bWe hypothesized that interactions between fusarium head blight-causing pathogens and herbivores are likely to occur because they share wheat as a host plant. Our aim was to investigate the interactions between the grain aphid, Sitobion avenae, and Fusarium graminearum on wheat ears and the role that host volatile chemicals play in mediating interactions. Wheat ears were treated with aphids and F. graminearum inoculum, together or separately, and disease progress was monitored by visual assessment and by quantification of pathogen DNA and mycotoxins. Plants exposed to both aphids and F. graminearum inoculum showed accelerated disease progression, with a 2-fold increase in disease severity and 5-fold increase in mycotoxin accumulation over those of plants treated only with F. graminearum. Furthermore, the longer the period of aphid colonization of the host prior to inoculation with F. graminearum, the greater the amount of pathogen DNA that accumulated. Headspace samples of plant volatiles were collected for use in aphid olfactometer assays and were analyzed by gas chromatography-mass spectrometry (GC-MS) and GC-coupled electroantennography. Disease-induced plant volatiles were repellent to aphids, and 2-pentadecanone was the key semiochemical underpinning the repellent effect. We measured aphid survival and fecundity on infected wheat ears and found that both were markedly reduced on infected ears. Thus, interactions between F. graminearum and grain aphids on wheat ears benefit the pathogen at the expense of the pest. Our findings have important consequences for disease epidemiology, because we show increased spread and development of host disease, together with greater disease severity and greater accumulation of pathogen DNA and mycotoxin, when aphids are present. P lant interactions with pathogens or insects are usually studied in isolation, even though plants are exposed to both in the field. It is likely that organisms that share the same host plant can alter its condition and thereby interact with each other. Although Fusarium pathogens and grain aphids cause substantial damage to wheat and coincide, little is known about interactions between these species.Fusarium head blight (FHB) is a disease of small grain cereals that occurs globally and is caused by a complex of species from the genera Fusarium and Microdochium (1, 2). The most aggressive species causing FHB is Fusarium graminearum, a potent producer of the mycotoxin deoxynivalenol (DON), which is harmful to human and animal health if consumed (3). European legislation has set the acceptable limit of DON in grain marketed for human consumption at 1,250 g kg Ϫ1 (4) in unprocessed cereals. Consequently, FHB epidemics result in large economic losses from the combined effects of reductions in yield, increased mycotoxin contamination, and reduced quality and marketability of the crop (5).In the spring, ascospores and/or conidia of Fusarium graminearum are released from crop residues and are spread by wind or rainsplash. Wheat is most susceptible to FHB du...