When Pesticides No Longer Work

Understanding Resistance

Resistance is defined as a change in the sensitivity of a pest population to a pesticide. This means even the correct application of the pesticide fails to control the pest. Resistance can develop when the same pesticide or a pesticide with a similar mode of action are used over and over again.

It was presumed the pests changed or mutated to become resistant. However, it is not the individual pest (insect, weed, or microorganism) that changes, but the population.

When a pesticide is applied to a crop or treatment site, a tiny proportion of the pest population (for example, one insect or weed in 10 million) may survive exposure to the pesticide due to its genetic makeup. When the surviving pests breed, some of their young will inherit the genetic trait that confers resistance to the pesticide. Through subsequent generations, the less-susceptible individuals in the population increase.

A case in point: DDT

We saw this with the use of DDT on bed bugs during the second world war. DDT was used extensively to control pests within troop quarters from 1939. By 1940, bed bugs were already proving resistant to the potent chemical. After DDT was banned in 1972, people started using pyrethroid pesticides, like deltamethrin and lambda-cyhalothrin. Pyrethroids kill insects in a similar way as DDT. Both pyrethroids and DDT target the same part of an insect’s nervous system. Because of this, some bed bugs that were resistant to DDT were also resistant to pyrethroids, even if they had never been around pyrethroids before. This is called cross-resistance and is a contributing factor in our current resurgence of bed bug issues.

How a Professional Pest Controller Deals with Pesticide Resistance.

When the efficacy of a treatment drops, the pest controller will generally use higher rates and more frequent applications until the pesticide provides little or no control. This is NOT the way a professional pest controller will do things.

Identify your pest correctly

Older broad-spectrum pesticides may have worked on any living thing and didn’t require exact identification, but new chemistries are designed to kill
only the target pest and have less effect on beneficial insects.

Get the Timing Right

Timing is critical. Correct timing of a pesticide application can be the
difference between success and failure. New pesticide chemistries may
target only a certain life stage of the pest, like eggs or larvae, so timing the
application to the correct life stage is more critical than ever. Even with older
chemistries, contact insecticides may only be effective on a certain life stage,
like scale crawlers. It takes time to scout and determine exactly when the
crawlers are exposed but applying too early or too late will be a waste of time,
money and pesticide. Some pesticides should only be applied a certain number of times or amount per year, so you must time the application to maximize its effectiveness.

Sulfuryl Fluoride (ProFume) has proven efficacy on ALL life stages of pest but Research by the Queensland Department of Agriculture and Fisheries, with GRDC investment, has shown that effective control of all insect life stages with sulfuryl fluoride requires seven-day exposure for grain above 25°C, and 10-day exposure for grain between 20°C and 25°C. When grain is below 20°C, fumigation with sulfuryl fluoride is unlikely to be 100% successful as the insect life cycle is too slow, meaning the gas is not absorbed by the adolescents.

Coverage is Paramount

Coverage is another critical aspect of treatments.  If the pesticide does not reach the target pest, it will be ineffective. This may not be critical with systemic pesticides, but it is important when using a contact pesticide.

Factors to consider when applying a contact pesticide:

• Are you applying too quickly?
• Are the droplet sizes/nozzles right for the application?
• If using an airblast sprayer, are the nozzles correctly selected and adjusted for the various levels of height?
• Is the wind blowing too much at the time of application?
• Do you need to add an adjuvant to alter the droplet size or spread?

We can all be lazy, but don’t let the importance of coverage escape your attention.

Factors to consider when applying fumigant treatments

All fumigants, including sulfuryl fluoride, require gas-tight sealable storage to effectively control insects at all life stages. Unsealed storages will not hold a high-enough gas concentration for a long-enough period of time to kill the eggs, larvae and pupae.

Unlike methyl bromide which can be sealed using regular duck tape, sulfuryl fluoride requires a specialised tape such as Fumico’s specifically engineered PINK TAPE. 

Environmental Conditions

Wind and rain can wreak havoc on any pest treatment.  Temperature extremes can inactivate chemicals or, as in the case of oil based chemicals, make it phytotoxic to the crop. Temperatures also effect the development of pests: cooler conditions slow the life cycle while warmer conditions offer a robust breeding, developing phenomenon. Cooler environments will need longer exposure times to irradicate pests through the entire life cycle.

Sulfuryl fluoride is a heavy gas at 3.7 times the density of the atmosphere. Most applicators will introduce the gas into the top of a storage, anticipating it will settle to the bottom. Best practice is to have some form of sealed recirculation system to gently transfer gas through the storage to ensure an even concentration throughout the grain bulk for the duration of the fumigation.

Without recirculation, it will take longer for the gas to reach all parts of the storage and, as it sinks, it will likely leave the top of the storage with a sub-lethal gas concentration.

Pesticide Storage

Unsuitable storage conditions can impact the quality of the pesticide and harm application effectiveness. Some pesticides, especially biologicals or biorationals, have stringent storage requirements and can be inactivated
above or below certain temperatures. Some pesticides were just not created
to maintain effectiveness if stored for too long or under conditions above
40° C – which many areas of South Africa can experience regularly.

Some pesticides have a use-by date. Pesticides should be stored on a “first-in, first-out” basis.

Sulfiryl Fluoride (ProFume Fumigant Gas) is classed as a hazardous chemical and therefore transportation and storage of cylinders are strictly controlled.

Rotate Chemical Treatments

Never rely on a single pesticide class. Rotate modes of action, especially
when there is more than one generation of the pest in a year. For example, rotating phosphine fumigation with sulfuryl fluoride will protect against the development of phosphine resistance.  Read More about Sulfuryl Fluoride can break phosphine resistance…

Best Pest Control Practice

Your goal should always be to apply pesticides at the recommended rate with thorough coverage. Using a higher rate is illegal and may not be more effective. Using a lower rate can also be ineffective. Applying pesticides
without calibrating your equipment or checking the deposition uniformity can
be dangerous and wasteful.

“If at first you don’t succeed, try again” does not apply to pesticide applications of the same chemical. If control with a pesticide fails, do not retreat with the same chemical or another with the same mode of action.

Re-examine your checklist:

• Did you properly identify the pest?
• Did you use the correct dosage and timing?
• Is yourequipment properly calibrated and
  do you have good deposition?
• Were environmental conditions correct for
  the application?
• Was your pesticidequality still good and not out of date?

If you answer yes to all these, then try another pesticide with a different mode of action or a tank mix with two different modes of action. Pest resistance is something we all need to work together to avoid