A research project which was started during Anna’s Tropical Ecology Course in Wanang, and extended further by our assitants to higher elevations of Mt Wilhelm, resulted into new publication entitled: Exogenous application of methyl jasmonate to Ficus hahliana attracts predators of insects along an altitudinal gradient in Papua New Guinea. The paper will be publiseh in Journal of Tropical Ecology.

Abstract

In many plants, the defence systems against herbivores are induced, and may be involved in recruiting the natural enemies of herbivores. We used methyl jasmonate, a well-known inducer of plant defence responses, to manipulate the chemistry of Ficus hahliana along a tropical altitudinal gradient in order to test its ability to attract the enemies of herbivores. We examined whether chemical signals from MeJA-treated trees (simulating leaf damage by herbivores) attracted insect enemies in the complex settings of a tropical forest; and how this ability changes with altitude, where the communities of predators differ naturally. We conducted the research at four study sites (200, 700, 1700 and 2700 m asl) of Mt. Wilhelm in Papua New Guinea. Using dummy plasticine caterpillars to assess predation on herbivorous insect, we showed that, on average, inducing plant defences with jasmonic acid in this tropical forest increases predation twofold (i.e. caterpillars exposed on MeJA-sprayed trees were attacked twice as often as caterpillars exposed on control trees). The predation rate on control trees decreased with increasing altitude from 20.2% d^-1 at 200 m asl to 4.7% d^-1 at 2700 m asl. Predation on MeJA-treated trees peaked at 700 m (52.3%  d^-1 ) and decreased to 20.8% d^-1 at 2700 m asl. Arthropod predators (i.e. ants and wasps) caused relatively more attacks in the lowlands (200–700 m asl), while birds became the dominant predators above 1700 m asl. The predation pressure from birds and arthropods corresponded with their relative abundances, but not with their species richness. Our study found a connection between chemically induced defence in plants and their attractivity to predators of herbivorous insect in the tropics.

Figure 2. Mean daily (N = 4) predation by the two groups of predators on artificial caterpillars exposed (N = 5) on individual control (N = 24) (a) and MeJA-treated (MeJA; N = 24) saplings (b) of Ficus hahliana at four study sites on Mt. Wilhelm in Papua New Guinea. Sites with significantly different attack rates by arthropods and birds are marked by asterisks (*** P = 0.001,* P = 0.05; results of Tukey post hoc test). Altitudes with significantly different (P < 0.05) incidences of attack by individual predators within the control and MeJA treatment are denoted by different letters: small letters = predation by arthropods, capital letters = predation by birds (results of Tukey post-hoc test).