By John P. Roche, Ph.D.
The fall armyworm (Spodoptera frugiperda) is native to the Americas, but in the past six years has become invasive to Africa, Asia, and Australia. Because the species causes large amounts of crop damage, knowledge of which plants are preferred by fall armyworms will be invaluable for control strategies.
To assist with this effort, researchers at the University of Queensland tested egg laying and larval performance of fall armyworm on five crop species in Australia: maize (i.e., corn, Zea mays), sorghum, peanut, cotton, and pigeon pea. Their results were published in December 2022 in the Journal of Economic Entomology, part of a new special collection, titled “World-Scale Ecology and Management of Fall Armyworm.” The collection features nine new research articles and eight others published since 2019.
A previous study on fall armyworm listed 353 plant species that are hosts for the insect. However, for many of those plants, evidence was lacking that they truly qualify as host plants.
“There is a large body of literature reporting that many plant species were at risk from fall armyworm across its invasion front,” says Trevor Volp, research scientist at the Queensland, Australia, Department of Agriculture and Fisheries, a Ph.D. candidate at the University of Queensland, and lead author on the new study.
To truly be a host plant, Volp and University of Queensland colleagues Myron Zalucki, Ph.D., and Michael Furlong, Ph.D., say fall armyworm has to choose a plant species to lay its eggs on, and then the larvae have to feed on the plant, finish their development on the plant, and become reproductive adults. To investigate host preference in fall armyworms, the team performed experiments on egg laying, larval performance, and larval growth.
In their egg-laying experiment, they placed groups of 10 moths, half of each sex, in a glass container with three plants of one crop species. After four nights, the investigators located the egg masses and counted the eggs. They found that fall armyworm laid significantly more eggs on maize than on any of the other four crops.
In one of their larval-performance tests, they attached egg masses to crop plants in the lab and counted the number, stage, and weight of larvae after five days. Larval survivorship was highest on maize and lowest on cotton, and larval development up to five days was fastest on maize and slowest on cotton. Ninety-five percent of larvae on maize had reached the third instar stage by the fifth day, whereas only 22 percent of larvae on cotton had become third instars.
In a second larval-performance experiment, the researchers attached egg masses to crop plants in the lab and recorded larvae number and stage after eight days. The team found that survival at eight days was greatest on maize (67.5 percent) and lowest on cotton (17.5 percent). More than half of the larvae on maize, sorghum, and peanut had reached the fourth instar. On pigeon pea, most of the larvae had only reached the third instar stage, and on cotton most larvae had only reached the second instar stage.
In a third larval-performance test, they maintained larvae on an artificial diet for the first three instars and placed the fourth instars on plants. After seven days, they took larvae that survived off of plants, weighed them, and recorded their stage.
In their growth-rate experiment, the researchers recorded stage of development after 14 days and found that the highest percentage of larvae had pupated on sorghum plants (100 percent), and the lowest percentage pupated on cotton plants (30 percent).
Importantly, Volp and colleagues saw feeding damage from fall armyworm on all of their maize and sorghum plants, but on only some of their peanut, pigeon pea, and cotton plants. In Australia, feeding damage on crops from fall armyworm in the field has been recorded for maize, sorghum, and peanut but not for pigeon pea or cotton. The majority of the crop damage in agriculture has been to maize and sorghum.
Which of the five plants studied should be considered host plants? Volp and colleagues conclude that maize and sorghum are host plants, whereas pigeon pea, peanut, and cotton are food plants. Volp says, “Our findings—overwhelming oviposition preference for maize and superior larval performance on maize—align with the infestation patterns we are seeing in the field.”
The investigators propose that fall armyworms have two feeding modes: (1) a defoliator mode, in which fall armyworms lay eggs on a crop and the larvae hatch, feed, and develop on the plant, and (2) an armyworm mode, in which fall armyworms lay eggs on grasses and then older instar larvae switch to feeding on other crops. They suggest that many of the plant species that were previously listed as host plants are damaged by fall armyworm in its armyworm mode but do not serve as hosts through the insect’s whole life cycle and thus are not truly host plants.
Population dynamics probably influence oviposition behavior as well, the researchers say. For example, if large numbers of fall armyworms are in a habitat with many non-preferred plant species available, fall armyworms may lay eggs on and cause damage to non-preferred plants.
Click here to see more...