Several species of fungus in the genus Fusarium are known to infect insects, while some also infect plants. Researchers in China report the first observation of the species Fusarium concentricum infecting an insect—in this case a key moth pest of Chinese fir trees. Shown here are dead larvae and pupae of the moth Polychrosis cunninhamiacola infected with Fusarium concentricum (a-e), along with front and rear views of a petri dish with the fungi isolated from the dead insects (f-g). (Image originally published in Qiu et al 2023, Journal of Insect Science)

By Andrew Porterfield

Andrew Porterfield

Members of the fungal genus Fusarium have long been known as pests to agricultural and ornamental plants. But what if the species Fusarium concentricum could be damaging to insect pests? The fungus had never been observed as an insect pathogen, until now.

Researchers from the Guangdong Academy of Forestry and South China Agricultural University recently found that Fusarium concentricum was pathogenic to the moth Polychrosis cunninhamiacola, a pest that causes severe economic losses to cultivated Chinese fir. Their study, published this week in the Journal of Insect Science, marks the first time F. concentricum has been seen as an insect pathogen.

Hua-Long Qiu, Ph.D., research assistant in the lab of Jin-Zhu Xu, Ph.D., at the Guangdong Academy of Forestry, and colleagues collected fungus-infected cadavers of moth larvae and pupae in a fir forest in northern Guangdong Province. The researchers isolated fungal spores, cultured them, and then identified the fungal strain as F. concentricum. They validated the identification with genetic sequencing to align with the Tef-1alpha gene, a commonly used marker to delineate Fusarium species.

The moth Polychrosis cunninhamiacola is a significant pest of Cunninghamia lanceolata fir trees in China. The moth damage starts when females lay their eggs on the tree’s leaves. Hatched larvae then destroy the top buds of new shoots, creating multiple shoots at lower heights and trunk bending, which hinder growth and wood quality. Shown here are a P. cunninhamiacola early-instar larva (a), late-instar larva (b), pupa (c), and adult (d) and attacked C. lanceolata branches producing multiple shoots at lower heights, indicated with arrows (e, f). (Image originally published in Qiu et al 2023, Journal of Insect Science)

The Chinese fir Cunninghamia lanceolata is native to China and is one of the fastest growing trees in the southern provinces of the country. It is valued for its ability to grow in poor soil, its rapid growth rate, and the quality of its wood. However, seven plant diseases and five species of insects have threatened the trees, including the P. cunninhamiacola moth.

The moth damage to fir trees starts when females lay their eggs on the tree’s leaves. Hatched larvae then destroy the top buds of new shoots, creating multiple shoots at lower heights and trunk bending, which hinder growth and wood quality. Typically, control of the moth has been carried out with chemical insecticides, which causes pollution over the fir’s vast geographical range and kills parasitoid wasps and other natural enemies of moths. Fir forest managers have since looked to new pest control methods.

Fusarium species are known plant pathogens, but nine species have been found to be pathogenic to insects. Xu’s research team isolated F. concentricum from dead moths and tested its toxicity against live moths, as well as other insect pests: the butterfly Danaus chrysippus and the fire ant Solenopsis invicta.

The researchers collected adult P. cunninghamiacola from a fir seed garden in Shaoguan, China, as well as D. chrysippus larvae and S. invicta workers. They then centrifuged insect samples in a fungal spore suspension. Next, they recorded mortality of the insects. Mortalities were 81.48 percent for fungus-exposed moth, 81.03 percent for the plain tiger butterfly, and just 8.62 percent for the fire ant. The lethal time to death (LT50) was 5.94 days for the moth, 5.73 days for the butterfly and 14.17 days for the fire ant. The Fusarium strain used did not infect any branches or leaves of the host fit plants, however.

The research shows that the Fusarium strain could be part of an integrated pest management program, at least for the Chinese fir.  “As far as we know, this is the first record of natural infection by F. concentricum in insects,” the researchers write. They emphasize, however, the importance of using molecular techniques such as sequencing and mass spectrometry to determine the exact species, given that many Fusarium species and strains are toxic to plants.

Fortunately, “a large body of knowledge regarding mycotoxins produced by Fusarium now exists, which should facilitate the selection of benign Fusarium strains by surveying the presence of mycotoxins through high sensitivity detection methods,” they write.

The right strain of Fusarium, then, could be used as an effective and less toxic pesticide to protect Chinese fir stands. “Further bioassays are needed against a wider range of insect species which are known to exist in the fir field where this strain was obtained … to provide a more realistic preview of expected outcomes on non-target insects. Future studies could evaluate mycotoxin production against non-target insects and associated potential effects on plants and humans, as to expand our understanding of the potential to exploit host-Fusarium-environment interactions.”

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.