The Diamondback Moth That Kept Coming Back

Likely cause: The pattern - good early control, then failure after repeated pyrethroid sprays - points to selection for resistance in diamondback moth, possibly combined with poor coverage and late spraying of large larvae. DBM is known to develop resistance quickly. The field has had six lambda-cyhalothrin applications, which already exceeds the label maximum of four per season. The presence of larvae of all sizes and only partial parasitism suggests surviving populations are reproducing.

Doubling the rate is not acceptable. It exceeds the label, increases residues and selection pressure, and is unlikely to restore control if resistance is present. Adding a second pyrethroid is also a poor choice because it shares the same mode of action and will not 'reset' resistance.

A tank mix with a carbamate is not a resistance reset. If the carbamate has a different mode of action and is registered for cabbage and DBM, it may be part of a rotation, but only at the label rate and with the correct PHI. The crop is 10 days from harvest, and the lambda-cyhalothrin PHI is 7 days - but the maximum number of applications has already been exceeded, so further lambda-cyhalothrin is illegal regardless of the PHI. The farmer must not use the borrowed product without confirming its registration, label and PHI.

Resistance management should reduce selection and protect beneficial insects:

• Scout at least weekly and record larvae per plant, size classes and parasitism. Treat only when the action threshold is reached, not on a calendar.
• Rotate mode-of-action groups rather than product names. Use the IRAC group number on the label. Do not follow one pyrethroid with another pyrethroid.
• Limit any one group to a defined number of applications per crop, and never exceed the label maximum.
• Use registered biological or selective options where available, such as Bacillus thuringiensis products, and conserve the parasitoids already present.
• Improve coverage: calibrate the sprayer, use the correct water volume and nozzle, and spray smaller larvae.
• Keep a refuge or untreated area where practical, and destroy crop debris after harvest.

Record keeping must be rebuilt. Each spray record should show: date and time; field block; crop stage; pest and count; product name, active ingredient, IRAC group and lot; rate; water volume; area treated; nozzle and pressure; weather; applicator; start and finish times; PHI; harvest date; and any observations of poor control. The borrowed product should be listed with its name and label information, or noted as unverified. Records should be kept for at least the period required by the authority and be available to the buyer or inspector.

The borrowed product is a serious conduct issue. The farmer does not know what it is, whether it is registered for cabbage, what its PHI is, or what PPE is required. Using it without checking the label is illegal and could leave residues. He should not repeat this, and he should not describe the crop as compliant if he cannot document every product used.

For the hotel buyer, the farmer has a responsibility to supply safe produce and to be honest about his pest-control history. If he cannot establish the identity and PHI of the borrowed product, he should not sell the crop until the situation is clarified. He should not falsify records or invent a PHI. For the roadside market, the same safety duty applies even if the buyer asks fewer questions.

The professional response is to disclose the uncertainty to the buyer if necessary, keep the crop out of sale if residue status is unknown, and seek advice from the PCA or an extension officer. He should also tell the neighbour why borrowing pesticide is unacceptable, and he should train workers to check labels before every use. The long-term cost of a residue incident or a customer illness is far greater than the value of one cabbage crop.