Interactions between arbuscular mycorrhizal fungi and foliar‐feeding insects inPlantago lanceolataL.

Abstract

SUMMARYA field experiment was conducted to investigate whether infection by arbuscular mycorrhizal (AM) fungi has any effect on herbi\-ory by foliar-feeding insects. Plants of Plantago lanceolata L. were grown in a randomized block design and naturaJ le\'e)s of mycorrhjza! infection reduced by the application of tbe granular fungicide lprodione. Plant growtb responses were examined and herbivore bioassays performed by rearing both a chewing and sucking insect on tbe lea\'es of mycorrhizal and fungicide-treated plants.Fungicide application successfully reduced mycorrhiza] infection, and this led to reductions in foliar biomass, caused by a lower leaf number. However, fungicide-treated plants suffered consistently higher levels of damage by generalist chewing and leaf-mining insects, which colonized the plants. The chewing insect bioassay confirmed the field results, in tbat larvae oi Arctia caja L. (Lepidoptera: Arctiidae) consumed more leaf material from plants in wbicb infection was reduced.Tbere was no evidence that AM fungi altered food quality for the chewing insect. Instead, infection caused an increase in the carbon/nutrient balance, which in turn led to increased levels of the carbon-based feeding deterrents, aucubin and catalpol.The sucking insect, Mysus persicae (Sulzer) reacted in an opposite fashion to the chewer, with performance being greater on mycorrhizal plants. Again, there was no evidence that an alteration in food quality was the cause, and m this case infection may result in changes in leaf morpbolog\-wbich benefit tbe insect.We suggest that under conditions of high light and low nutrient availability, AM infection can alter the carbon/nutrient balance of plants, leading to an increased allocation to carbon-based defences. This can have important consequences for insect herbivore performance and the patterns of herbivory in field situations.

Intraspecific Variation in Plant Defense Alters Effects of Root Herbivores on Leaf Chemistry and Aboveground Herbivore Damage

“…Wireworms were collected and weighed during root washing. Since wireworms might have affected colonization of roots by arbuscular mycorrhizal fungi (AMF) with subsequent effects on leaf chemistry and herbivore damage Gange 2001;Gange and West 1994), random subsamples of approximately 1.5 g fresh roots were taken, stored in ethanol (50%), stained, and assessed for mycorrhizal fungi colonization (for details see Wurst et al 2004). Shoots and roots were frozen at −80°C, freeze-dried, weighed, and subsamples were ground for analyses of N and C, glucose, and IG.…”

Performance of the thistle gall fly, Urophora cardui, in relation to host plant nitrogen and mycorrhizal colonization

“…Not surprisingly, fertilization elevated stem N, but the reduction in mycorrhiza] colonization by fungicide addition resulted in a N increase in stem tissue comparable to that of the fertilizer addition. IVlycorrhizas have been shown to reduce p]ant N in P. lanceolata (Gange & West, 1994) and Cucumis sativa (cucumber) (Trimble & Knowles, 1995), although in the latter case the eifect was short-lived. More importantly, the treatments had much less effect on the N concentration of gall tissue.…”

“…Furthermore, these significant effects were obtained when other factors known to affect larval performance (Lalonde & Shorthouse, 1985) were taken into account. There is an accumulating body of evidence that suggests AM fungi can increase the resistance of plant tissues to insect herbivores (Rabin & Pacovsky, 1985;Gange, Brown & Sinclair, 1994;Gange & West. 1994).…”

“…Arbuscular mycorrhizal (AM) fungi can also aiTect the C: N ratio, which in turn can affect levels of secondary compounds and insect performance (Gange & West, 1994). In the latter study, Gange & West (1994) found that mycorrhizal Plantago lanceolata had lower levels of total N than did plants in which colonization was experimentalK' reduced, and other reports also suggest that AM colonization can tower soluble N levels (Trimble & Knowles, 1995). Mycorrhizal fungi are a considerable carbon drain on a host, w ith 10-20 " o of annual photosynthate being directed to the fungus (Jakobsen & Rosendahl, 1990) and this, coupled with the lowering of plant N concentration, could have serious consequences for a gall-forming insect which does not have the abilit>' to move and seek alternative food sources.…”

Testing for mycorrhizal fungi-plant-ant indirect effects

“…Which if any of these factors was responsible for the increase in damage in M' plants is unknown, but reduced toxicity and/or increased palatability are less likely, because mycorrhizal plants are typically able to mount more effective chemical defences to generalist herbivores compared to non-mycorrhizal plants (reviewed in Gehring and Whitham 2002). For example, the presence of mycorrhizal fungi leads to the increased production of the feeding deterrents aucubin and catalpol in Plantago lanceolata plants (Gange and West 1994).…”