Moth caterpillar embryos and parasitoid egg infection as revealed in vivo and visualized by micro-CT scanning

Abstract

The Lepidopteran pest Trichoplusia ni and the parasitoid wasp Trichogramma brassicae represent a fascinating biological system, important for sustainable agricultural practices but challenging to observe. We present a nondestructive method based on micro-CT scanning technology (CT: computed tomography) for visualizing the internal parts of caterpillar embryos and of emerging parasitoids from infected eggs. Traditional methods of microscopic observation of the opaque egg contents require staining or dissection. To explore the biological system nondestructively, we optimized the application of micro-CT scanning to construct 3-D images of insects in vivo.

Keywords: Lepidopteran pest; Trichogramma wasp; X-ray tomography; embryo imaging; parasitoid biology.

Fig. 1.

Cross-sections of a Trichoplusia ni egg micro-CT scanned at (a) 12 h (day 1-time 1), (b) 36 h (day 2-time 1), (c) 60 h (day 3-time 1), and (d) 84 h (day 4-time 1) after oviposition. The density heatmaps show the development of the caterpillar embryo from the blastoderm at 12 h (red+yellow) in (a), with the complete formation of the organ tissue (green) at 60 h with the body positioned ventral concave in (c) and by 84 h (d) the caterpillar is nearing eclosion.

Fig. 2.

Trichoplusia ni 1st instar caterpillar emerging from the egg during micro-CT scanning on day 3-time 2. Fixed central axial cross-section showing: a) Dorsal artery and malpighian ampulla (red+yellow) and hindgut (green), b) convoluted midgut (zigzag green interspersed with blue), and c) brain and suboesophageal ganglion (red+yellow).

Fig. 3.

Parasitoid wasp, Trichogramma brassicae, newly emerged from a Trichoplusia ni egg during a micro-CT scanning session. a) Wasp midgut (peripheral green) shows large portions of high-density material (red+yellow) inside. This is evidence of the presence of concentrated host material within the gut of the insect, as host material is stored throughout larval development as urate crystals and released into the gut as uric acid in the early adult stage (Jarjees and Merritt 2002). b) Other structures that show up as high-density (red+yellow) are the thoracic ganglion and ventral nerve chord.