Instar and sex determination for Macrosaccus morrisella (Lepidoptera: Gracillariidae)

The soybean tentiform leafminer, Macrosaccus morrisella (Fitch) (Lepidoptera: Gracillariidae), is a new potential pest of soybean; however, little is known about its biology. This study determined the number of instars, how to distinguish those instars, and how to differentiate the sexes of M. morrisella. Soybean plants oviposited on by M. morrisella in the laboratory were sampled daily to assess the development of the insects by measuring larval head capsule width, body length, and leaf area mined. Cages containing a subset of plants were then checked daily until the first emergence of adults. In addition, 20 pupae and resulting adults were sexed. The frequency distribution of head capsule widths of M. morrisella larvae showed 4 nonoverlapping peaks. However, examination of individuals comprising the third peak showed 2 distinct mouthpart morphologies, which were a sap-feeding 3rd instar and a tissue-feeding 4th instar with overlapping distributions of head capsule widths. Inspection of tentiform mines containing individual larvae revealed the presence of up to 4 shed head capsules, confirming the existence of 5 instars. Leaf area mined ranged from 0.28 to 498.45 mm² per larva. Pupae of M. morrisella were sexually differentiated by observing the presence of the gonads through the cuticle in males and its absence in females, and this was confirmed by examination of the frenula of the resulting adults. This study fills an important gap in our understanding of M. morrisella biology, which is crucial for developing integrated management strategies for this potential pest of soybean in North America.