Streamlining leaf damage rating scales for the fall armyworm on maize

Abstract

The fall armyworm (Spodoptera frugiperda, Lepidoptera: Noctuidae), which is native to the Americas, has recently invaded Africa and Asia. There, it has become a major pest of maize (Zea mays). The variety of methods used to assess feeding damage caused by its caterpillars makes it difficult to compare studies. In this paper, we aim at determining which leaf damage rating scales for fall armyworm are most consistently used for which purposes, might provide most possibilities for statistical analyses, and would be an acceptable compromise between detail and workload. We first conducted a literature review and then validated the most common scales under field and laboratory conditions. Common leaf damage scales are the nominal “yes-no damage scale” that only assesses damage incidence, as well as difficult-to-analyse ordinal scales which combine incidence and severity information such as the “Simple 1 to 5 whole plant damage scale”, “Davis’ 0 to 9 whorl & furl damage scale”, or “Williams’ 0 to 9 whole plant damage scale”. These scales have been adapted many times, are sometimes used incorrectly, or were wrongly cited. We therefore propose simplifications of some of these scales as well as a novel “0.0 to 4.0 fall armyworm leaf damage index” which improves precision and possibilities for parametric data analyses. We argue that the choice of a scale to use should depend on the desired level of detail, type of data analyses envisioned, and manageable time investment.

Insecticide seed treatment against corn leafhopper: helping protect grain yield in critical plant growth stages

“…In addition, the search, development, and use of maize hybrids tolerant to corn stunt disease is no less important than efforts against other key pests 70–74 . Accordingly, the visual stunt rating scale developed here may be helpful, like the damage scales used for other key maize pests 75 . Some maize stunt rating scales have been proposed and utilized 76–80 despite lacking clear validation with published empirical data supporting the correlation with corn yield.…”

“…[70][71][72][73][74] Accordingly, the visual stunt rating scale developed here may be helpful, like the damage scales used for other key maize pests. 75 Some maize stunt rating scales have been proposed and utilized [76][77][78][79][80] despite lacking clear validation with published empirical data supporting the correlation with corn yield. Notably, the rating scale we used here exhibited accuracy and a strong relationship with the corn realized yield, consistently fitting the greenhouse and field data and indicating its robustness.…”

Laboratory and field trials reveal the potential of a gel formulation of entomopathogenic nematodes as biocontrol against the fall armyworm (Spodoptera frugiperda)

“…2. Plant damage was assessed using the Davis scale (Davis whorl & furl damage scale; Davis et al, 1992) as described in Toepfer et al (2021). Then we searched for surviving caterpillars.…”

“…15 cm distance) to mimic the effect of the dew. Six days post treatment, we evaluated plant damage using the Davis scale (Davis whorl & furl damage scale; Davis et al, 1992) as described in Toepfer et al (2021), where a score of “0” represent an intact plant while a score of “9” represent a plant that is almost totally destroyed. Subsequently we counted the number of surviving caterpillars.…”

Laboratory efficacy of selected synthetic insecticides against second instar invasive fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae) larvae

“…Damage occurs in skeletonized leaves and severely windowed whorls in the first instar larvae, who scrape leaves and display pin-hole symptoms and window-pane eating symptoms, but damage results in the later vegetative stages in skeletonized leaves and window-pane feeding symptoms. If the environmental conditions for pest establishment are favorable, this insect might cause a 100% crop loss in maize if it is not controlled on time [ 12 , 13 ].…”