Richmond D S, Bigelow C A
Department of Entomology, Purdue University, West Lafayette, IN 47907, USA.
Environ Entomol. 2009 Aug;38(4):996-1004. doi: 10.1603/022.038.0406.
This study examined how variability in Neotyphodium endophyte-grass associations influences black cutworm Agrotis ipsilon Hufnagel performance and susceptibility to the entomopathogenic nematode Steinernema carpocapsae (Weiser). Second-instar cutworm larvae were confined to greenhouse pots containing four different tall fescue Schedonorus phoenix (Scop.) Holub cultivars. After 1 wk, larvae were recovered from the pots, weighed, and individually exposed to 20 infective juvenile nematodes. Nematode-induced mortality was monitored for 72 h after exposure. Endophyte infection levels and ergot alkaloid concentrations varied between tall fescue cultivars, but endophyte infection level was not a significant predictor of ergot alkaloid concentrations in above-ground plant tissue. Larval survival also varied between cultivars, but neither endophyte infection level nor ergot alkaloid concentration was a significant covariate. Larval susceptibility to the entomopathogenic nematode varied between cultivars at 48 and 72 h after exposure. In all but one cultivar (Plantation), cumulative mortality at 72 h decreased significantly as ergot alkaloid concentrations increased. Neither larval biomass nor endophyte infection levels in tall fescue were significant predictors of larval susceptibility to the nematode. Results show that variation in endophyte-plant associations can influence black cutworm performance and susceptibility to entomopathogenic nematodes and that susceptibility to the nematode H. bacteriophora may be partially tied to ergot alkaloid levels in the insects' food. Findings further support the assertion that black cutworm may use certain endophyte-mediated toxins, particularly ergot alkaloids, as a form of acquired chemical defense against nematode-induced septicaemia.
本研究考察了内生真菌——新麦角菌与禾本科植物的组合变异性如何影响小地老虎（Agrotis ipsilon Hufnagel）的生长性能及其对昆虫病原线虫小卷蛾斯氏线虫（Steinernema carpocapsae (Weiser)）的易感性。将二龄小地老虎幼虫置于装有四种不同高羊茅（Schedonorus phoenix (Scop.) Holub）品种的温室花盆中。1周后，从花盆中取出幼虫，称重，并分别暴露于20条感染性幼虫线虫中。暴露后72小时监测线虫引起的死亡率。高羊茅品种间内生真菌感染水平和麦角生物碱浓度各不相同，但内生真菌感染水平并非地上植物组织中麦角生物碱浓度的显著预测指标。不同品种间幼虫存活率也有所差异，但内生真菌感染水平和麦角生物碱浓度均不是显著的协变量。暴露后48小时和72小时，不同品种间幼虫对昆虫病原线虫的易感性各不相同。除一个品种（种植园）外，在所有品种中，随着麦角生物碱浓度的增加，72小时时的累积死亡率显著下降。高羊茅中的幼虫生物量和内生真菌感染水平均不是幼虫对线虫易感性的显著预测指标。结果表明，内生真菌与植物组合的变异性可影响小地老虎的生长性能及其对昆虫病原线虫的易感性，并且对嗜菌异小杆线虫的易感性可能部分与昆虫食物中的麦角生物碱水平有关。研究结果进一步支持了这样的观点，即小地老虎可能利用某些内生真菌介导的毒素，特别是麦角生物碱，作为一种后天获得的化学防御机制来抵御线虫引起的败血症。
