Low Impact of Fall Armyworm (Spodoptera frugiperda Smith) (Lepidoptera: Noctuidae) Across Smallholder Fields in Malawi and Zambia

Abstract

Fall armyworm (Spodoptera frugiperda Smith), a serious pest of cereals from the Americas, has spread across sub-Saharan Africa and Asia since 2016, threatening the food security and incomes of millions of smallholder farmers. To measure the impact of S. frugiperda under different management approaches, we established on-farm trials across 12 landscapes (615−1,379 mm mean annual rainfall) in Malawi and Zambia during the 2019/2020 and 2020/2021 seasons. Here we present the results from our conventional tillage, monocrop maize, no pesticide treatment, which served to monitor the background S. frugiperda impact in the absence of control measures. Median plot-level S. frugiperda incidence ranged between 0.00 and 0.52 across landscapes. Considering severe leaf damage (Davis score ≥5), the proportion of affected plants varied between 0.00 and 0.30 at the plot scale, but only 3% of plots had ≥10% severely damaged plants. While incidence and damage severity varied substantially among sites and seasons, our models indicate that they were lower in high tree cover landscapes, in the late season scouting, and in the 2020/2021 season. Yield could not be predicted from S. frugiperda incidence or leaf damage. Our results suggest S. frugiperda impacts may have been overestimated at many sites across sub-Saharan Africa. S. frugiperda incidence and damage declined through the cropping season, indicating that natural mortality factors were limiting populations, and none of our plots were heavily impacted. Long-term S. frugiperda management should be based on Integrated Pest Management (IPM) principles, including minimising the use of chemical pesticides to protect natural enemies.

Understanding the impact of fall armyworm (Spodoptera frugiperdaJ. E. Smith) leaf damage on maize yields

“…However, infestation is a poor indicator of yield loss, because natural mortality, driven by natural enemies and other factors such as rainfall [40], is highly variable. Hence, infestation level imparts very little useful information [41].…”

“…It is also worth stressing that, despite the screen-house conditions and the fact that we removed transplants, dead or wilting plants and those infected with Maize Streak Virus from the sample, leaf damage score explained only a very small proportion of the variance (<3%) in grain yield. It may be argued that our sample was dominated by plants that experienced low levels of leaf damage (Figure 2), but these are typical of field conditions [41]. Grain yield varies from plant to plant in a screen-house primarily due to small differences in genetics, soil conditions, establishment success and access to light as the plants grow.…”

“…These thresholds are still not well understood [34], but a focus on the proportion of plants with severe leaf damage or infestation with late stage larvae (e.g. 4-6th instars) may be informative because these provide an index of the number of FAW larvae surviving to the late stages [41].…”

“…However, infestation is a poor indicator of yield loss, because natural mortality, driven by natural enemies and other factors such as rainfall [40], is highly variable. Hence, infestation level imparts very little useful information [41]. Assessments based on leaf damage hold more promise, although maize can also recover from high levels of damage [42,43].…”

“…The degree to which yield loss from ear damage can be predicted from leaf damage measured earlier in the crop cycle is still currently limited. In an on-farm experiment measuring FAW impact across 12 landscapes in Malawi and Zambia [41], FAW incidence and leaf damage declined from 3 WAE to 6 WAE and both leaf damage and ear damage were limited. However, where FAW populations are very high, they could potentially escape natural mortality factors if natural enemy populations are unable to track, or growth lags behind, pest populations.…”