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Uninvited Guest

Jacob Dayton

It’s November and the sweet corn has been harvested. I’m roaming the field, scanning stalks for a single round entry hole. This hole is a clue that an overwintering European corn borer caterpillar (ECB) is hiding inside. I slice the stalk open and bingo I see the caterpillar. I add it to my growing collection and continue.

Left: Round entry/borer hole in a corn stalk. Right: Bingo! An ECB is overwintering inside the stalk.

At the end of the day, I take the caterpillars back with me to the laboratory at Tufts University. To simulate winter, I stow them in the refrigerator. March arrives and I move the overwintering ECB caterpillars into a spring-like environment; every three days, I check on them and track how long it takes different ECB to “wake up” and finish development. After 15 to 36 days, most eventually pupate and eclose as moths.

Male (Left) and female (Right) European corn borer moths found at the Farm Hub.

However, sometimes these overwintering caterpillars reach a different ending.

One day there will be a caterpillar, chilling inside its web cocoon. And the next day, I’ll find the cocoon stuffed with many small, whiteish-yellow parasitoid larvae. Within the cocoon, the brown head capsule is the only remaining evidence of the (former) ECB caterpillar. What a rude way for guests to treat their winter host! These small larvae will eventually develop into adult parasitoid wasps, Macrocentrus cingulum

Left: European corn borer caterpillar, individual parasitoid larva, and (former) ECB cocoon now containing >30 parasitoid larvae. Right: Adult Macrocentrus wasp with embryo ovipositor highlighted.

Macrocentrus cingulum are not originally native to the United States. In fact, after the initial U.S. infestation of European corn borers in 1917 (Vinal and Caffrey 1919), the government intentionally released 24 exotic parasitoid species between 1920 and 1938 to control the growing ECB pest problem (Baker et al. 1949). Macrocentrus cingulum was one of only three parasitoids to successfully establish in the U.S. as biocontrol for ECB populations.

Macrocentrus are synchronized to ECB’s pest cycle and adult wasps parasitize two generations of ECB each growing season. In June/July, wasps will inject a single egg into developing caterpillars. This egg undergoes polyembryonic development, meaning it divides into multiple clonal eggs and larvae. Each of these growing Macrocentrus larva complete three stages of development inside the caterpillar before eating out to finish their fourth larval instar. After pupating, adult wasps will emerge and inject eggs into the second generation of ECB caterpillars in August. These injected parasitoids will overwinter inside the hibernating ECB, before developing and exiting the caterpillar in the following spring (reviewed in Morrison 2011).

On average, 30Macrocentrus wasps emerge per caterpillar (range: 8-49; Zaman et al. 2010). Larger, heavier caterpillars host more parasitoid wasps than smaller caterpillars (Zaman et al. 2010). This suggests that Macrocentrus wasps may preferentially parasitize larger female caterpillars, who normally weigh more than males (Zaman et al. 2010). Similarly, the parasitized Farm Hub caterpillars also tend to be larger too.

Average mass (mg) of male, female, and parasitized larvae collected at Farm Hub in November 2022. Different letters above 95% confidence interval bars denote significant differences between groups (P < 0.05).

In contrast to Macrocentrus’ conspicuous life cycle, parasitism of overwintering Farm Hub ECB by Macrocentrus is more modest: only 6% of ECB sampled in 2021 (n = 32) and 24% in 2022 (n = 33). As a result, European corn borers consistently remain a dominant pest of corn at the Farm Hub across the growing season.

Three European corn borer caterpillars in a single stalk at the Farm Hub.

ECB tunnel into all parts of the corn stalk and ear; this reduces yields by driving stalk or tassel breakage and/or compromising quality of the corn ear.

Examples of European corn borer damage to the corn stalks and ears at the Farm Hub.

Ultimately, although Macrocentrus’s contribution towards ECB biocontrol is small, they still represent one (interesting!) piece of the many biological, cultural, and chemical strategies employed for ECB pest management. 

Jacob Dayton is a Ph.D. candidate in Biology at Tufts University. He studies the genetic mechanisms regulating the timing of daily behaviors and seasonal emergence in European corn borers.

References

Baker, W.A., Bradley, W.G.,  & C.A. Clark. 1949. Biological control of the European corn borer in the United States. USDA Technology Bulletin 983:185.

Morrison, W.R. 2011. Macrocentrus cingulum(=grandii) natural enemy factsheet. In Biological Control: A guide to Natural Enemies in North America (Anthony Shelton, editor).

Vinal, S.C., & D.J. Caffrey. 1919. The European corn borer and its control. Massachusetts Agricultural Experiment Station Bulletin 189.

Zaman, F.U., Calvin, D.D., Rajotte, E.G., & D.V. Sumerford. 2010. Can a specialist parasitoid, Macrocentrus cingulum (Hymenoptera: Braconidae), influence the ecotype structure of its preferred host Ostrinia nubilalis (Lepidoptera: Crambidae)? Journal of Economic Entomology. 103(2): 249-256.

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