Host Plant Selection Is Driven by Plant Availability Rather Than Nutritional Quality or Parasitoid Pressure for an Alpine Caterpillar in the Field

ABSTRACT Herbivores often feed on plants that do not maximize their developmental performance. Bottom‐up plant availability, top‐down natural enemy pressure, or limits in host discrimination may shape host plant selection. We studied host plant selection of the grassland caterpillar Gynaephora alpherakii on the Qinghai‐Tibetan Plateau through field surveys, laboratory choice experiments, and controlled feeding trials to determine the relative importance of these mechanisms. In the field, caterpillars fed predominantly on sedges (Cyperaceae) relative to grasses (Poaceae), but host plant selection closely followed local plant availability, indicating that realized feeding patterns were mainly driven by local resource abundance rather than intrinsic preference. Neither parasitism rate nor parasitism risk was associated with host plant feeding distributions, indicating no detectable effect of population‐level parasitism pressure on caterpillar feeding distributions under the field conditions investigated. The laboratory choice experiment revealed that caterpillars recognized and avoided the toxic non‐host Stellera chamaejasme , whereas preference between the common host plants, the grass Deschampsia cespitosa and the sedge Kobresia capillifolia , was weak and differed only among the largest caterpillars, suggesting that host plant choice may change during development. Feeding trials further showed that grasses with higher nitrogen and phosphorus concentrations supported greater survival and faster growth relative to sedges, despite being consumed less frequently in the field. These findings indicate that caterpillars show little evidence of strong intrinsic preference among common host plants and that realized host plant selection was more closely associated with bottom‐up plant availability than with population‐level measures of top‐down parasitoid pressure in the studied alpine meadow system. The potential performance benefits of feeding on higher‐quality host plants might be moderated under natural conditions by having access to multiple food resources throughout development.