Cabbage looper (Trichoplusia ni Hübner) labial glands contain unique bacterial flora in contrast with their alimentary canal, mandibular glands, and Malpighian tubules

Abstract

In recent years, several studies have examined the gut microbiome of lepidopteran larvae and how factors such as host plant affect it, and in turn, how gut bacteria affect host plant responses to herbivory. In addition, other studies have detailed how secretions of the labial (salivary) glands can alter host plant defense responses. We examined the gut microbiome of the cabbage looper (Trichoplusia ni) feeding on collards (Brassica oleracea) and separately analyzed the microbiomes of various organs that open directly into the alimentary canal, including the labial glands, mandibular glands, and the Malpighian tubules. In this study, the gut microbiome of T. ni was found to be generally consistent with those of other lepidopteran larvae in prior studies. The greatest diversity of bacteria appeared in the Firmicutes, Actinobacteria, Proteobacteria, and Bacteriodetes. Well‐represented genera included Staphylococcus, Streptococcus, Corynebacterium, Pseudomonas, Diaphorobacter, Methylobacterium, Flavobacterium, and Cloacibacterium. Across all organs, two amplicon sequence variants (ASVs) associated with the genera Diaphorobacter and Cloacibacterium appeared to be most abundant. In terms of the most prevalent ASVs, the alimentary canal, Malpighian tubules, and mandibular glands appeared to have similar complements of bacteria, with relatively few significant differences evident. However, aside from the Diaphorobacter and Cloacibacterium ASVs common to all the organs, the labial glands appeared to possess a distinctive complement of bacteria which was absent or poorly represented in the other organs. Among these were representatives of the Pseudomonas, Flavobacterium, Caulobacterium, Anaerococcus, and Methylobacterium. These results suggest that the labial glands present bacteria with different selective pressures than those occurring in the mandibular gland, Malpighian tubules and the alimentary canal. Given the documented effects that labial gland secretions and the gut microbiome can exert on host plant defenses, the effects exerted by the bacteria inhabiting the labial glands themselves deserve further study.

Tomato defences modulate not only insect performance but also their gut microbial composition

“…The cabbage looper gut bacterial community was predominantly composed of bacteria belonging to the Enterobacteriaceae family. This is consistent with previous findings in cabbage loopers reared on wheat germ diet, Arabidopsis thaliana, S. lycopersicum, and Brassica oleracea 28 , 44 , 62 , suggesting that Enterobacteriaceae spp. may be a constituent group .…”

“…Frass (insect feces) was used as a proxy for insect gut-associated bacteria. This choice was supported by previous studies on cabbage-fed T. ni that showed similar bacterial communities across different organs (i.e., the alimentary canal, Malpighian tubules, and mandibular glands) 44 . Similarly, in Lymantria dispar grown in the wild and the lab, small differences were found in bacterial abundance in frass and gut 45 .…”

“…Even under these homogenizing conditions, insect gut bacterial communities were not a mere reflection of the leaf microbiome, supporting that physicochemical conditions of the gut are a crucial component in shaping the bacterial communities. How those conditions are changed by plant structural and chemical defences, and whether the effect is direct or indirect by affecting insect physiology requires further investigation 44 , 61 , 64 .…”

“…are known to be overrepresented in terpene exposed caterpillars and induce higher mortality in Protaetia brevitarsis seulensis (Coleoptera) due to their pathogenic nature 68 , 69 . Bacteria of the genus Pseudomonas inhabit cabbage looper midguts and have been linked to weaker immune systems in other insects 44 , 70 , 71 . However, the presence of Pseudomonas spp.…”