Larvicidal factors contributing to host-plant resistance against sunflower moth

Abstract

One of the most effective economical and environmentally acceptable forms of agricultural pest control consists of built-in resistance or immunity of the host plant toward potential predators. By establishing the nature of host-plant resistance, one may hope to facilitate the selection of crops possessing immunity of this sort. Sunflower moth (Homoeosoma electellum, H.) is a major pest of sunflower (Helianthus annus, L.) an increasingly important crop in the United States. Sazyperov [1] and Kiewnick [2] have suggested that the resistance of sunflower seed to damage by European sunflower moth (H. nebulella, H.) is associated with the presence of phytomelanin or "armored layer" in the seed coat which becomes recognizable after chromic-acid treatment [2, 3]. The term "armored layer" implies physical resistance to penetration of the seed coat by the larvae. Correlation between the presence of a phytomelanin layer and reduced larval damage was also reported in the United States [3]. We find evidence that host-plant resistance to the sunflower moth (H. electellum) is caused by chemical factors. We have observed that sunflower-moth larvae in their first and second instars feed mostly on florets of the host plant. As the larvae grow, they proceed to attack the immature seeds. On occasions some larvae do penetrate the seeds of resistant varieties, although the damage is less severe for them than for susceptible varieties. The penetrabilities of immature sunflower seeds from both resistant and susceptible varieties, measured by a tensile tester, are not significantly different. On the other hand the young larvae died within a week after being fed on a synthetic diet [4, 5] containing 5% ofhexane extract from florets of a resistant variety (H 2135) while normal development occurred with larvae fed on a diet containing 20% of the extraction residue. Liquid chromatography (silica gel) of this extract yielded two fractions which inhibited larval growth. Crystallization gave two isomeric diterpenoid acids (Fig. 1) which were shown to be the previously described trachyloban-19-oic (I) and ( ) kaur16-en19-oic (II) acids by comparison with published data [5, 6]. Quantitative GLC analysis (1.8 m OV-17 at 200~ of I and II as methyl esters indicated that dried sunfower florets maycontain in some cases over 5 % in combined amount of these compounds. This result depends upon the maturity of the floret as well as the particular variety of sunflower, and tests are underway to correlate levels of I and II with field resistance to moth damage.