Validation of a model for estimating leaf-feeding by insects in soybean

Abstract

A computer model (AUSIMM) estimate of the total leaf area consumed by soybean, Glycine max (L.) Merrill, leaf-feeding insect larvae was compared with measured leaf area loss after feeding by endemic field infestations in 11 field experiments. Estimates of percentage defoliation and total leaf area, both at the onset of feeding by endemic insect infestations and after the insect feeding period, were also compared with yield. Insect infestations consisted of mixed sizes of green cloverworm, Plathypena scabra (F.) (Lepidoptera: Noctuidae), velvetbean caterpillar, Anticarsia gemmatalis (Hübner) (Lepidoptera: Noctuidae), or soybean looper, Pseudoplusia includens (Walker) (Lepidoptera: Noctuidae). Insect infestations occurred over about a 14-day period during the R5 and R6 growth stages (beginning of seed formation to pod fill). Leaf consumption varied from ∼ 1000 to 14 000 cm 2 per row-m depending on the species, number and size of larvae. The model accurately predicted leaf area consumption over all experiments. Predicted consumption increased linearly with measured leaf area loss ( p = 0.027, r 2 = 0.47). Estimated percentage defoliation at the onset of feeding by endemic infestations was ∼ 8%, and was 5% and 20–68% after the insect feeding period in treated and untreated plots, respectively. Yield decreased with an increase in estimated percentage defoliation, but the relationship was not significant. Total leaf area varied from ∼9000 to 27 000 cm 2 per row-m among experiments. Yield increased linearly with total leaf area, both at the onset of insect feeding ( p = 0.004, r2 = 0.41) and after the insect feeding period. Total leaf area remaining after insect feeding was the best indicator of soybean yield ( p = 0.002, r 2=0.46). A 30% increase in total leaf area after feeding resulted in a 168.2 kg ha −11 (2.5 bushels acre −1) increase in yield.