Grasshoppers are host to a remarkably wide range of parasites, from worms that hijack their behavior to fungi that dissolve their exoskeletons from the outside in. The diversity is striking: nematodes, hairworms, parasitoid flies, ectoparasitic mites, microsporidian protozoans, and entomopathogenic fungi all target grasshoppers at various life stages. Some of these parasites quietly sap nutrients; others kill outright. A few even turn grasshoppers into unwitting vehicles for their own offspring, manipulating the insect’s movements in ways that look almost intentional.
Hairworms That Drive Grasshoppers Into Water
Among the most dramatic grasshopper parasites are hairworms, threadlike creatures in the phylum Nematomorpha. The hairworm Spinochordodes tellinii develops inside grasshoppers of the species Meconema thalassinum, growing to fill much of the body cavity. Once the worm reaches maturity, something strange happens: the grasshopper begins seeking water. Infected individuals become far more likely to jump into streams, pools, or puddles, a behavior rarely seen in healthy grasshoppers. This is not accidental. The adult hairworm needs an aquatic environment to reproduce, and the infected insect’s altered responses to water deliver it there.
Researchers have confirmed this behavioral manipulation in both field and laboratory settings. A two-year observation study at an outdoor swimming pool documented infected insects from nine different species jumping into the water, followed by hairworms emerging from their bodies. Controlled experiments comparing infected and uninfected crickets (close relatives of grasshoppers that harbor the same type of parasite) found that parasitized individuals were significantly more likely to jump into water than healthy ones.
Proteomics work on infected grasshoppers has shed light on the mechanism. The hairworm appears to produce proteins that mimic those involved in the host’s central nervous system signaling, essentially rewiring the grasshopper’s orientation toward light and water. The grasshopper does not “choose” to drown itself; its decision-making apparatus has been chemically subverted. From an evolutionary standpoint, the grasshopper is just a temporary vessel that the worm discards once it reaches water.
Parasitoid Flies That Develop Inside Living Grasshoppers
Several species of flesh flies in the genus Blaesoxipha are important parasitoids of grasshoppers and locusts. The adult fly deposits larvae on or near a grasshopper, and those larvae burrow inside and develop by feeding on the host’s internal resources. Unlike a simple parasite that takes a share of nutrients and leaves the host alive, a parasitoid almost always kills its host when the larvae are ready to pupate.
Research on Blaesoxipha atlanis parasitizing the migratory grasshopper Melanoplus sanguinipes found that the fly larvae do not cause obvious physical damage to the host’s organs. Instead, they function as a metabolic sink, diverting energy and nutrients away from the grasshopper’s own growth and reproduction. When multiple larvae develop inside a single host, each larva ends up smaller and less likely to survive. Female grasshoppers, being larger, tend to harbor more parasitoid larvae than males.
A separate study of Blaesoxipha japonensis parasitizing Parapodisma grasshoppers in Japan confirmed that female hosts carried higher larval densities, and that the weight of each resulting pupa dropped as the number of larvae sharing a single host increased. The parasitoid’s fitness, in other words, is constrained by how many siblings it has to compete with inside the same grasshopper.
Grasshoppers also face attack from nemestrinid flies, which parasitize from a different angle. In one documented case covering roughly 200,000 acres, a 99 percent reduction of an outbreak of Circotettix grasshoppers coincided with heavy parasitization by the fly Neorhynchocephalus sackenii, which was found in about 70 percent of grasshoppers collected in net sweeps. The host population crashed because parasitized individuals lived shorter lives and laid far fewer eggs.
Mermithid Nematodes
The large roundworm Mermis nigrescens has a life cycle closely tied to grasshoppers. On rainy mornings in spring and summer, adult worms deposit eggs on plant leaves. Grasshoppers eat the eggs along with the foliage, and the juvenile nematode hatches inside the gut, penetrates the gut wall, and grows in the body cavity for weeks or months. The worm can reach impressive lengths for something living inside an insect, sometimes exceeding the grasshopper’s own body length. Eventually it exits, often after a rain or when the host is injured. The sight of a long, dark worm emerging from a grasshopper is startling enough that it has generated folklore in farming communities for centuries.
Mermithid infection tends to sterilize female grasshoppers, diverting the resources that would have gone into egg production toward nourishing the developing worm instead. Males are similarly debilitated. From a population standpoint, mermithid nematodes reduce the reproductive output of infected cohorts even when they do not directly kill the host.
