The Southeast Asian transboundary haze contains a mixture of gases and particles from forest fires and negatively impacts people's health and local economies. However, the effect of the haze on organisms other than humans has not yet been...
moreThe Southeast Asian transboundary haze contains a mixture of gases and particles from forest fires and negatively impacts people's health and local economies. However, the effect of the haze on organisms other than humans has not yet been sufficiently studied. Insects are important members of food webs and environmental disturbances that affect insects may impact whole ecosystems. Here we studied how haze directly and indirectly affects the survival, growth, and development of insects by rearing Bicyclus anynana butterflies under artificially generated smoke as well as reared in clean air but fed on plants previously exposed to smoke. Direct haze exposure significantly increased the mortality of caterpillars, increased larval development time, and decreased pupal weight, while indirect haze exposure, via ingestion of haze-exposed food plants, also affected development time and pupal weight. No smoke particles were found in the tracheae of subjects from the smoke treatment suggesting that the increase in development time and mortality of B. anynana under smoke conditions might be due to toxic smoke gases and toxic food, rather than particulate matter. These results document significant deleterious effect of haze smoke to the development, adult size, and survival of insects, key players in food-webs. The Southeast Asian (SEA) transboundary smoke plumes, commonly known as "haze", occur when commercial tree plantation developers and local farmers, mainly from the Indonesian islands of Sumatra and Borneo, clear forest lands using fire 1,2. The phenomenon peaks during the intermonsoon dry season between June to October and is exacerbated when there are prolonged draughts during El Nino years 1,3. When carried by the monsoon winds, the haze affects local dwellers as well as those in neighboring countries such as Malaysia and Singapore 4-7. The haze impacts both human health and local economies 8,9. Field studies have identified over 100 compounds in the smoke from forest fires. They include carbon dioxide and monoxide (CO 2 and CO), methane (CH 4), nitrogen oxides (NOX), sulphur dioxide (SO 2), polycyclic aromatic hydrocarbons (PAHs), and smoke particles (also known as particulate matter, PM) 10. Particulate matter less than 2.5 μm in aerodynamic diameter (PM2.5), especially, can invade the smallest airways in the lungs, aggravating respiratory and cardiovascular diseases 11-13. For instance, the 1997 SEA haze episode reached hazardous levels of ground-level particulate matter concentrations, and was followed by an increase in respiratory problems in Singapore, together with a 2% annual increase in regional adult cardiovascular mortality 12. In addition, haze has brought economical losses because of falls in tourism, aviation, industrial investment and production 6. Despite the high number of studies on the effect of the haze on human health and lifespan, its impact on other species and on ecosystems has rarely been addressed. Herbivorous insects are primary consumers that link plants to higher-level consumers in food webs. They are also key players in many ecological functions 14 , such as in plant pollination 15 , seed dispersal 16,17 , or soil aeration 18. Because some insects have high sensitivity to environmental change, they are used as bioindicators of the health of a habitat or an ecosystem (e.g. 19,20). Butterflies, in particular, are used for these purposes because, in addition to their sensitivity to environmental disturbances, they are easy to identify and monitor. In Indonesian forests, for instance, butterfly species richness and abundance was directly linked to human activities such as forest logging and fires, presumably via direct destruction of their habitat 21,22. In addition to direct impacts, forest fires can have indirect effects on butterflies as well as other insects via the generation of smoke plumes. No study, however, has investigated the effects of smoke haze on insects. One way in which haze might affect insects is by impacting their respiratory system, as it does for humans. Insects breath air via spiracles, valve-like openings on the side of their bodies that connect to internal tracheal tubes 23,24 which branch repeatedly into finer tracheoles that eventually reach every cell inside the insect's body,