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By Esther Ngumbi
Several African countries, including Zimbabwe, Zambia, Namibia, Kenya, and Malawi, are in the middle of a war. It’s not against other countries, but against the invasive pest called the fall armyworm, which is spreading across the African continent like wildfire, devastating maize and other crops. This invasive pest poses a major threat to food security and agricultural trade in African countries, so it’s not surprising that it has been making headlines.
Fall armyworm is native to Central and South America and was first identified in West Africa in 2016. It can survive on a wide range of hosts, including cotton and soybeans, but prefers grass-based host plants like maize, millet, sugarcane, and sorghum. It is a strong flier and has been reported to travel thousands of miles a year in search of warmer climates. Adult insects can lay up to 1,000 eggs during their lifetimes. Its larvae eat voraciously and have similar feeding characteristics to those of the cotton red bollworm and the maize stalk borer, pests that are difficult to control with pesticides.
What’s more, invasive insect pests like the fall armyworm are expected to become even more of a problem. Fueled by climate change and global trade, scientists predict that invasive species, including crop-damaging insects, will continue to advance into new territories and cause more havoc. Global climate change will accelerate the emergence of new insect pests and plant diseases as well as the redistribution of commonly known pests.
As happens with many invasive and emerging pests, the appearance of the fall armyworm brings up many questions that remain unanswered. For instance, it will take some time for scientists to understand how this insect came to Africa and how it is different from the native African armyworm, including different breeding patterns. Scientists also have to figure out how best to control this pest if it persists or spreads.
To respond to the crisis, the U.N. Food and Agriculture Organization in partnership with the Government of Zimbabwe, the International Red Locust Control Organization for Central and Southern Africa, and other Africa-wide groups, convened an emergency meeting last month to discuss proven scientific methods to manage and stop the spread of the fall armyworm.
The meeting culminated several proposals, including the use of pheromone traps. Moreover, national agricultural research organizations agreed to ramp up their monitoring and surveillance capacities.
Scientists recommend the use of integrated pest management (IPM) as one of the best possible strategies. This approach seeks to minimize the use of chemical pesticides and uses locally relevant, available, and scientifically proven multidisciplinary tools. These include resistant and disease-free seeds, crop rotation, bio and chemical pesticides without harsh environmental effects, habitat manipulation techniques such as trap crops, landscaping and companion plantings to enhance activity of biological control agents, pheromone traps to monitor of insect pests, and seed treatment with beneficial soil microbes, which increase the ability of plants to fight off pests and adapt to other climate change stressors, including drought.
At Auburn University, we have researched the use of soil microbes, specifically plant growth-promoting rhizobacteria (PGPR), to improve crop growth while helping plants to fight damaging insect pests. Our research has used cotton and turf grass as model plants and Spodoptera exigua, known as the beet armyworm, and Spodoptera frugiperda, the fall armyworm, as model insects.
Plants treated with beneficial soil microbes can deter the oviposition of adult insects, subsequently resulting in fewer laid eggs. Results from a study done by Nangle et al. using the beet armyworm showed that female beet armyworms preferred to oviposit on untreated cotton plants compared to microbes-treated plants. Furthermore, fewer eggs were laid on microbes-treated plants compared to the untreated plants. Coy et al., who investigated the effect of microbe’s treatment of bermudagrass on the oviposition behavior of the fall armyworm, obtained similar results. They revealed that the fall armyworm laid more eggs in grasses in the control plants than they laid in microbes-treated Bermuda grass. These two studies confirm that microbes may help in the battle against the fall armyworm.
Second, beneficial soil microbes can negatively impact feeding as well as larval development and performance. Insect larvae feeding on microbes-treated plants develop much slower and gain less weight, which may result in mortalities that reduce the effect these larvae will have on plants. Other microbe strains negatively impact the ability of larvae to transform into pupa, hence, fewer larvae are able to complete the lifecycle to become adults. Coy et al. showed that treatment of Bermuda grass with microbes (PGPR) negatively impacted larval development and pupal weight.
Third, some microbe strains change the volatile organic compounds that plants emit. Treatment of plants with microbes changes the fragrance or volatiles that plants produce, with important consequences for foraging natural enemies, like ladybugs, that use these volatiles to locate their insect host. Indeed, work from my research showed that microbes can change plants’ chemistry, and these changes have important ramifications for natural enemies foraging for their hosts.
Alternatively, microbes can have a direct toxic effect against insect larva. In this scenario, also called antibiosis, plants are directly sprayed with beneficial soil microbe’s formulation. Sprayed plants are then no longer attractive to insects. At the same time, the larval development and performance of pests that come in direct contact with these plants are negatively affected.
For African nations to succeed in battling the invasive and devastating fall armyworm, we will need to use all of the available tools that are developed from beneficial soil microbes. Microbes are our allies and are a key instrument for dealing with the extremities that come with a changing climate, including the rise of new insect pests.
Esther Ngumbi is a post-doctoral researcher at the Department of Entomology and Plant Pathology at Auburn University in Alabama. She serves as a 2017 Clinton Global University (CGI U) Mentor for Agriculture and is a 2015 Food Security New Voices Fellow at the Aspen Institute.
[Photo courtesy of sicrump]
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