TOXICITY OF ALKALOID FRACTIONS FROM Psychotria spp. (RUBIACEAE) AGAINST Atta sexdens FOREL, 1908 (HYMENOPTERA: FORMICIDAE)

ABSTRACT Leaf-cutting ants are the main pests in forest plantations. The most commonly used chemical control for the ants is toxic bait. However, the active ingredients in these baits have been restricted by forest certification organizations, justifying the search for new active compounds to control these insects. Thus, this work aimed to evaluate the formicidal activity of alkaloid fractions from the roots, stems, branches and leaves of Psychotria hastisepala and Psychotria leiocarpa (Rubiaceae) against Atta sexdens Forel, 1908 (Hymenoptera: Formicidae). The alkaloid fractions were obtained from the crude methanolic extracts of P. hastisepala and P. leiocarpa by the acid-base extraction method employing liquid-liquid partitions. The fractions, previously solubilized in propanone, were applied topically to the pronotum of worker ants. Bioassays were conducted in a completely randomized design, with six replicates of each treatment and 10 ants per replicate. Insect survival was assessed daily for 21 days. The median lethal time was estimated by Weibull models. The root fractions from P. hastisepala presented the highest toxicity, followed by those from the branches, stems and leaves. Among fractions from P. hastisepala roots, the most toxic were a fraction that apparently is rich in lipophilic compounds and nonbasic alkaloids and a fraction that is likely rich in quaternary benzophenanthridine alkaloids, protopine alkaloids and tertiary bases. Psychotria leiocarpa fractions were not toxic to this insect. Psychotria hastisepala is potentially useful for the development of new products for A. sexdens control.

摘要 切叶蚁（Leaf-cutting ants）是人工林的主要害虫。当前防治此类蚂蚁最常用的化学防控手段为毒饵（toxic bait），但这类毒饵中的有效成分已被森林认证机构（forest certification organizations）限制使用，因此亟需研发新型活性化合物以防控该类昆虫。为此，本研究旨在评估尖萼九节（Psychotria hastisepala）与光果九节（Psychotria leiocarpa，茜草科（Rubiaceae））的根、茎、枝、叶中的生物碱组分对塞氏切叶蚁（Atta sexdens Forel, 1908，膜翅目（Hymenoptera）: 蚁科（Formicidae））的杀蚁活性。研究采用酸碱萃取法结合液液分配步骤，从两种植物的粗甲醇提取物中分离得到生物碱组分；将前述组分预先溶于丙酮（propanone）后，以局部施药方式涂抹于工蚁前胸背板（pronotum）。生物测定采用完全随机设计方案，每个处理设置6次生物学重复，每次重复使用10头工蚁，连续21天每日记录试虫存活率，并通过Weibull模型（Weibull models）估算半数致死时间（median lethal time）。结果显示，尖萼九节的根组分杀蚁活性最高，其余依次为枝、茎与叶组分；在尖萼九节的根组分中，两类组分杀蚁活性最为突出：一类疑似富含亲脂性化合物（lipophilic compounds）与非碱性生物碱（nonbasic alkaloids），另一类大概率含有季铵型苄菲啶生物碱（quaternary benzophenanthridine alkaloids）、原阿片碱类生物碱（protopine alkaloids）与叔胺碱（tertiary bases）。光果九节的各组分对该试虫均无杀蚁活性。综上，尖萼九节具备开发用于防治塞氏切叶蚁的新型植保药剂的潜力。