Fumigation is a method of pest control involving the injection or incorporation of a chemical (a fumigant) that turns into a gas in the soil to reduce pathogens including nematodes and weed seeds before planting. Some fumigants may also impact other organisms in the soils such as overwintering insects. Fumigation is expensive and only performed on high-value crops with significant soil-borne pest issues such as tobacco, ginseng, root vegetables, tomatoes and strawberries. It is occasionally used on other crops such as tree fruit and nurseries when there are significant disease issues in the soil.
Fumigants are formulated as granules, liquids or gas, but all convert to gas when injected into the soil. Because they move through the soil as a gas, they are highly volatile and require stringent adherence to label instructions to ensure they do not prematurely escape the soil, which can cause human safety and environmental issues and reduce efficacy of these expensive products.
Fumigants vary in their efficacy against different soil pests. Optimal choice of fumigant will depend on the crop, the pests of concern and the cost relative to the value of the crop. There are also different methods of application that vary in their efficacy and cost. Methods of application are:
Most crops requiring fumigation use this method. It involves injecting or incorporating fumigants only in a band under the row where the crop will be planted. This provides protection to the developing roots in the bulk of the root zone, while reducing the cost by not fumigating the areas between the rows or beds. The downside to banded applications is the potential that the pathogen can spread from the untreated area into the fumigated area. For example, fast growing fungi such as Rhizoctonia may re-colonize the treated area rapidly after banded fumigation. Also, if the crop is not planted directly over the treated bands, there may be sections of plants within the row that are unprotected.
Broadcast fumigation involves treating the entire field rather than just a band under the row. Despite the name, most fumigants are still injected into the soil, but shanks are spaced at even intervals over the whole area. While broadcast fumigation is more expensive, it is more suitable for crops without distinct rows such as ginseng and strawberry crown production.
Whether broadcast or banded, most fumigants are injected either under the row or at even intervals across the application equipment using a shank to slice the soil ahead of the injection tube. Applicators may vary the depth of the shank, or have the fumigant injected at different depths, depending on the label, the crop and the target pathogen.
Some fumigants (e.g. Basamid) come in a granular formulation that is spread on the soil and then incorporated using a rototiller. Once the granules come in contact with moisture, they convert into a gas. It is important to incorporate granular fumigants as soon as possible after broadcasting to ensure they do not convert to a gas while still on the surface. Granular fumigants are more difficult to incorporate as deep into the soil as injected fumigants, and depending on soil moisture levels, may take longer to release the gas and this can reduce the concentration of fumigant at any one time and therefore affect efficacy.
Good Fumigation Practices
Preparing for Fumigation
Regardless of the product being used or the method of application, soil preparation and proper application procedures are essential for achieving optimal product efficacy. Improper fumigant application can lead to drift issues that can have major consequences for the grower and the industry. Detailed instructions for preparing sites and applying fumigants are provided on fumigant labels. These are legal requirements that must be followed, but are also beneficial to grower because they ensure the product remains in the soil and maximizes efficacy.
Soil preparation prior to fumigant application is critical for achieving maximum control of soil‑borne pests. Fumigants cannot easily penetrate large clumps of soil and organic matter. Debris can prevent an adequate seal of the soil, allowing the fumigant to escape from the soil before it has a chance to kill pests. Soil cultivation may be necessary well before fumigation to ensure breakdown of organic matter. This is in addition to the cultivation that is necessary just prior to fumigation.
Prior to fumigation, cultivate the area to be fumigated thoroughly, breaking up clods and loosening soil deeply. If a soil crust forms between this cultivation and fumigation, lightly cultivate the soil again immediately before application of the fumigant.
Soil moisture in the top 15-20 cm must be at levels stipulated by the fumigant label prior to and during application. Typically, the soil should be moist without being close to saturation since saturated soils do not have enough air spaces to allow the fumigant to move through the soil. Exact moisture levels required are specified on the product label. Soils that are too dry can allow the fumigant to escape prematurely and prevent an adequate seal of the soil surface. If necessary, irrigate the soil before fumigation to ensure good soil moisture.
The efficacy of fumigants depends on strictly adhering to good agricultural practices including applying under the right air and soil temperatures, sealing the fumigant into the soil effectively, avoiding periods of adverse weather conditions, and waiting the appropriate length of time to open the soil and plant the crop.
High air and soil temperatures during fumigation can make the fumigant more volatile and escape the soil too quickly. Conversely, cold air and soil temperatures can prevent the fumigant from moving through the soil properly and may trap the fumigant in the soil causing damage to the subsequent crop. Generally, optimal fumigant efficacy occurs when soil temperatures at the depth of injection are between 10 and 25°C. Exact air and soil temperature restrictions are stipulated on each product label.
Do not apply fumigants during periods of dead calm or high winds as these can cause significant drift issues. DO NOT apply fumigants when a temperature inversion is occurring or is predicted to occur within 48 hrs after application is complete, as fumigant vapours may drift. For more information on how inversions affect drift of pest control products, search "inversions" at www.sprayers101.com.
Immediately after fumigation seal the soil to prevent escape of the fumigant. The best method for sealing the soil is covering it with plastic tarps. This is required for the broadcast application of chloropicrin fumigants in some crops. For other crops or fumigants, if tarping is not possible, roll, cultipack, or disrupt the chisel trace using pressed sealers or bed shapers immediately behind the fumigant applicator. Consult the product labels for sealing requirements for specific fumigants and crops. For products not applied under a tarp, light watering after application will further prevent the escape of the fumigant from the soil.
