The Basics of Pesticide Resistance

Dr. Fred Fishel, Professor – UF/IFAS Agronomy

When Pesticides Don’t Work

• Improper pest identification

• Incorrect pesticide dosage

• Improper application timing

• Pesticide doesn’t reach target pest

• Unfavorable environmental conditions

• State of poor pesticide conditions

• Pesticide resistance

Terminology

• Resistant: Pest was originally susceptible to pesticide; over time control lost through the selection of resistant individuals.

• Tolerant: The inherent ability of a species to survive following a pesticide treatment – was never susceptible.

Terminology

• Mode of action: describes the biochemical processes by which the pesticide poisons the pest (for example, disrupting photosynthesis).

• Target site of action (mechanism of action): the exact location of inhibition, such as interfering with the activity of a specific enzyme within a metabolic pathway.

Terminology

• Cross resistance: resistance to 2 or more pesticides that share the same mode of action.

– Dotted duckweed (Landoltia punctata)

– Diquat and paraquat (WSSA Group 22)

• Multiple resistance: resistance to 2 or more pesticides with different modes of action.

– Palmer amaranth (Amaranthus palmeri)

– Imazapic and glyphosate (WSSA Groups 2 and 9)

Resistance in General

“Biotype”

“The Pesticide Treadmill”

1. Pest population rises

2. Apply pesticides

3. Pests develop resistance

4. Apply more pesticides

5. Resistance becomes more prevalent

6. Switch to new pesticide (if possible)

History of Resistance (Organic Pesticides)

• 1947 – insecticides

– Housefly

– Organochlorines (DDT)

– Organochlorines have long since been banned for use

• 1960s – fungicides

– Powdery mildew

– Benzimidazoles (benomyl)

– DuPont voluntarily canceled benomyl due to high legal costs

History of Resistance (Herbicides)

• 1957 – weeds

– 2,4-D resistant dayflower in sugarcane field

Herbicide Resistance Mechanisms

Sequestered in vacuole

X Z

X

Help! Get me outta

here!

X Y

X

Please! Let me in!

I can’t do my

job!

X

X

My key won’t

fit!

More than 450 resistant biotypes globally

9 of the top 15 most common herbicides prone to resistance are ALS inhibitors

Herbicide Resistance in Florida

Year Species Situation Herbicide(s)

1985 American black nightshadeTomato Paraquat

1996 GoosegrassDiquat, paraquat2001 Dotted duckweed

Aquatic2002 Hydrilla Fluridone

2008

Palmer amaranth Cropland

Imazapic, pyrithiobac-Na

2013Glyphosate

Glyphosate, imazapic

2014 Ragweed parthenium Industrial Glyphosate

Resistance Concerns

Circa mid-1990’sBecause glyphosate is non-selective, Monsanto has insisted that glyphosate resistant weeds will not appear with the use of this herbicide. With the imminent release of glyphosate resistant corn and soybeans, this hypothesis will soon be tested on potentially millions of acres. Resistant weed populations have been reported already in locations outside the USA.

Managing Resistance in General

• Rotate herbicide applications using different modes of action

• Use tank- or pre-mixes of different MOAs

• Avoid unneeded applications

• Follow label rates

• Use non-herbicidal alternatives if possible

• Monitor changes in weed control results

Playing by the Numbers

Spraying by the Numbers

• Weed Science Society of America herbicide classification system

– Approximately 30 herbicide modes of action identified

Spraying by the Numbers

Aquatic Herbicide Mode of Action Group Numbers According to the WSSA

WSSA group number Common name Mechanism of action

2Bispyribac-sodium,Imazapyr, Imazamox, Penoxsulam

Enzyme inhibitor (ALS)

4 2,4-D, Triclopyr Growth regulator (organo-auxin)

9 Glyphosate Enzyme inhibitor (EPSPS)

12 FluridonePhotosynthesis inhibitor (pigment synthesis – PDS)

14Carfentrazone, Flumioxazin

Enzyme inhibitor (PPO)

22 Diquat Photosynthesis inhibitor

27 TopramezonePhotosynthesis inhibitor (pigment synthesis – HPPD)

NCEndothall, Hydrogen peroxide, Copper

Photosynthesis inhibitor (and other effects)

Spraying by the Numbers

Thank you for your

attention!