Maintaining understory vegetation in oil palm plantations supports higher assassin bug numbers

• Published in: Ecological solutions and evidence Vol. 4; no. 4
• Main Authors: Stone, Jake, Advento, Andreas Dwi, Pashkevich, Michael D., Aryawan, Anak Agung Ketut, Caliman, Jean‐Pierre, Hood, Amelia S. C., Foster, William A., Naim, Mohammad, Pujianto, Purnomo, Dedi, Suhardi, Tarigan, Ribka Sionita, Rambe, Tuani Dzulfikar Siguga, Widodo, Rudy Harto, Luke, Sarah H., Snaddon, Jake L., Turner, Edgar C.
• Format: Journal Article
• Language: English
• Published: Hoboken John Wiley & Sons, Inc 01.10.2023; Wiley
• Subjects: Abundance; assassin bugs (Reduviidae); Biodiversity; Biodiversity loss; biological control agents; Chemical pest control; Complexity; Crops; Ecological function; Forests; habitat heterogeneity; Harvesting; Herbicides; Heterogeneity; integrated pest management (IPM); Life span; oil palm agroecology; Pest control; Pesticides; Pests; Plantations; Population decline; Species richness; Spraying; tropical agriculture; Understory; understory vegetation; Vegetable oils; Vegetation; Southeast Asia; Sumatra; Indonesia
• ISSN: 2688-8319; 2688-8319
• DOI: 10.1002/2688-8319.12293

The expansion of oil palm agriculture across Southeast Asia has caused significant biodiversity losses, with the reduction in habitat heterogeneity that accompanies the conversion of forest to oil palm being a major contributing factor. However, owing to their long commercial lifespan, oil palm plantations can support relatively high levels of vegetation complexity compared to annual crops. There is therefore potential for the implementation of management strategies to increase vegetation complexity and associated within‐plantation habitat heterogeneity, enhancing species richness and associated ecosystem functioning within productive oil palm landscapes. This study focusses on two species of asassin bugs Cosmolestes picticeps and Sycanus dichotomus, which are important agents of pest control within oil palm systems. Using a Before‐After Control‐Impact experimental manipulation in Sumatra, Indonesia, we tested the effect of three alternative herbicide spraying regimes and associated vegetation complexity treatments on assassin bug numbers. Our treatments encompass a range of current understory vegetation management practices used in oil palm plantations and include removing vegetation only in areas key to harvesting (“Normal”), removing all understory vegetation (“Reduced”), and allowing native vegetation to regrow naturally (“Enhanced”). We assessed both the long‐term (18 months) and short‐term (within 2 weeks) effects of our treatments following herbicide spraying. Pre‐treatment, we found high numbers of assassin bugs of both species in all plots. Long‐term post‐treatment, the abundance of both C. picticeps and S. dichotomus declined in reduced understory plots, although this decline was only significant for C. picticeps (98%). In contrast, there were no significant differences in the post‐treatment abundance of either species in the short‐term. These results suggest that the long‐term decline in assassin bug abundance was likely to be caused by loss of vegetation, rather than any immediate effects of the herbicide spraying. Our findings have clear management implications as they demonstrate that maintaining vegetation in oil palm understories can benefit an important pest control agent. Ekspansi pertanian kelapa sawit di Asia Tenggara telah menyebabkan hilangnya keanekaragaman hayati secara signifikan, dengan berkurangnya heterogenitas habitat yang menyertai konversi hutan menjadi kelapa sawit menjadi salah satu faktor penyebabnya. Namun, karena umur komersialnya yang panjang, perkebunan kelapa sawit dapat mendukung tingkat kompleksitas vegetasi yang relatif tinggi dibandingkan tanaman tahunan. Oleh karena itu, terdapat potensi penerapan strategi pengelolaan untuk meningkatkan kompleksitas vegetasi dan heterogenitas habitat perkebunan, meningkatkan kekayaan spesies dan fungsi ekosistem terkait dalam lanskap kelapa sawit yang produktif. Penelitian ini berfokus pada dua spesies kepik pembunuh Cosmolestes picticeps dan Sycanus dichotomus, yang merupakan agen penting pengendalian hama dalam sistem kelapa sawit. Dengan menggunakan manipulasi eksperimental Sebelum‐Sesudah Kontrol‐Dampak di Sumatera, Indonesia, kami menguji pengaruh tiga cara penyemprotan herbisida alternatif dan perlakuan kompleksitas vegetasi yang terkait terhadap jumlah kepik pembunuh. Perlakuan kami mencakup serangkaian praktik pengelolaan vegetasi lantai yang saat ini digunakan di perkebunan kelapa sawit, termasuk menghilangkan vegetasi hanya di area yang penting untuk dipanen (“Normal”), menghilangkan semua vegetasi lantai (“Reduced”), dan membiarkan vegetasi asli tumbuh kembali secara alami (“Enhanced”). Kami menilai efek jangka panjang (18 bulan) dan jangka pendek (dalam 2 minggu) dari perlakuan yang kami berikan setelah penyemprotan herbisida. Dari masa pra‐perlakan, kami menemukan sejumlah besar kepik pembunuh dari kedua spesies di seluruh plot. Setelah perlakuan dalam jangka panjang, kelimpahan C. picticeps dan S. dichotomus menurun pada plot vegetasi lantai “Reduced”, meskipun penurunan ini hanya signifikan pada C. picticeps (98%). Sebaliknya, tidak ada perbedaan signifikan dalam kelimpahan kedua spesies setelah perlakuan dalam jangka pendek. Hasil ini menunjukkan bahwa penurunan kelimpahan kepik pembunuh dalam jangka panjang kemungkinan besar disebabkan oleh hilangnya vegetasi, dan bukan akibat langsung dari penyemprotan herbisida. Hasil studi kami menunjukkan bahwa memelihara vegetasi lantai dalam lahan kelapa sawit dapat memberikan manfaat bagi agen pengendalian hama yang penting. Reduced understory vegetation complexity in oil palm plantations leads to a significant drop off in numbers of assassin bugs, which are major pest control agents within the system. This suggests that maintaining understory vegetation in oil palm has the potential to boost both biodiversity and pest control.