Biotic Stress and Yield Loss

feeding insects. However, the impacts of adult feeding on green corn silks duringplant flowering can be equally important from a yield perspective. High adultpopulations have been observed to trim silks back to the ear tips, resulting in partialkernel set and poorly filled ears. 77 With scatter-grained ears, kernel size is increased,while yields are reduced. 22Little has been written concerning the impact of adult corn rootworm silk feedingon corn yields. Leva 80 reported that with open pollination, three beetles per earon inbred corn and five beetles per ear on hybrid corn could significantly reduce earweight and kernel number per ear. Research to date indicates that silk length plays asignificant role in determining severity of corn rootworm feeding damage, withinjury tied more closely to the “mechanics of pollination” than to physiologicalchanges within the plant. 77 Silk lengths of 0.75 to 1 inch (inbred corn) or 0.5 inch(hybrid corn) have been determined as bordering on minimum lengths required toassure optimal pollination. 774.3.5.5 Japanese BeetlesAdult Japanese beetles, Popilla japonica, can disrupt corn reproduction through silkclipping and silk feeding behaviors similar to those observed for adult corn rootworms.Although Japanese beetles will feed gregariously on numerous plant species,corn in the silking stage (R1) is a highly preferred host. Adult beetle populationstherefore will concentrate in corn fields during silk emergence, hindering pollinationand seed set.22, 26Yield losses associated with Japanese beetle are tantamount to those observedfor other silk clipping insect pests. Average silk length during pollination (as regulatedby beetle feeding) is likely a reliable indicator of potential yield impacts. Theimpacts associated with reduced silk lengths are most probably similar to thoseobserved for corn rootworm.4.3.5.6 GrasshoppersGrasshoppers, Melanoplus spp., were also discussed earlier as foliage feeding pests.Although it is known that grasshoppers will readily feed on corn foliage, a distinctpreference for corn silks is regularly observed. Heavy grasshopper populations willregularly eat corn silks down to the cob, a practice that interferes with pollinationthrough mechanical injury to kernels. 26Injury first appears at field margins, as grasshoppers rarely originate from withinfields. 82 As with corn rootworm and Japanese beetles, silk length during pollinationand yield reductions are likely associated with injury that results in silks beingtrimmed to critically short lengths. 784.4 SUMMARY AND CONCLUSIONSWelter 6 noted that what is perhaps most interesting about the pattern of plantresponses to herbivory is not the uniqueness of these responses, but rather the similaritybetween plant responses to a diversity of biotic and abiotic stresses. Parallel