The use of toxic baits can result in the build-up of chemical residues in soil and water, as well as bioaccumulation in animals. These build-ups can have negative impacts on non-target organisms and human health. In this section we describe several tests available for monitoring pesticide residues, contact information of laboratories that provide these services, and things to consider when designing a sampling protocol for pesticide residues.
One of the most common methods for detecting pesticide residues in samples is gas chromatography (GC) combined with mass spectrometry (MS).
The first part of this technique, gas chromatography, separates the sample (ie soil or water sample) into its component chemicals based on how volatile they are (how easily a chemical evaporates at normal temperatures). It does this by measuring how fast each chemical moves through the column dissolved in an inert gas (part of the GC machine), as more volatile chemicals move more quickly than less volatile chemicals.
The second part, mass spectrometry, identifies and quantifies (measures the amount of) the chemicals of interest based on their structure.
There are different protocols and considerations depending on if you are sampling soil or water.
A closed GS/MS machine (Polimerek, CC BY-SA 4.0, via Wikipedia Commons)
Though specialized sampling techniques are not necessary when using this method, extreme care must be taken when designing a sampling protocol to ensure accurate results.
Collected soil should be sifted through a 4 mm sieve to remove rocks and other debris. Samples must be stored immediately in lightproof containers (this can be achieved by wrapping sample containers in tinfoil) and frozen after collection to minimize breakdown of the target chemicals. Multiple replicate soil samples (ideally a MINIMUM of 20) can be combined to provide a single composite, or representative sample from the site being tested.
Things to consider when designing a sampling protocol for detecting pesticide residues in soil:
- Soil depth to sample
- Where to sample
Due to the specialized nature of pesticide residue testing, we strongly recommend consulting experts when planning your soil sampling protocol.
The first thing to decide when designing a soil sampling protocol is how deep to sample from. This will depend on how far the pesticide is likely to have moved down through the soil since it was applied. You can estimate this by knowing the pesticide’s physiochemical properties, such as water solubility (how much a chemical will dissolve in water), whether it is hydrophilic (attracted to water) or hydrophobic (repelled by water), and how it adsorbs to soil (how strongly the pesticide sticks to the outer surface of the soil particles).
If a pesticide has high water solubility, is hydrophilic and doesn’t strongly adsorb to soil then it will move deeper into the soil with the movement of water and be deposited in the lower soil layers. In contrast, if a pesticide has low water solubility, is hydrophobic and strongly adsorbs to soil, it will remain in the upper soil surface layer. Degradates (the molecules the pesticide breaks down into) tend to be more polar than their parent compounds, and are more likely to migrate deeper into the soil.
Since it is water that causes movement of pesticide residues through the soil, the amount of rainfall and the movement of water in the area that was treated also need to be considered when deciding upon sampling depth.
The soil surface layer is usually considered to be the top 5 cm. While the deeper layers are 5-10 cm, 10-15 cm, 15-20 cm etc.
Where to sample
Even deciding where to sample for pesticide residues is a complicated process. Of course you will want to sample in the area that was treated with the pesticide, but any residues are unlikely to be evenly distributed across this site. The movement of water over the soil surface also moves soil particles which can have pesticide residues attached. This results in areas of high concentration of pesticides where these particles are deposited and areas of low concentration where they were taken from.
In the case of granular ant baits, pesticide residues are likely to also be concentrated where the ants brought the granules back to their nests.
Many thanks to Grant Northcott, Plant & Food Research, for providing information on soil sampling.
Sampling water for pesticide residues is more difficult because of how quickly samples from tropical waters degrade. Water samples need to be collected in specialised bottles and refrigerated immediately after collection and the pesticide residues extracted within TWO DAYS of collection using the specialised techniques described below. Because of this constraint we recommend hiring a consulting company to collect and extract water samples from sites.
When sampling for organic compounds such as pesticides, you need to use amber glass bottles and you need to collect a large volume of water from the sampling site. The reason for this is that many of the compounds bind to plastics. Some pesticides will even bind to the glass surface but using a large volume minimizes the risk of compounds adhering to the glass.
Four litre amber coloured glass solvent bottles which have had a very high purity solvent in them (HPLC grade or similar) are usually used. These could potentially be sourced from the laboratory that will be doing the analysis, as laboratories use a large number of these bottles.
Protocol for pre-cleaning bottles before sampling:
- Rinse the internal surface of the bottle and the inside of the cap with HPLC grade or better acetone.
- Repeat using Dichloromethane (HPLC grade or similar)
- Repeat using Methanol (HPLC grade or similar)
Once the bottles have been rinsed with all three solvents then let the solvent evaporate in a fume hood. All rinsing should also be done in a fume hood and the solvents should be disposed of using appropriate waste collection containers and collected by someone who is approved to deal with organic waste. All bottles should be pre-rinsed before going into the field to collect samples and all solvent residues should be evaporated.
The sampling process really depends on what you are looking for – sometimes you can use special types of plastic bottles, depending on the types of compounds you want to analyse.
To take a grab-sample, simply go into the water and take a sample from around 10 cm below the surface of the water by submerging the bottle. This is so that water only flows into the bottle and no solvent residue can flow out. The sample should be taken at approximately 1-2 metres in from the riverbank / shoreline at about knee depth.
Once the bottle is full, the bottle is capped and then stored on ice until it reaches the lab for extraction. Compounds need to be extracted from the sample and put into a solvent within 48 hours of collecting the sample (ideally, within 24 hours and less time is better).
Many thanks to Zak Murray, Victoria University of Wellington, for providing information on water sampling.
Any samples being sent to New Zealand for analysis need to be shipped under quarantine conditions with a Ministry for Primary Industries (MPI) Biosecurity sample import permit. This permit should be acquired before sampling is done to ensure samples can be analyzed as quickly as possible. Information about quarantine conditions and how to apply for an import permit can be found on the MPI website.
Information on how to ship to Samoa or Australia will be added soon.
Contact: Dr. Pousui Fiame Leo
SROS can provide tests for the following chemicals:
Contact: Dr. Grant Northcott
Plant & Food Research can provide tests for the following chemicals:
Hill Laboratories requires water samples to be in 500 mL glass bottles and sediment samples to be in 300 mL glass containers.
Hill Laboratories can provide tests for the following chemicals:
ALS can provide tests for the following chemicals:
In soil: Diazinon
Another, more recently developed method to test for pesticide residues uses aptamers. Aptamers are short nucleotide (DNA or RNA) or peptide (short chains of amino acids, which are the building blocks of protein) molecules that bind or connect to a specific target molecule. They have been used in a variety of ways including the treatment of disease and detecting cancer cells and bacteria. Pesticides are made up of specific molecules that aptamers can also be designed to detect.
Samples can include sediment extracts, solutions or homogenised tissues, depending on the system. Samples must be collected in a specific way, which is described in this protocol.
Commercially available aptamer tests are not yet available. However, AuramerBio will have some formats available later this year.
Contact: Dr. Shalen Kumar for more information on this work, or to enquire about aptamer testing kits.
Currently aptamers are available to test for specific neonicotinoid molecules:
- Clothianidin (CTND)
- Imidacloprid (ICPD)
- Thiacloprid (TCPD)
In future an assay should available to test for any neonicotinoid.
In the very near future aptamers will also be available for:
For those also interested in herbicides an aptamer assay is also planned for Glyphosate.