Soil should remain undisturbed for 7–14 days, depending on the product, after fumigation. This is to allow time for the product to contact the target organisms, move through the soil profile and break down to more inert chemicals. Once the tarps are removed or seal is broken, the remaining fumigant in the soil must be allowed to escape before a crop is planted. Cultivation can aid in the escape, but growers must follow label directions for post-fumigation practices. Depending on the product, the soil type and soil moisture levels, it can take over 30 days after fumigation for the fumigant to have escaped and for it to be safe to plant the crop.
Transplants can be killed or severely damaged if the fumigant is still in the soil at the time of planting. The danger of fumigant remaining in the soil is greatest in low, poorly drained areas, in heavy soils, or when soils remain cool and saturated after fumigation, especially when higher rates of fumigants are used.
For transplanted crops where planting occurs shortly after fumigation it is important to ensure that it is safe to plant before proceeding. Once all label re-entry restrictions have passed, cultivate the soil as per label instructions to aid escape of any remaining vapours. It may be necessary to confirm it is safe to transplant by transplanting a young susceptible plant (e.g. tobacco, tomato) and examine for injury after a day or two before proceeding. Re-opening the soil may be necessary if damage occurs to the test plants. See labels for specific information.
Label Requirements for Fumigants
Fumigant labels have detailed requirements for use that involve more extensive activities before, during and after application than those of most other pest control products. These requirements are intended to further limit user exposure and increase protection of workers, bystanders and the environment. General requirements are outlined below but do not replace the detailed instructions found on the most current product label.
There are some specific terms used on fumigant labels which are defined below:
Fumigation Management Plan (FMP)
A fumigation management plan (FMP) is an organized, written description of the steps involved to ensure a safe and effective fumigation and must be prepared by the applicator or site owner.
The application block is the area of the field that is being fumigated.
Fumigant Buffer Zone
A fumigant buffer zone is an area around the perimeter of the application block that extends equally in all directions. The size of the buffer zone area will depend on the product, the application method, product rate and field size and are stipulated on the product label. Note: A fumigant buffer zone is different from a buffer zone on other pest control product labels.
Application Block Period
The application block period is the time after fumigation during which entry into the application block is prohibited except by certain personnel specified on the product labels wearing specific safety equipment (e.g. emergency responders). This period begins at the start of application and expires at least 5 days after application is complete but may be longer depending on application methods.
Buffer Zone Period
The buffer zone period is the time after fumigation during which entry into the buffer zone is restricted to personnel specified on the product labels (e.g. applicators, emergency personnel). This period begins at the start of applications and expires a minimum of 48 hrs after the application is complete.
Emergency Response Plan
An emergency response plan is a section of the FMP which provides a written plan to identify procedures to respond to incidents (e.g. equipment failure, spills, sensory irritation outside the buffer zone) related to fumigation.
Any person involved in the use of fumigants is considered a fumigant handler.
The following outlines some key label requirements. Refer to fumigant labels for the complete lists.
- Complete an FMP prior to the start of any fumigant application to document all information related to the application and ensure that label requirements are met. A separate FMP is required for each application block and it must be present at the block during all fumigant handling activities.
- At least one week and no more than four weeks before the application, provide residences and businesses that are close to the buffer zone (distances specified on the product labels) with details related to the application, which are stipulated on the product label.
- Follow mandatory good agricultural practices species on the label before, during and after application, including avoiding conditions that may lead to drift (e.g. temperature inversion).
- Ensure all fumigant handlers hold an appropriate pesticide applicator certificate or license recognized by the province.
- Follow the application block period as stipulated on the label. Fumigant applicators must verbally warn workers of the application.
- Post fumigant application signs (as specified on the product labels) at all entrances to the application block prior to the start of the application but no earlier than 24 hrs prior. Note that this applies to the edge of the actual application block and not at the main entrance to the field, unless the whole field is treated. These must remain for the duration of the application block period but must be removed within 3 days of the end of that period.
- Establish the buffer zone for your fumigant application as per label instructions. Labels contain charts to identify the buffer zone distance required for the specific application method and size of field. All non-handlers, including field workers, residents and pedestrians must be excluded from the buffer zone during the buffer zone period except for transit (e.g. vehicle or bicycle). Buffer zones cannot contain any residential area (including lawns, parks, playground, trails, roadways or rights of way), land not owned or operated by the owner of the application block, or building that may be occupied during the entire buffer zone period.
- Post buffer zone signs (as specified on the product labels) along the perimeter of the buffer zone, at typical points of entry and along likely routes where people not under the owner’s control may approach the zone (e.g. at intervals along a public roadway or residential area). Buffer zone signs are in addition to application block signs. Since buffer zones must be a minimum of 8 m regardless of application method, the buffer zone sign should never be beside the application block sign.
- Buffer zones signs should be posted within 24 hrs of the start of application and must remain posted until the buffer zone period has expired.
- Follow all site monitoring requirements specified on the product labels to ensure drift is not occurring after application. Monitoring is typically required for the duration of the buffer zone period. If monitoring indicates a fumigant escape, activate the emergency response plan.
- Complete a post-application summary within 30 days of the application, which outlines and documents all information related to the application such as weather conditions, incidents, repairs, air monitoring, and any deviation from the FMP.
- Signed copies of both the FMP and the post-application summary must be retained by the fumigant applicator and site owner/operator for 2 years.