I’ll be the first to point out that I am not a trained entomologist, but … I dabble. I have run up against a fair number of insects (pest or otherwise) over the course of my career, and, as a result, I can recognize a good number of them, or at least get within the right postal code when it comes to narrowing down their identity.

To simplify things, insects go through a series of transformations or metamorphic stages on their way from the youngest form (often, but not always, an egg) to the adult form. When you want to manage an insect pest population (a nice way of saying “control”, but with lower expectations), it is important to know when an insect is at it’s most vulnerable. It is also important to understand when applying a control measure is just being petty, vindictive, or wasteful since A) the stage is not causing any harm, or B) the damage is done and the control will have little to no effect, or C) the control method you are using doesn’t actually work on that stage.

Types of Metamorphosis

Insects vary in the number of steps or stages in their developmental process. Depending on the type of insect, they can either have a Complete Metamorphosis or a Simple Metamorphosis.

In Complete Metamorphosis, there are four life stages, starting with an egg, different types of larvae, a pupal (pupa) stage, and then an adult. There are a number of different groups of common pests that undergo this type of metamorphosis. They include broad groups like moths, butterflies, beetles/weevils, and flies. It also includes insects that aren’t pests, like bees, wasps, and midges.

In Simple Metamorphosis, there are basically three developmental stages, starting with eggs, then an immature stage called a nymph, followed by an adult stage. The types of insects or organisms (since some aren’t true insects) that undergo simple metamorphosis include things like aphids, true bugs, mites, scales, and leafhoppers. For some of these pests, eggs are only produced at some points in the season, with live young being produced most of the time.

For either type of metamorphosis, there may be developmental steps within each life stage. For example, an insect may go through a number of “instars”, where there is a change in size and perhaps form, as the immature form moves up to the next level. This might involve “moulting” or shedding the outer layer/exoskeleton/skin. There might be 3-5 nymphal or larval instars before the insect finishes development or moves onto the next life stage. For example, Spotted Wing Drosophila, an invasive and aggressive vinegar fly goes through three larval instars before becoming a pupa.

What are the stages and what are their roles?

The starting point for most insect life cycles is the EGG. The egg usually represents a combination of male and female genetic material. In some insects, there will be eggs produced without fertilization, with a fertilized egg being less common. Regardless, an egg can be an overwintering stage (being hardy and able to withstand the elements) or can be the starting point for each subsequent generation. Eggs can hatch after a variable period of time, depending on temperature and the species of insect.

Eggs hatch to release some immature form of insect or organism. For those insects that undergo complete metamorphosis, some type of LARVA emerges. For moth and butterflies (the Lepidoptera), this a caterpillar, which eats and eats and gets bigger and bigger. For flies, the eggs hatch to release a maggot, which has a similar focus to the other larval forms. It eats and eats. Other insects, like beetles, weevils, and sawflies produce a worm-like larva, that, you guessed it, eats and eats. Once larvae reach a certain size and age, they will find a spot to form a cocoon or some other type of pupal chamber, or they might drop to the ground for this stage.

For insects that use simple metamorphosis, eggs hatch to release a NYMPH. Some nymphs look very much like the adult form, only less advanced and lacking some key parts. Others look very different, resembling some strange, armored vehicle from the future (e.g., Hawthorn lace bugs). Either way, nymphs feed and go through several instars until they reach maturity.

Nymphs can vary in how they act. Some nymphs are mobile for their entire life, whereas others have brief periods when they move around (called a “crawler”) before they settle down in one spot permanently (e.g., some types of scale insects).

For insects that undergo complete metamorphosis, the final stage of development before they reach maturity and adulthood is the PUPA. Pupae is a stationary and immobile phase, where the insect finishes growing and developing all the adult parts, before emerging. The pupal stage is an overwintering stage for many insects, from which the adults emerge.

For those insects with more rapid life cycles, you might see pupae during the season. I recall the first time I saw a Colorado Potato Beetle as a pupa. The larval stages all look like little orange squishy slug-like things, but the pupal stage happens underground. When we dug one up, it looked like the orange larva was wearing a transparent Colorado Potato Beetle suit.

ADULTS are the mature stage for all insects. Some adults are a big problem and can do some major damage. But the focus of most adults is mating and reproducing. Some adults don’t do any damage to plants, just feeding on pollen and nectar, as they fuel up for the reproductive effort.

For some insects, controlling the adult stage is critical to interrupting an increasing population, as it means that eggs aren’t laid. For others, the adults aren’t really doing a whole lot, so many of the controls that require ingestion through feeding don’t work. The larval stages are often the targets of control measures, if they happen to be in a place that can be reached easily.

When do they occur?

The timing of each stage can vary quite a bit. If an insect overwinters in an area, you might either see the adults emerge at some point in the spring or summer, typically coinciding with their food sources, or those of their offspring. If an insect flies or travels into an area from somewhere else, the start of the cycle might be a bit later, depending on the season. Once a population gets started, it all depends on the weather conditions, as those each affect the rate of development of each stage, and how quickly a pest population can go from start to finish and repeat.

Cheat Sheet Charts

To help me to get a sense of when to potentially expect certain pests (and therefore be more alert and watchful of them), I adapted and built some charts a few years ago, putting in some of the more common insect pests of fruits and vegetables that occur on the prairies. This way, knowing when a pest might be present, you can plan your monitoring, and also make plans for controlling them.