Results

As of mid-March 2026 I have documented nearly 800 associations involving an endophagous insect species feeding inside a stem or stemlike structure of a plant species in the study area. I lumped some records together -- such as mutiple host plant species in a host genus that all accommodate the same species of insect -- so the total number of unique associations between an insect species and a plant genus sits at roughly 710 currently (March 14, 2026).

These 710 records involve about 265 host plant genera, spanning grasses, forbs, vines, shrubs, and trees. Approximately 82 of these records occurred on woody plants, and the remaining 628, comprising about 88.5% of the total, occurred on herbaceous plants. The stem was the most common plant part affected (77% of records), followed by petiole (16%) and midrib (6%), but there may be overlap in these figures since some insects migrated between plant parts over the course of their feeding.

Of the feeding mode labels I assigned to the insects in the 710 records, 50% were borers for all or part of their feeding activity, 22% were miners, and 28% were local feeders (with the percentages again not mutually exclusive, especially between the miners and borers since some insects conducted both mining and boring). The feeding mode definitions I used in the survey are specified in the Introduction.

For approximately 690 records, I was able to determine the basic taxonomy of the insect. Diptera accounted for 45.9% of the total, followed by Lepidoptera (25.2%), Coleoptera (21.3%), Hymenoptera (6.5%), and Hemiptera (1.0%). Beetles and moths in woody stems are probably somewhat undercounted, since I intentionally chose not to focus on these, but the relative abundance of the orders is mostly similar even if we only consider records from herbaceous stems, in which I did not deliberately neglect any taxa. (Hymenoptera take up a significantly smaller proportion in herbaceous stems because the ultra-speciose cynipids in oak trees are not being counted in that case). At the family level, the two most abundant insect taxa in the survey both belonged to Diptera: agromyzids (22%) and cecidomyiids (18%) together accounted for 40% of all records. Among beetles, mordellids (7%) were the most common, followed by curculionids (6%) and erotylids (2%). Frequently encountered moths included noctuids (5%), tortricids (4%), and gracillariids (4%). Cynipid wasps made up 3% of the total; eurytomids, 2%.

The dominance of flies in stems and stemlike structures is perhaps surprising at first glance, since beetles and moths in stems (especially woody stems) seem to be better studied, and the "classic" image of a stem borer in the minds of the general public is probably a caterpillar or beetle larva. Nevertheless, such a preponderance of flies may be less surprising when one considers recent findings about the relative diversity of the major insect orders. As Forbes et al. (2018) summarize: "[A] mass-barcoding study of Canadian insects found both Hymenoptera and Diptera were more diverse than Coleoptera...After Hymenoptera, the Coleoptera may not even be the second most-speciose order; several recent inventories of species diversity suggest that the Diptera may hold that title" (p. 9).

I have compiled information about some notable patterns I observed among endophagous insects in stems and stemlike structures during the survey. The icon after the link in each list entry below indicates the type of document ( = HTML image gallery, = audio voice recording). Audio links go to the individual files on the Dropbox website.

• Cohabitation - A few examples of multiple endophagous insects sharing a single hostplant stem or stemlike structure.
• Parasitoids - A selection of parasitoids encountered during the survey.
• Exit holes - A comparative photographic study of the emergence holes left by larvae and adults escaping from stems and stemlike structures. Also see "Exit strategies" audio recording, below.
• Stem stumps - Identifies insects that create stem stumps, describes the features of a stem stump, and speculates about their possible function.
• Grazers vs. borers - What is the proper term for an insect that feeds and travels inside a hollow stem? Are these insects borers, and if not, what similarities do they show to borers?
• Shreddings - Some borer larvae generate shreddings as part of their activities inside the stem. This recording discusses where shreddings may be found, which insects create them, and how the larvae use them. (Reference: Haack et al. 2017)
• Exit strategies - When a fly borer larva is done feeding inside a stem, what does it do next and how does the insect eventually get out of the stem? Here I discuss common "exit strategies" of borer flies that I observed in the survey.

For a table of all insect records from the survey, with links to individual record pages, navigate to Browse by Hostplant. Or, if you prefer, view a recent compilation of some of these data (minus the images) as a single PDF.