The sudden appearance of massive insect swarms often leads people to question if California is experiencing a “locust” outbreak. This concern highlights widespread confusion over the terminology used to describe these insects and their behavior. While California deals with large-scale, destructive insect populations, the biological distinction between a true locust and a common grasshopper is significant. Clarifying this difference is necessary to understand the nature of the swarms seen across the state’s agricultural and rangeland areas. The insects responsible for these damaging events are native species that require a specific, informed approach to monitoring and control.
Defining the Difference Between Grasshoppers and Locusts
All locusts are technically a type of short-horned grasshopper belonging to the family Acrididae, but only a few species worldwide can transform into the true “locust” phase. The difference is not one of species, but of behavior and physiology, driven by population density. When certain species of grasshoppers become overcrowded, they undergo a phenomenon known as density-dependent phase polyphenism.
This dramatic transformation causes the insect to switch from a solitary, shy phase to a gregarious, social phase. The change involves shifts in color, morphology, and behavior, resulting in stronger wings suitable for long-distance migration. Solitary grasshoppers remain local and pose minimal threat, but a high-density gregarious locust forms immense, coordinating swarms that can devastate vast areas of vegetation.
The True Locust Status in California
Based on the biological definition of true locusts, California does not currently host an established population of these destructive, phase-transforming insects. The famous Old World species, such as the Desert Locust (Schistocerca gregaria), are not established in North America. Historically, the United States did contend with a true locust species, the Rocky Mountain Locust (Melanoplus spretus), which formed massive swarms.
However, the Rocky Mountain Locust was last observed in 1902 and is now considered extinct, with its disappearance attributed in part to the cultivation of its native breeding grounds. While California is home to the Gray Bird Grasshopper (Schistocerca nitens), a relative of the Desert Locust, this local species does not exhibit the same level of density-dependent swarming behavior.
California’s Swarming Grasshopper Species
The most problematic insects for California agriculture are native grasshoppers that occasionally experience population explosions, mimicking the damage of true locusts. A common culprit is the Pallid-winged Grasshopper (Trimerotropis pallidipennis), a species frequently responsible for swarms in the desert and rangeland areas of Southern California. These outbreaks, which may last a year or two, are associated with environmental factors like above-average rainfall in the preceding fall and spring.
Another highly destructive group belongs to the Melanoplus genus, which includes the Differential Grasshopper (Melanoplus differentialis) and Melanoplus devastator. These short-horned grasshoppers are widespread and can consume up to 250% of their body weight daily, making them a significant threat to crops like hay, onions, and garlic. When populations boom, these grasshoppers move in large numbers into irrigated fields and gardens in search of food. While technically not grasshoppers, Mormon Crickets also pose a similar threat in swarming masses.
Ecology and Management of Outbreaks
The environmental triggers for California’s grasshopper outbreaks often follow a boom-bust cycle related to precipitation and temperature. Periods of prolonged drought reduce natural predators and suppress fungal pathogens. These periods are often followed by favorable, wet conditions that promote mass hatchings. This rapid increase in population density, combined with subsequent food scarcity as the landscape dries, forces the insects to migrate into agricultural areas.
Management strategies focus heavily on early intervention when the grasshoppers are still small, flightless nymphs. University of California Cooperative Extension advisors recommend monitoring for high-density populations in uncultivated areas, typically during April and May. Control measures include the targeted application of insecticides like diflubenzuron (Dimilin), a chitin inhibitor that prevents successful molting, or carbaryl bait. Coordinating treatment across vast rangelands is challenging, requiring widespread cooperation among landowners to prevent re-infestation.