Pests and Diseases: How do They Grow?

Insect growth depends on temperature.

Unlike warm‑blooded animals, insects can’t regulate their body temperature. Insects develop more slowly when it’s cold, more quickly when it’s warmer, and may not develop at all below certain threshold temperatures. Each different insect species will grow at a different rate, and will respond differently to changes in temperature. 

It’s a similar story with pathogens, the microbes that cause disease on our crops. Different pathogens will have different growth rates and responses to environmental conditions.

Pathogen development can be affected by things like temperature, relative humidity, and leaf wetness.

Why is this important to us?

If we understand the relationship between a specific insect and the temperature of its environment, we can use temperature data to estimate how far along that insect is. This can help us make management decisions that are often more responsive than relying on a calendar date or waiting for symptoms to appear, because temperature can vary year-by-year.

We can do exactly this using pest and disease models. Read below to learn how these models work, and how to use them.

Pest Models: The Basics

A pest model is a mathematical tool that uses temperature data to estimate how far along an insect is in its development. Each pest has its own unique model, because each pest has unique temperature requirements for developing. 

Each pest model acts a bit like a recipe. If you have a recipe for chocolate chip cookies, including the baking temperature and baking time, you will know when to take them out of the oven. Similarly, by modeling the life cycle of a pest, pest models help us make timely management decisions, like when to plant, scout, exclude, or spray.

There are a few details about pest models that you should know when using them: 

• Degree Days
• Biofix Date
• Minimum and Minimum Temperatures

Degree Days

In order to track insect growth using temperature, we need a way to measure heat energy as it accumulates throughout the season. A degree day (DD) is a unit that measures how much heat energy a pest can use for growth and development each day.

• Degree days can accumulate each day throughout the season, like cups of water filling a bucket.
• On cooler days, less degree days will accumulate. On warmer days, more will accumulate. 
• Degree days can be calculated in Fahrenheit or Celsius. VDIFN uses Fahrenheit degree days by default, which you’ll see listed as FDD.

With careful research, scientists have been able to align accumulated degree days with insect developmental events or life stages. For example: 

• For cabbage looper, the first small larvae appear around 325 FDD, and feeding peaks around 520 FDD. 
• Squash vine borer egg-laying peaks around 1000 FDD. 
• Adult Japanese beetles emerge starting around 970 FDD, with peak feeding around 1600 FDD. 

For a comprehensive list of degree day events for common insect pests in Wisconsin, visit Degree Days for Common Insect Pests.

Another way to think about this is like cups filling a bucket. Instead of asking “How many cups do we need to fill this bucket?”, we can ask “How many degree days need to accumulate before squash vine borers start laying their eggs on my squash?”

Biofix Date: Starting the Stopwatch

Each pest model has a start date, or “biofix date”. Think about the biofix date like starting a stopwatch – after the biofix date, degree days start to accumulate each day.

Most pest models use January 1 as a biofix date, but some models use other, mid-year dates or scouting events. For example, the biofix for the Cabbage Looper model is automatically set to May 15th.

Minimum and Maximum Temperatures

Each insect pest has a minimum temperature needed to grow, and a maximum temperature where they grow as quickly as possible. Each pest model uses these unique values. Below the minimum temperature, degree days will not accumulate. Above the maximum temperature, degree days will accumulate at a maximum rate.

Disease Models: The Basics

Disease models are similar to the pest models described above in many ways. There are two unique components of disease models that you should know when using them:

• Disease Model Units
• Crop Emergence Date or Last Fungicide Application Date

Disease Model Units: Measuring Environmental Conditions

Like how pest models use degree days, disease models also use units to turn environmental data into something that we can measure and relate to pathogen development. Instead of degree days, disease models can use a variety of units, including the following:

• Physiological days (p-days)
• Disease Severity Values (DSVs)
• Daily Infection Values (DIVs)
• Cumulative Disease Severity Index (CDSI)

Researchers have been able to align these units with disease development. For example: 

• The risk of Early Blight is highest 400 p-days after crop emergence. Preventive fungicide applications should be initiated once 300 p-days are reached to protect the crop during the highest risk period.
• For Onion Botrytis Leaf Blight, recommendations are tiered. When the Cumulative Disease Severity Index (CDSI) is between 21 and 31, risk is in the “warning threshold”, and the recommendation is not to spray unless rain is predicted or overhead irrigation is used. Once CDSI goes above 31 into the “high risk” zone, initial spray application is recommended as soon as possible.