Ectoparasitic Mites
Not all grasshopper parasites live inside the body. Tarsonemid mites are tiny arachnids that attach externally, often hiding under the hindwings of their hosts. A taxonomic study described an entirely new group of these mites, collected from the hindwings of pygmy grasshoppers across an enormous geographic range including Bolivia, Ecuador, Honduras, southern India, Japan, Papua New Guinea, South Africa, and northeastern Australia. At least seven new species were identified, all specialized for life on tetrigid grasshoppers.
Ectoparasitic mites typically feed on the host’s hemolymph (the insect equivalent of blood) or on secretions near wing bases and leg joints. While individual mite loads may not kill a grasshopper, heavy infestations can impair flight, reduce body condition, and make the host more vulnerable to predators and other stressors. The sheer global diversity of these mites suggests that the association between mites and grasshoppers is ancient.
Entomopathogenic Fungi
Fungi in the genera Metarhizium and Beauveria are among the most important natural enemies of grasshoppers. These are not parasites in the classical sense, since they kill their hosts rather than living off them long-term, but their infection process is deeply parasitic in character. Spores land on the grasshopper’s cuticle, germinate, and produce specialized structures that physically punch through the exoskeleton using a combination of mechanical pressure and cuticle-dissolving enzymes.
The enzyme cocktail these fungi deploy is finely tuned. Research on Metarhizium species infecting desert locusts found that the fungal protease PR1 broke down different body regions at different rates, with the softer abdominal cuticle of developing adults being the most susceptible and the harder wing cuticle being the most resistant. More recent work on Metarhizium acridum identified a transcription factor (MaFTF1) that actually acts as a brake on virulence. When researchers knocked out this gene, the modified fungus penetrated locust cuticle faster, developed its infection structures more quickly, and killed its hosts about a day sooner than the unmodified strain.
Once inside, the fungus proliferates throughout the body cavity, consuming soft tissues and producing toxins. Death typically follows within one to three weeks, depending on the fungal species, the dose of spores, and the grasshopper’s own defenses. After the host dies, the fungus often sporulates on the outside of the carcass, turning it into a fuzzy green or white spore factory that can infect other grasshoppers nearby.
How Grasshoppers Fight Back
Grasshoppers are not passive victims. One of their most interesting defenses against fungal pathogens is behavioral fever, a strategy where infected individuals deliberately raise their body temperature by basking in the sun or selecting warmer microhabitats. Unlike mammals, grasshoppers cannot generate metabolic heat internally, so they use the environment as a thermostat.
A field study of Senegalese grasshoppers (Oedaleus senegalensis) infected with fungal pathogens found that parasitized individuals shifted their thermoregulatory behavior, raising their preferred body temperature to a new set point of about 42 °C. This was described as the first evidence of behavioral fever in response to a microbial infection in any natural insect population. Laboratory work on the migratory locust Locusta migratoria infected with Beauveria bassiana confirmed that infected individuals maintained body temperatures one to three degrees higher than healthy controls over a 10-day period.
Whether this fever actually helps the grasshopper survive is complicated. The initial field study found that the temperature increase provided little obvious therapeutic benefit under the conditions tested. However, the researchers noted preliminary evidence from other work suggesting that host thermoregulation could be a significant constraint on the pathogen and might limit its effectiveness under certain environmental conditions. Temperature sensitivity varies by pathogen species: normal thermoregulatory basking was found to reduce the virulence of Beauveria bassiana but had no apparent effect on Metarhizium acridum, suggesting that the usefulness of behavioral fever depends on which fungus the grasshopper is fighting.
Beyond fever, grasshoppers possess a cellular immune response that can encapsulate foreign objects inside the body. When a nematode egg hatches in the gut or a fungal hypha breaches the cuticle, immune cells called hemocytes can swarm the invader and wall it off in a melanized capsule. Research on pygmy grasshoppers tested whether this encapsulation response varied by color morph or sex and found no significant difference, indicating that the baseline immune investment is fairly uniform across individuals regardless of their external appearance.
Grasshoppers as Middlemen for Other Parasites
Grasshoppers do not just suffer from their own parasites. They also serve as intermediate hosts, carrying parasites destined for entirely different animals. One well-studied example involves the eyeworm Oxyspirura petrowi, a nematode that infects the eyes of wild quail and other ground-feeding birds. The worm’s larvae develop inside grasshoppers and other arthropods; when a bird eats an infected grasshopper, the larvae migrate to the bird’s eye and mature into adults.
