Competitive displacement of maize stem borers by invasive fall armyworm
By Loka Mounika and Santhrupthi B.
If a species has to establish in a new locality, it has to have some inherent competitive abilities. That means it has to adjust to local environmental conditions, and also it has to displace any native species which is already occupied. The niche within which it feeds and survives has to be free from competitors. If not, the species cannot survive in a competing environment in an exotic location.
In India, recently invaded fall armyworm (FAW), Spodoptera frugiperda has to compete with native lepidopterans to become successful in an exotic environment. Both the invasive FAW and the native pink stem borer (PSB), Sesamia inferens, belong to the same feeding guild, and hence they are intraguild competitors. Furthermore, FAW is an aggressive pest known to have a cannibalism/predation habit and, therefore, it is a sound model system to study the ecological interactions.
In a recent study published in current science, lead author, Kalleshwaraswamy and his lab at UAHS, Shivamogga, thrown a light on the displacement of native maize stem borers after the invasion of fall armyworm (FAW), Spodoptera frugiperda. What makes FAW far more superior in eliminating native stem borer, such as pink stem borer, Sesemia inferens established.
FAW is an aggressive pest known to have a cannibalism/predation habit, making the species more competitive in an exotic environment. They reported an increased incidence of FAW since its first introduction year of 2018, whereas the incidence of other lepidopteran pests reduced. The incidence of fall armyworm has surpassed in the next successive years due to its super-competing ability. In laboratory interspecific competition studies, FAW had 94.11% and 100% survival in the competition scenarios of large versus small and large versus large PSB, respectively. In field studies, the survival of FAW was 90% even against large Sesamia larvae. This clearly revealed that large FAW larvae are active and competitive enough to kill Sesamia larvae, ultimately removing them from the maize whorl. Even small larvae of FAW were competitive due to its faster growth on maize and a shorter life cycle than native Sesamia. The survival of Sesamia in competition with FAW was significantly lower, indicating less capacity to prevent FAW from entering its niche. Their preliminary study also shows a high incidence of PSB in rabi, sorghum, and rice fields, which were not known to be seriously infested by this stem borer, indicating the possible host shift by native S. inferens. Recently, a similar report was published in Africa where FAW displaces cereal stem borer from Maize to sorghum crop.
G. F Gause marks that ‘No two species with identical ecological requirements can coexist in the same place’ in his Competitive Exclusion principle.
The principle is now a widely studied and accepted phenomenon operating in nature. Competitive displacements are common when invasive species succeed in the exotic environment. Many such displacements have occurred and are known to imbalance the native species. The invasive species with no or fewer natural enemies can occupy other native species within a short span. But the question is whether natural enemies of native stem borers will have a tough time in the absence of their host need to be studied.
The native stem borers, which once ruled the maize ecosystem, by giving nightmares to maize growers, now leaving the space for exotic fall armyworms. THEY ARE DISPLACED, most likely to shift its niche completely from maize. There are a lot of ecological implications, including changes in pest scenario are expected in the days to come due to this type of displacement.
More information on the study can be found at
Loka Mounika and Santhrupthi B. are PG scholars from the Department of Entomology, University of Agricultural and Horticultural Sciences, Shivamogga- 577 204, Karnataka, India
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