Grasshopper populations in Colorado fluctuate cyclically, often appearing unusually high during certain years across the state’s plains and rangelands. These periodic outbreaks are driven by the interplay of weather patterns and the insect’s biology. Colorado’s environment is uniquely suited to foster these population spikes, which can cause noticeable damage in agricultural and residential areas. Understanding the environmental and biological mechanisms behind these swings reveals why the state is prone to high-density years.
Environmental Factors Driving Population Spikes
Colorado’s semi-arid climate and expansive geography create optimal conditions for population explosions. Outbreaks are triggered by a sequence of mild winters followed by warm, dry springs and hot summers. This climate sequence boosts the survival rate of overwintering eggs and accelerates the development of newly hatched nymphs.
Warm, dry conditions reduce the mortality factors that usually keep grasshopper numbers in check. Cool, wet weather is destructive to newly hatched nymphs, making them vulnerable to drowning and fungal pathogens. Drought conditions suppress these natural enemies and diseases. Prolonged heat also speeds up the grasshoppers’ growth rate, allowing them to mature faster and extend their feeding period, resulting in a larger adult population.
The state’s geography, featuring vast stretches of rangeland and undisturbed soil, further contributes to the problem. Female grasshoppers prefer to lay their egg pods in dry, firm, untilled ground, such as roadsides, fence rows, and open rangeland, which are abundant across the eastern plains. These open spaces provide the ideal habitat for large-scale movement and breeding. When conditions align over back-to-back dry years, the environment supports a dominant herbivore presence that can quickly deplete available forage.
The Life Cycle and Rapid Reproduction
The grasshopper’s life cycle capitalizes on favorable environmental conditions, enabling rapid population growth. Most pest species spend the winter as eggs, deposited in clusters called egg pods, buried beneath the soil surface. Each female can lay up to 25 egg pods, with each pod containing 10 to 300 eggs, demonstrating massive reproductive potential.
The eggs enter a state of arrested development, known as diapause, until spring. A combination of temperature and moisture cues their hatching. Warmer soil temperatures allow for an earlier and more synchronized hatch, meaning millions of tiny, flightless nymphs emerge simultaneously. These nymphs undergo incomplete metamorphosis, shedding their exoskeleton five or six times over 40 to 60 days before reaching adulthood. The lack of a vulnerable pupal stage means they are actively feeding and growing for a greater portion of their life cycle, quickly developing into destructive adults visible during mid-summer outbreaks.
Key Pest Species in Colorado
Colorado is home to over 100 grasshopper species, but only about a dozen are considered agricultural pests, with a few responsible for the most significant outbreaks. These pest species are categorized by their preferred diet, which influences the damage they inflict. Species include grass specialists, “mixed feeders” that consume both grasses and broadleaf plants, and “forb specialists” that prefer broadleaf plants.
The most damaging and widely distributed species is the migratory grasshopper, Melanoplus sanguinipes. This early-hatching, mixed feeder is capable of long-distance flight and can rapidly colonize new areas. The two-striped grasshopper (Melanoplus bivittatus) is another common pest; it is one of the largest in the genus and prefers tall, lush herbaceous vegetation found in ditch banks and crop borders. The Packard grasshopper (Melanoplus packardii) is a third key species that focuses on legumes and other forbs. The feeding habits of these species determine which plants are most at risk, with vegetable crops like lettuce, carrots, and sweet corn being particularly favored.
Managing Localized Grasshopper Infestations
For homeowners and gardeners facing a localized infestation, management strategies focus on preventing damage and targeting the insects when they are most vulnerable. Since grasshoppers are highly mobile, control is most effective when they are young, flightless nymphs concentrated near hatching sites in late spring or early summer. Targeting these early stages reduces the population before significant damage occurs.
Short-term, non-chemical controls include using physical barriers, such as fine netting or row covers, to protect valued plants. Cultural practices can also help, such as tilling small areas of undisturbed soil in the late fall to expose overwintering egg pods to predators and harsh winter weather. Chemical options for localized control include baits containing carbaryl or products with the insect growth regulator diflubenzuron, which interferes with the nymphs’ molting process. Homeowners may also use a “trap crop” strategy by planting preferred foods like lettuce away from their main garden to concentrate the insects, making spot treatment easier.