On 22 July 2021, I collected about a dozen half-ripe seedheads of the large tussock grass, tall fescue, Schedonorus arundinaceus (Schreb.) Dumort. (syn. Festuca arundinacea Schreb., Lolium arundinaceum (Schreb.) Darbysh.) growing in coastal pasture at Hestefaelleden near Copenhagen, Denmark (55.570881°N 12.558981°E), and brought them home in a spacious ziplock plastic bag. This is my standard procedure for collecting gall midge larvae living in grass spikelets, where they feed on the ripening fruits (caryopses) without leaving any externally visible signs of their presence. No immature gall midges appeared. Instead, after four days, the emergence of large numbers of tiny, green, metallic chalcid wasps appeared. A single male gall midge of the genus Stenodiplosis emerged as well, but not a single larva left the spikelets. The emerging wasps left neat circular exit holes in the grass glumes. Dissection of spikelets revealed partially eaten-out caryopses. The large number of bored spikelets and caryopses and the almost complete absence of other insects, as evidenced by dissecting numerous spikelets under the microscope, provide strong evidence that the wasp larvae had been feeding on the unripe grass fruits as herbivores (they could be called seed predators as they destroy their 'prey'). The reared specimens were sent to Richard R. Askew, who identified them as Mesopolobus pseudolaticornis von Rosen, 1966. Hans von Rosen described the species based on material reared from the same host plant species ('seeds of Festuca arundinacea') collected on 26 August 1963 at Lomma, Scania, Sweden, ca. 30km in a direct line from my collection site. von Rosen did not observe any other insect larvae in the grass seedheads, on which the wasp larvae could have been parasitoids. He specifically noted that 'based on the data available so far, it is not impossible, however unlikely, that M. pseudolaticornis leads a phytophagous lifestyle and produces more than one generation per year'. Graham (1969) caught specimens of M. pseudolaticornis in a coastal marsh at Lodmoor, near Weymouth, Dorset, England. Their host was unknown to him, but the presence of Schedonorus arundinaceus at the site is quite likely. Mesopolobus Westwood is a large, cosmopolitan genus, with most of its members parasitoids of insects from several orders (Diptera, Lepidoptera, Hemiptera, Hymenoptera). It is quite possible that secondary phytophagy-evolution of phytophagous daughter lineages from parasitoid ancestors-has arisen within the genus, much the same as secondary phytophagy has arisen multiple times within the superfamily Chalcidoidea and has been proposed as a driver of chalcid diversification (Blaimer et al. 2023). Within Pteromalidae, at least one example of secondary phytophagy is known: Blascoa ephedrae Askew, 1997, in which the solitary larva feeds on a developing seed of Ephedra nebrodensis Tineo in Spain (Askew & Blasco-Zumeta 1997). Whether M. pseudolaticornis is obligately phytophagous and whether its closest relatives within the genus are also phytophagous remains to be discovered. Material examined 30♂♂48♀♀, em. 26-30.vii.2021, Coll. 22.vii.2021, Hestefaelleden, Amager, Denmark