27), although the conditions for entry from different countries may vary. For example, asparagus imported from Tunisia and Morocco is allowed entry only into North Atlantic ports, presumably because the climate in this region of the US is less suitable for associated pests. Fumigation of Tunisian or Moroccan asparagus is not a mandatory condition for entry (27).
The USDA-APHIS-PPQ currently requires that asparagus imported from Australia, New Zealand, Thailand, or Peru be fumigated with methyl bromide as a precondition for entry into the US (27,28) to eliminate potential infestations of Halotydeus destructor (Acarina: Penthaleidae) from Australia and New Zealand, Scirtothrips dorsalis (Thysanoptera: Thripidae) from Thailand, or Copitarsia spp. (Lepidoptera: Noctuidae) from Peru (28). However, asparagus from Australia and New Zealand does not have to be fumigated if the regulatory agency in the country of origin certifies that the asparagus was grown in an area free of H. destructor. Methyl bromide fumigation is required for asparagus if quarantine pests are found during inspection, regardless of the country of origin, and methyl bromide fumigation has proven effective in controlling the detected pest (3).
Four treatment schedules specified by the USDA-APHIS-PPQ are pertinent to the importation of fresh asparagus (28). Each treatment schedule mandates the minimum concentrations of methyl bromide fumigant over a 2-h interval to control specified insects within different temperature ranges (Table 2). In general, the prescribed concentration of fumigant increases as temperatures decrease. To control thrips and noctuids on asparagus, PPQ treatment T101-b-1 is required, except for asparagus from Thailand, which may harbor S. dorsalis. To control S. dorsalis on asparagus from Thailand or H. destructor on asparagus from Australia or New Zealand, treatment T101-b-1-1 is specified and requires greater concentrations of methyl bromide than are used against noctuids and other thrips. Fumigation for S. dorsalis and H. destructor must take place at air temperatures > 15.6°C. Two additional methyl bromide fumigation treatments recently have been developed that are "pest specific/host variable" (28). These PPQ treatments are designed to control targeted groups of insects, regardless of the host with which they are associated; however, elements of this treatment are modified slightly for different host plants to ensure that the quarantine treatment complies with usage requirements described on the methyl bromide label (28). For insects that are loosely associated with a commodity (i.e., are "hitchhikers") or that feed on the outer surface of a plant (specifically, "thrips, aphids, scale insects, leafminers, spider mites, lygaeid bugs, ants, earwigs, and surface-feeding caterpillars"), PPQ treatment T104-a-1 applies (Table 2). For asparagus, this treatment is identical to the protocol for the control of noctuids and most thrips (i.e., T101-b-1). For mealybug control, PPQ treatment T104-a-2 is appropriate and is identical to the protocol for control for S. dorsalis and H. destructor (i.e., T101-b-1-1).
Table 2. Concentrations of methyl bromide prescribed at different temperatures to control insect pests of imported asparagus from admissible countries [reproduced with minor modification from the APHIS-PPQ "Treatment Manual" (28)]. Initial dosage and concentrations at 0.5 and 2 h must be attained to satisfy treatment requirements.
a Methyl bromide treatment schedule, T101-b-1
b Methyl bromide treatment schedule, T101-b-1-1
c Methyl bromide treatment schedule, T104-a-1; only those requirements applicable to asparagus are presented
d Methyl bromide treatment schedule, T104-a-2, only those requirements applicable to asparagus are presented
Methyl Bromide Injury on Asparagus
Damage to commodity quality caused by methyl bromide fumigation varies depending on the commodity or cultivar fumigated (11). The kinds of damage methyl bromide can cause to asparagus includes discolored lesions, decay at the ends of the spear, or off-flavors. Moderate to high doses of methyl bromide (44 to 48 g/m3) may adversely affect asparagus quality and palatability (2). Methyl bromide does not seem to cause unique damage to asparagus, but seems to accelerate natural degradation processes, perhaps through increased rates of respiration and ethylene production (2). Regardless, damage may not be evident for a week or more after fumigation, so rapid distribution of asparagus may lower the influence of fumigation on its marketability (2).
Several insect species that are not known to occur in the United States attack asparagus in Senegal. Of these, seven species have been identified that could remain associated with spears of asparagus during international trade. Thus, potential importation of asparagus from Senegal would provide a new pathway for the introduction of these pests into the US. Senegalese asparagus currently is not admissible into the US. This status is unlikely to change unless appropriate management steps can be identified, steps that would lower the risk of pest introduction to acceptable levels.
Potential quarantine pests of concern on Senegalese asparagus may be classified generally as noctuids, thrips, mealybugs, or scales. Currently, four PPQ treatment schedules involving methyl bromide fumigation are designed to control these insects on asparagus and provide quarantine security (Table 2). Thus, workable quarantine treatments exist to safeguard against the introduction of pests on asparagus from Senegal.
The impact of moderate to high concentrations of methyl bromide (particularly concentrations associated with PPQ treatments for the control of S. dorsalis, H. destructor, and mealybugs) on the quality of certain asparagus cultivars is not known. Further testing to assess the potential damage caused by methyl bromide to improved cultivars of asparagus should be sponsored by the Senegalese government or US importers. Although such information is important to develop and maintain markets in the US, it is not essential to maintain phytosanitary security.
Mandatory fumigation of Senegalese asparagus should not be a prerequisite for importation of this commodity into the United States. USDA-APHIS officers currently inspect shipments of commodities for contaminating pests and maintain long-term records of pest interceptions. In accordance with existing regulations, Senegalese asparagus should be fumigated any time pests are observed. However, should pests be found in numerous shipments from Senegal, routine fumigation may be necessary. To prevent this unfortunate outcome, we encourage Senegalese researchers to develop a series of complementary pest-control tactics for exported asparagus that would help preserve the phytosanitary security of the US.
The authors thank Steven A. Clarke (University of Minnesota) and the US Agency for International Development (Grant No. HFM-G-00-02-00041-00) for making this project possible.
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