Morphometric analysis of instar variation in Tenebrio molitor (Coleoptera: Tenebrionidae)

Authors: Morales Ramos, Juan; KAY, SASHA - University Of Georgia; Rojas, Maria - Guadalupe; Shapiro Ilan, David; TEDDERS, WALKER - Southern Insectaries

Submitted to: Annals of the Entomological Society of America
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2014
Publication Date: 3/1/2015
Publication URL: http://handle.nal.usda.gov/10113/60850
Citation: Morales Ramos, J.A., Kay, S., Rojas, M.G., Shapiro Ilan, D.I., Tedders, W.T. 2015. Morphometric analysis of instar variation in Tenebrio molitor (Coleoptera: Tenebrionidae). Annals of the Entomological Society of America. 108(2):146-159.

Interpretive Summary: The yellow meal worm (Tenebrio molitor) is a commercially produced insect that has many uses including production of insect killing nematodes, food for pets, birds, and other animals. Meal worms could potentially be used as animal food, especially poultry and catfish and they are also human food in some countries. The yellow meal worm has the unusual characteristic of displaying a variable number of molts during their development. This characteristic has make it difficult for industry to predict and synchronize their development during production. Our research is an attempt to use sophisticated statistical methods to analyze head capsule, mandible, and body weight measurements to obtain a method for predicting or identifying their developmental stage. Cluster, discriminant, and frequency distribution analyses failed to provide a high level of confidence on identifying larval instars using body measurements. However, head capsule and body weight measurements were useful to determine the number molts a given larva (late in their development) would require to attain the pupal stage with a 95% confidence. This results could be used to develop methods to separate larvae by either weight or head size in a mass production facility. Methods for mass separation of late meal worm larval instars could fasilitate commercial separation of pupal stages, which are more suitable for human consumption.

Technical Abstract: Measurements of head capsule, mandible, metanotum, and body weight were done on larvae of Tenebrio molitor L. (Coleoptera: Tenebrionide) from the second to the last instar. Instar number varied from 14 to 18, but 15 or 16 instars were the most common. The value of dimensional measurements was evaluated as a tool for instar determination and to improve understanding of instar variation in T. molitor. Three methods of analysis were used including discriminant, cluster, and frequency distribution analyses. Stepwise regression analysis was used to determine the optimal combination of dimensional variables. Head capsule width, left mandible length, and body weight at the beginning of stadia were the most significant variables impacting instar. Discriminant analyses of instar using these three variables resulted in 47.74% misclassifications indicating a high degree of dimensional overlapping among instars. Dimensional overlap and variability increased in older instars. Cluster 15-group analysis using measurements of head capsule width and mandible length resulted in the lowest level of group variability; however, there was low degree of correspondence between cluster groups and instars. Frequency distribution analysis of head capsule width revealed 11 peaks, but only the early 8 peaks corresponded with observed instars. All three methods of morphometric analysis failed to identify all instars of T. molitor correctly. Frequency distribution analysis provided the best match, but only among instars 2 to 10. It appears that instar variation in the larval development of T. molitor occurs after the tenth instar. A model of T. molitor developmental plasticity is proposed.