In the amaranth growing region of Tulyehualco (Valley of Mexico) stem tunneling attributed to larvae of the curculionid Hypolixus truncatulus has been reported. The aim of this study was to test the effect of cultivar and plant densities on stem tunnelling. A bifactorial experiment with two cultivars and two plant density was carried out. The cultivars were Frondosa and Tulyehualco, and plant densities were 62 500 and 380 000 plants ha-1. Vegetative growth, number or larvae, percentage of stem tunnelling and yield data were collected from 10 plants in each of 16 experimental plots. All sampled plants showed stem tunnelling damage. Hypolixus truncatulus and Amauromyza abnormalis were identified from larvae stages. Both species have been reported in Mexican Amaranth cultures. The shorter cultivar (Frondosa) had 70% of the stem affected by larvae tunnelling, in contrast to 80% in the taller cultivar (Tulyehualco); statistical differences were significant (P<0.05). The number of larvae at harvest was significantly larger in the taller cultivar (18 larvae) compared to Frondosa (12.5 larvae). High plant density resulted in a significant reduction of stem diameter for both cultivars; however, there were no differences (P>0.05) in number of larvae per plant and in stem tunnelling damage in the two plant densities. These results are in disagreement with the hypothesis that a reduction in stem diameter caused by a change in amaranth canopy should result in a smaller larvae population and damage by stem borers. The number of larvae and percentage of stem tunnelling damage did not affect grain yield and biomass production parameters. Results suggest that the shorter length of the production cycle for the early cultivar (Frondosa) might favour a smaller larvae population and a lesser damage in stem tunnelling.
