Caterpillars are the larval stage of moths and butterflies. While a few are a natural part of a healthy ecosystem, an infestation involves a population explosion causing significant, widespread damage. Outbreaks occur when factors align, providing larvae with unlimited food and favorable conditions while removing the natural checks that keep their numbers low. The primary causes are rooted in the availability of specific food sources, environmental changes that favor survival, and the disruption of natural enemies.
Host Plant Specificity
A major factor driving a caterpillar infestation is the abundance of a specific host plant, as most species are highly specialized feeders. A female moth or butterfly only lays eggs on the particular plant species its offspring has evolved to eat. This reliance means that a large concentration of a preferred plant acts as a massive, uninterrupted nursery and food supply for that specific caterpillar species.
Monocultures, whether in agriculture or landscaping, create an ideal environment for an outbreak by providing a vast, dense food patch. For example, the tomato hornworm focuses primarily on nightshade plants like tomatoes and peppers. Similarly, the invasive gypsy moth heavily prefers oaks, and a dense oak forest can quickly be defoliated during an outbreak. When the host plant is widely available, the caterpillar population can reproduce rapidly, enabling quick population growth and subsequent infestation.
Environmental Conditions Driving Outbreaks
External physical factors, particularly weather and climate, directly influence the survival and reproduction of caterpillar populations, often triggering large-scale outbreaks. Temperature is a significant regulator because warmer conditions accelerate their life cycle. A shorter time from egg to adult means a species can complete more generations in a single season, leading to a much larger population overall.
Mild winters allow a higher percentage of overwintering eggs or pupae to survive, ensuring a larger starting population in the spring. Furthermore, a warm, moist spring provides optimal conditions for young larvae to thrive and for host plants to grow quickly, offering an immediate and abundant food source. Conversely, extreme weather variability can stress host plants, making them more susceptible to insect attack.
Absence of Natural Population Controls
The final element required for an infestation is the absence of biological and human-mediated factors that normally regulate caterpillar numbers. In a balanced ecosystem, caterpillar populations are kept in check by a network of natural enemies. These include generalist predators like birds, ladybugs, lacewings, and ground beetles.
More specialized controls are provided by parasitoids, often tiny wasps or flies that lay eggs inside or on the caterpillar’s body. The developing larvae then consume the host, representing a major source of mortality. Pathogens, including specific fungi, bacteria, and viruses, also play a substantial role in population collapse, particularly at high densities.
For example, the bacterium Bacillus thuringiensis (Bt) is a natural control agent. Outbreaks of species like the Douglas-fir tussock moth often end when a naturally occurring virus decimates the larval population.
Human activity frequently disrupts this natural balance, allowing caterpillar populations to escape control. The widespread use of broad-spectrum pesticides often eliminates beneficial predators and parasitoids more effectively than the pest. This removal of natural enemies can lead to pest resurgence, where the caterpillar population rebounds to higher levels than before treatment.
Habitat loss and urban planning can also reduce predator populations, such as birds, leading to increased caterpillar abundance in localized areas. Additionally, introducing non-native caterpillar species without their co-evolved natural enemies can result in devastating outbreaks that native predators are not equipped to handle.