Physical characteristics of walnut husk in relation to infestation by walnut husk fly (Rhagoletis completa)

The walnut husk fly (WHF), Rhagoletis completa (Cresson), is a serious pest of walnuts and has increased in importance over the past 5 to 10 years. Early season WHF infestation results in reduced nut weight, edible yield weight and kernel colors. In addition, early season WHF infestation results in increased adhering husk, mold and shell staining in all cultivars. Seasonal and cultivar susceptibility is known to occur with WHF infestation in walnuts. Various physical characteristics such as trichome density, husk firmness, thickness, surface texture, nut size and color were analyzed on nine walnut cultivars in 2013 and 2014. These nine cultivars were then exposed to 15 gravid females for 72 h. There was a significant negative correlation between average trichome density and stings with eggs present among the cultivars. This indicates that high trichome density inhibits WHF infestation. During both years, 'Earliest' and 'Chandler' were examples of two cultivars that had a relatively high trichome count and low WHF infestation. In addition, there was a significant negative correlation between the mean red to green color range measurement of the nuts and the total number of stings with eggs found on each cultivar over the season. These characteristics can be incorporated into the evaluation of potential new cultivars for release to the growers. These nut characteristics are stable phenotypically over time and can be used in a breeding program. Initial crosses between 'Chandler' and 'Earliest' were made in the spring of 2017.