Molecular screening of grasshoppers in the wild found that about 23 percent of sampled individuals tested positive for O. petrowi larvae. At least six different grasshopper species were confirmed carriers, including common species like Melanoplus differentialis and Melanoplus femurrubrum. This has real implications for wildlife management: declining quail populations in parts of the American Southwest have been linked in part to eyeworm infections, and grasshoppers are a primary route of transmission.
When Parasites Crash Grasshopper Populations
Whether a disease or parasite actually reduces a grasshopper population is less straightforward than it sounds. A field experiment studying the fungal pathogen Entomophaga gryllii in Camnula pellucida grasshoppers found that the answer depends on the grasshopper’s life stage and population density. At high densities, disease mortality increased by about 60 percent compared to low-density conditions. But the total mortality rate in diseased grasshoppers at high density (roughly 87 percent) was barely higher than in disease-free grasshoppers at the same density (about 83 percent). The fungus was essentially killing grasshoppers that would have died anyway from food limitation, a phenomenon ecologists call compensatory mortality.
The picture changed for younger grasshoppers. In early developmental stages, disease mortality was additive, meaning it stacked on top of other causes of death and genuinely reduced survival. This suggests that parasites and pathogens are most effective at suppressing grasshopper numbers when they hit immature insects and when populations are not already stressed by overcrowding and starvation. For adult grasshoppers, disease often just swaps one cause of death for another without changing the bottom line.
Parasites as Pest Control Tools
The lethal efficiency of entomopathogenic fungi has not gone unnoticed by people who need to manage locust and grasshopper outbreaks. Metarhizium acridum is now the basis of commercial biopesticide products used in Africa, Australia, and Brazil. Oil-based spore formulations can be sprayed over rangeland in much the same way as chemical insecticides, but with a very different ecological footprint.
Field trials have consistently shown that these fungal biopesticides kill 70 to 90 percent of treated locusts within two to three weeks, with no measurable impact on non-target organisms. One trial reported mortality of caged desert locust nymphs reaching roughly 83 percent in treated plots compared to about 4 percent in untreated controls. Importantly, a recent study demonstrated that dried M. acridum spores retained their effectiveness even after ten years of storage, a practical advantage for countries that need to stockpile biocontrol agents between intermittent locust outbreaks.
Fungal biocontrol also interacts with natural predation in useful ways. Research on desert locusts sprayed with Metarhizium acridum found that birds preferentially consumed infected locusts, particularly females, which tend to be larger and slower as the infection progresses. This selective predation effectively accelerates the population decline beyond what the fungus alone would achieve.
The slow-kill timeline of fungal biopesticides, typically five days before locusts start dying with peak mortality at one to two weeks, is both their main limitation and an ecological advantage. Chemical insecticides kill faster but also wipe out beneficial insects, pollinators, and natural enemies of grasshoppers. Fungal agents are far more selective. The tradeoff is that farmers watching a swarm devour their crops understandably want immediate results, which creates tension between ecological best practices and on-the-ground urgency.
The Arms Race Between Grasshoppers and Their Parasites
The long coexistence of grasshoppers and their parasites has shaped both sides. Grasshoppers have evolved thickened cuticles in vulnerable body regions, behavioral fever responses, encapsulation immune reactions, and avoidance behaviors. Parasites, in turn, have evolved cuticle-degrading enzymes tuned to their host’s armor, behavioral manipulation strategies like the hairworm’s water-seeking trick, and immune evasion tactics including the suppression of host hemocyte activity.
Gut microbes add another layer to this relationship. The bacterial community living in a grasshopper’s digestive tract can influence how well the insect resists infection. Some gut bacteria produce antimicrobial compounds that help keep fungal pathogens and microsporidian parasites in check. Parasites, for their part, can disrupt the gut microbiome as part of their infection strategy, weakening the host’s first line of internal defense. This three-way interaction between parasite, host immune system, and resident microbiota is an active area of research, and the full picture is still emerging.
What is clear is that no single parasite has managed to wipe out grasshoppers as a group, despite millions of years of coevolution. Grasshopper populations boom and crash in response to weather, food supply, and disease acting together. Parasites are one thread in a larger ecological fabric, sometimes pulling populations down hard and sometimes barely making a dent, depending on timing, density, and which species are involved.