Pesticide applications in tree care, particularly tree sprays, need to be timed during the growing season to target the most susceptible stage of a tree insect pests development. To manage lilac ash borer, we time the spray to coincide with when the adult female moths are actively searching for places to lay eggs and when eggs are hatching into larvae on the trunk. For Kermes scale, the spray is timed to coincide with when immature nymphs (called crawlers) have hatched and are moving to new places on the tree. Ips bark beetle sprays should be applied as overwintering adults waking up and are attempting to reenter trees to mate and lay eggs, then again in 2-3 months as they produce multiple generations per year.
Weather Affects Insect Growth and Development
So when do these life cycle events happen and how can we use that information to apply pesticides in the most effective way possible? Insects are cold-blooded animals whose growth and development through their life cycle is dependent on external temperatures. In order to perform effective pesticide applications that target the most susceptible stage of a tree insect pests development, it is important to have a general understanding of when certain life cycle events happen in relation to what the recent weather has been. Simply assuming that an event will happen during a prespecified calendar timeframe (i.e. late April-early May) doesn’t take into account what the weather has actually been that year, which is what ultimately determines insect growth and development.
One option, which is the best if it is possible, is to monitor trees and tree insect pests directly and to apply pesticides when you observe the targeted life cycle stage. This is possible through the use of pheromone traps that attract and capture the adult stage of a tree insect pests life cycle. It’s possible to monitor pests such as aphids and mites using a “beat sheet”. This involves aggressively tapping a branch over a white sheet of paper to dislodge inhabitants, and then observing what is crawling around on the paper. Also, sticky traps (double-sided tape) wrapped around branches can be used to detect crawlers of scale insects.
Growing Degree Days as a Prediction Tool
Direct monitoring is not always possible, so arborists have gotten hip to a tool that orchard growers and farmers have used for many decades, Growing Degree Days (GDD). A GDD is a measure of accumulated heat in a day. In its basic form, it is calculated by taking the average of the low and high temperature for a day, then subtracting the predetermined base temperature. Base temperature is the approximate point at which a tree insect pests development starts and stops. For many insects (and the sake of simplicity), 50 degrees F is used as a base. An example would look like:
(80 degrees + 50 degrees / 2) – 50 degrees = 15 GDD
When the number of GDD’s are totalled for the season, starting at the biofix (the generally accepted date that minimal insect development happens before, regardless of temperatures) of March 1st, you get an idea of how hot that year has been up to that point. Since tree insect pests life cycle events happen at relatively predictable GDD thresholds, you can put this information together to get an idea of when these things are actually happening. Here are some common thresholds for insect pests we commonly treat for in Colorado:
Insect Pest | Life Cycle Stage | GDD50 |
Emerald ash borer | 1st adult emergence | 400-500 |
Lilac ash borer | adult flight | 325-350 |
Zimmerman pine moth | 1st larvae | 25-100 |
Elm leaf beetle | 1st generation larvae | 400-600 |
Elm leafminer | 1st generation larvae | 365-530 |
Ips bark beetle | 1st adults active | 100-150 |
Example Tree Insect Pest: The Ips Bark Beetle
Let’s take a look at Ips bark beetles for example. Generally speaking, we are trying to get the first trunk spray application done before the first generation of adults reach their peak activity. This is around 125 GDD. In Boulder, CO for the year 2024, that date was May 2nd. In the past 10 years, the earliest date this occurred was April 12th (2017) and the latest that occurred was May 6th (2018). This data comes from Colorado State University Climate Center’s Growing Degree Day Tool. This is a valuable resource for those wishing to track GDD in Colorado. I check it almost every day in the spring.
Limitations and Need for Further Research
Obviously, there are more factors at play in insect development than simply how warm the season has been, and there are some serious limitations of just simply relying on GDD as your sole tool for pesticide application timing. One of the biggest problems is that the data for when certain life cycle events for specific tree insect pests are inconsistent across sources. The same insect species can behave differently in different geographic locations and in different climates. Data for lilac ash borer adult emergence in Pennsylvania, Michigan, and California is going to differ from what is seen on the ground here in Colorado.
But it is a valuable tool in the kit, and hopefully more Colorado specific tree insect pests data becomes publicly available in the future. And as always, please contact us to schedule a consultation with an ISA Certified Arborist if you have questions or concerns regarding the trees on your property. I hope this has been an interesting read and that you have more questions now than when you started. Please shoot me an email if you would like clarification on any of this, or even if you think I am flat out wrong about something. I do enjoy reading those emails. Until next time, stay safe out there, and go set up a pheromone trap.