Rainfall, nitrogen and host plant condition: consequences for the processionary caterpillar, Ochrogaster lunifer

1. At the end of November in subtropical areas of Australia, second-instar larvae of the processionary caterpillar Ochrogaster lunifer (Lepidoptera: Thaumetopoeidae) initiate feeding on the peripheral shoots of acacias (first-instar larvae do not feed). Field surveys at ten localities in south-east Queensland showed that larval survival was highly variable both among and within localities. Within-locality variation in larval growth was low compared with variation among localities. Larval growth and survival rates were higher at coastal and island localities, where November rainfall was high, than at drier inland localities.
2. Potted Acacia concurrens were grown in the greenhouse under high and low watering regimes, with and without nitrogen-rich fertilizer. Plant vigour (height, foliar water content and quantity of flush growth) was significantly greater in high-water treatments than in low-water treatments. Watering also affected foliar nitrogen, with plants in the high-water/no fertilizer treatment having similar nitrogen levels to those in fertilized treatments. Fertilizer increased foliar nitrogen levels of plants in low-water treatments and increased the number of shoots in high-water treatments. Different treatments had no effect on leaf toughness. After the first 3 weeks of feeding, size and survival of larvae were significantly reduced on the small, less vigorous plants in low water treatments. These results do not support the plant stress hypothesis.
3. Early-instar larvae (instar II–IV) developed more quickly and grew larger when reared on flush leaves than when reared on senescent leaves of A. concurrens . As water uptake affects the quantity of flush growth available to early stage larvae as well as foliar quality, rainfall and water availability may have important consequences for the distribution and population dynamics of the moth at local and regional scales.