Grasshoppers are common herbivores across nearly all terrestrial ecosystems, making them hosts for a wide array of parasitic organisms. These parasites represent diverse taxonomic groups, including worms, flies, mites, and fungi, each employing unique strategies to complete their life cycle at the grasshopper’s expense. The relationship is highly specialized, often involving the parasite consuming non-essential tissues to prolong the host’s life until the parasite is ready to emerge.
Internal Invaders: Nematodes and Parasitic Flies
Among the most dramatic internal parasites are the Nematomorpha, commonly known as horsehair worms. These develop from microscopic larvae ingested by a grasshopper. Once inside, the larva penetrates the gut wall and settles into the body cavity (hemocoel), where it begins intense growth. The developing worm absorbs nutrients directly from the host’s hemolymph and internal tissues, growing into a long adult that can reach several times the length of the grasshopper itself.
Another common internal parasite is the grasshopper nematode, Mermis nigrescens. The adult nematode lays eggs on vegetation, and the grasshopper becomes infected when it consumes the contaminated foliage. The hatched larva burrows into the host’s body cavity, feeding on the hemolymph for one to three months. This prolonged internal feeding severely stresses the host, often resulting in functional sterility and premature death.
A third group of internal invaders are parasitic flies of the order Diptera, such as Acridomyia canadensis, whose larvae are endoparasites. The female fly lays its eggs directly inside the grasshopper, and the emerging maggot consumes the host from the inside.
External Pests and Fungal Pathogens
Grasshoppers are also targeted by external pests and microbial agents. Ectoparasitic mites, such as the red mite Eutrombidium locustarum, attach to the exoskeleton, typically near the wings, legs, and antennae. The larval stage uses specialized mouthparts to pierce the host’s cuticle and feed on the hemolymph. Heavy infestations significantly reduce the host’s fitness and reproductive capacity.
Fungal pathogens also play a significant role in mortality, most notably the species complex Entomophaga grylli. This fungus causes “summit disease,” beginning when spores penetrate the cuticle and colonize the host body.
Before the grasshopper dies, the fungus induces a precise behavioral change crucial for its propagation. It compels the insect to climb to the top of a plant stalk, where it tightly grips the stem. This high perch ensures that when the fungus erupts, it can forcibly discharge its spores into the air for widespread dispersal to new hosts.
How Parasites Manipulate Grasshopper Behavior
The most remarkable examples of parasitic control involve the behavioral manipulation induced by Nematomorpha (horsehair worms), which must return to water to reproduce. The worm needs its terrestrial host to seek out an aquatic environment to emerge and complete its life cycle. This shift from a naturally water-avoiding (hydrophobic) state to a water-seeking (hydrophilic) state is mediated by chemical warfare within the host’s nervous system.
The parasite alters the function of the grasshopper’s central nervous system by producing specific molecules. Proteomic analysis of infected brains reveals a differential expression of host proteins linked to neurotransmitter activity and geotactic behavior. This suggests the worm is directly interfering with the neurological signals that control movement and environmental orientation.
The hairworm produces molecules, including those belonging to the Wnt family, which influence the central nervous system. These parasitic molecules exhibit similarities to proteins naturally found in the insect host, suggesting a case of molecular mimicry. This biochemical tampering forces the grasshopper to abandon its terrestrial habitat and jump into the nearest body of water. The manipulation is highly focused, typically only compelling the grasshopper to enter the water when it is already in close proximity, allowing the fully grown worm to exit and find a mate.