棉铃虫低龄幼虫对花香挥发物的联系性学习

鳞翅目昆虫的寄主选择主要是成虫的任务,但幼虫也可以精细调节取食部位。低龄幼虫记忆一些与食物伴随的化学信息能够提高适合度。当幼虫被迫离开寄主植物后,最好是搜索与此前取食过的寄主气味相似的植物,以便节省寄主转移的生理代谢成本。棉铃虫幼虫嗜食作物花器,因此推测花香挥发物可以代表幼虫食物的典型化学信息。采用人工饲料为无条件刺激,在条件化训练开始时,使初孵幼虫取食时暴露在7种花香挥发物下,随后7 d逐日测试低龄幼虫学习表现。结果发现,在7次测试中,苯乙醛选择频次有3次显著多于对照,芳樟醇条件化组在2次测定中处理被选频次显著或极显著多于对照,苯乙醇条件化组仅1次对处理选择频次显著多于对照,其他4种挥发物无论训练时间长短均不能造成嗅觉偏好性的改变,说明棉铃虫在取食过程中是选择性采集食物关键化学信息。然而,无经历组无论训练时间如何均不对苯乙醛表现出嗅觉偏好性,证实气味偏好性的改变并非生理性成熟所致。总之,和同种的成虫以及若虫期比较活泼的其他种类相比,棉铃虫低龄幼虫嗅觉联系性学习表现较差,并且嗅觉偏好性变化与条件化训练时长没有明确关系,结合棉铃虫的生态学习性讨论了可能的原因。

Associative learning in early-instar larvae of Helicoverpa armigera in response to several floral volatiles

Associative learning is a basic mechanism required for herbivorous insects to adapt to highly variable environments. In most lepidopteran species, the adult stage is the main stage responsible for host plant selection, but newly hatched larvae can also contribute to host selection by moving a short distance from their birth position to feeding parts on plants. An understanding of mechanisms underlying olfactory associative learning in larvae would be helpful in enhancing the effectiveness of natural lethal factors on early instar stages via ecological interference and provide a scientific accordance for cultivating crop varieties that cannot be easily attacked by the larvae. Studies on olfactory associative learning in lepidopteran species have generally concentrated on the adult stage; little attention has been paid to early-instar larvae, probably because of their limited mobility and relatively simple life behaviors. Once the larvae are forced to leave their natal host plant, they need to move to plants with an odor profile that is similar to that of the previous host so that the larvae do not have to re-adapt to novel diet items and save the cost of physiological metabolism caused by host switch. As long as the memory of some key and stable cues is formed, the larvae can recognize and re-locate to these food plants, thus improving their fitness. Since the females of Helicoverpa armigera (Hübner) prefer to oviposit on or near the flowering parts of a plant and the larvae exclusively feed on flowering parts, we hypothesized that some key floral volatiles may represent the characteristic olfactory cues of their natural foods. In previous studies on larval classical conditioning, the subjects were starved enough time to enhance their feeding motivation, and the inter-stimulus intervals and inter-trial intervals were strictly controlled in the laboratory; thus, the ecological relevance of larval classical conditioning is weak. In this paper, we have reported the associative learning capability of and corresponding temporal pattern of learning performance in early-instar larvae of H. armigera (Hübner) (Lepidoptera: Noctuidae) to the association between conditioned stimuli (seven floral volatiles with biological meaning to H. armigera) and an un-conditioned stimulus (artificial diet). During a seven-day conditioning period, the larvae were continually exposed to the floral volatiles. Results of the conditioning show that the phenylacetaldehyde-conditioning group exhibited significant preference to phenylacetaldehyde. Phenethylalcohol- and linalool-conditioned groups exhibited significant preference to the corresponding conditioned stimuli. However, benzaldehyde, phenylmethanol, benzyl acetate, and geraniol did not produce a positive result at any conditioning time lengths; this suggested that the larvae selectively sensed and remembered the cue of some stable and key components during the feeding process, rather than responding to all the floral volatiles equally. In addition, the naïve group did not show any preference to phenylacetaldehyde in all the tests, regardless of the time length after 24 h of feeding; this suggested that the alteration of the olfactory preference during conditioning was not induced by physiological maturity. Taken together, H. armigera larvae exhibited olfactory learning capability, and the variation in olfactory preference had no relationship with the conditioning time length. Possible explanations for such findings have been discussed with respect to the ecological habits of the larvae.