Crop Emergence Date or Last Fungicide Application Date: Starting the Stopwatch

Instead of starting with a “biofix date” like pest models, disease models use one of two potential starting dates.

Emergence Date: For crops that were direct seeded, this is the date that those plants emerged. For crops that were transplanted, this is the date that transplants were planted in the field.

Last Fungicide Application Date: In some disease models, like the one for late blight, the start date may also be the last date that fungicide was applied.

VDIFN Tool Basics and Navigation

The Vegetable Disease and Insect Forecasting Network (VDIFN) automatically runs pest and disease models using gridded weather data downloaded daily from NOAA servers. This data includes daily minimum and maximum temperatures and relative humidity. After running a chosen model, VDIFN displays degree-day accumulations or disease severity values on a map as color-coded risk scores. These are based on the estimated risk to crops in our chosen model, ranging from very low to very high risk. 

This gives users a snapshot of risk on a particular farm or field, which can help inform timely decisions about management, instead of treating on a preset schedule or waiting for symptoms to appear. VDIFN can provide more concrete answers to questions like “when is the highest risk for onion botrytis at my farm?” or “when should I plant my sweet corn to avoid seedcorn maggots?”. These answers are often much more accurate than using a fixed calendar date, because they account for year-to-year changes in temperature and conditions.

VDIFN can also allow users to see patterns of risk across the state. For heat based models, including all insect pest models and the early blight disease model, risk will always progress from south/southwest to north as the season progresses. This can be used to visualize how risk will be heading towards a particular location over time. 

How to Use VDIFN: Custom Model Tool

The “Custom Model” tool has the ability to run and visualize any degree day model. Once a model is submitted, users can customize the color gradient type, amount of gradations, and minimum or maximum color points.

This tool can be used as an easy way to visualize the progression of the seasons, or to cross-reference the degree-days for plant or insect developmental milestones. For example, some insect pests like Squash Bugs do not currently have a degree day model built into VDIFN, but have degree day-based recommendations for scouting and treatment that can be found online at the UW-Madison Vegetable Crop Entomology website. 

For more information on custom modeling, click on the question mark button next to the “model types”. 

Next Steps and Additional Resources

The VDIFN tool enhances our understanding of vegetable pests and diseases, supporting good management practices throughout the season. VDIFN is most effective when used together with a strong integrated pest management plan, including strategies like:

• Crop rotation
• Resistant varieties
• Biological control & natural enemies
• Field sanitation & debris removal
• Scouting for pests and symptoms

For support with pest or disease identification, visit: 

• UW-Madison Insect Diagnostic Lab
• UW-Madison Disease Diagnostics Clinic

For vegetable pest management information, research, and resources for pollinators and beneficial insects, visit the UW-Madison Vegetable Crop Entomology Program website (Dr. Russ Groves).

For vegetable disease management information, research, and newsletter updates, visit the UW-Madison Vegetable Pathology Program website (Dr. Amanda Gevens).

For more resources on vegetable and fruit production, visit the UW-Madison Extension Crops and Soils Specialty Crops Homepage.

For questions and feedback, please contact Ariana Abbrescia (abbrescia@wisc.edu) and/or Ben Bradford (bbradford@wisc.edu)

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Reviewers: Dr. Amanda Gevens, Dr. Russ Groves, Rue Genger
Last reviewed on July 16, 2026