Immediate and long-term facilitative effects of cattle grazing on a polyphagous caterpillar

Mammalian herbivores induce changes in the composition, abundance, architecture and chemistry of vegetation which can affect insects in their habitat. Many studies addressed the long-term effects of mammalian grazing on insect herbivores, yet few examined the effects during grazing (or right after it takes place). We investigated the immediate and long-term effects of cattle grazing on the abundance and distribution of the herbivorous spring webworm caterpillar (Ocnogyna loewii), via excluding cattle (by fencing) within a grazed paddock. In addition, we estimated the caterpillar density in replicated grazed and non-grazed paddocks (maintained as so for dozens of years), in moderate and heavy grazing intensities. Since the caterpillars develop during the cold winter months, we predicted that cattle grazing would positively affect them by reducing plant height and increasing their exposure to direct warm sunlight. Therefore, we examined caterpillar preference for sun-exposed areas using shade-manipulation experiments. Overall, cattle grazing positively affected the caterpillars, increasing their numbers two-fold on average, regardless of grazing intensity. This effect was immediate, as the caterpillars rapidly responded to exclusion of cattle by moving away from non-grazed areas. Caterpillar growth rate was similar when feeding on grazed and non-grazed vegetation. Most caterpillars (over 80%) preferred sun over manipulated shaded microhabitats. Furthermore, we found that cattle usually do not ingest caterpillars while feeding. Cattle grazing likely benefited the caterpillars that develop under low temperatures by reducing plant cover, thus creating a warmer habitat. This study demonstrates how changes in vegetation structure caused by mammalian herbivores can rapidly and positively affect the abundance and distribution of herbivorous insects.