Drs. Emilie Regnier and Kent Harrison are weed scientists and Dr. Clive Edwards is an entomologist at The Ohio State University. They recently wrote this summary of research that they have conducted on giant ragweed and nightcrawlers, which explains the reason that giant ragweed is so persistent. I thought you would appreciate the opportunity to read this interesting information.

Introduction and Scope of Importance

Giant ragweed has been a weed of major importance in Ohio and Indiana corn and soybean fields for over 20 years, and it seems to be increasing westward and has been ranked as one of the ten most problematic weeds of agronomic crops in Illinois, Kentucky, Missouri, Minnesota, Nebraska, Oklahoma, and Tennessee.   It is the among the most competitive annual weeds of corn and soybean, with a weed population of one plant per 10 square feet causing crop yield losses ranging from 50 to 75%. Since the mid-20th century, giant ragweed has migrated from its native range in North America to Europe, South America, and Asia, where it is regarded as an exotic invasive weed of agricultural fields, field borders, grasslands and natural areas. Giant ragweed produces copious pollen in late summer and early fall that is a major cause of hay fever in humans.

Not your “typical” weed

Like many important agricultural weeds, giant ragweed has a summer annual life cycle that is completed between the months of March and October in Ohio. However, giant ragweed is unlike many of our major weeds in that its seeds are comparatively large, seed production per plant is relatively low, and once buried in soil, the seeds do not remain viable for long periods of time.

Giant ragweed seeds serve as food sources for various organisms including insects, rodents, and birds, and almost 90% of its unburied seeds are destroyed during the first six months after the plants shed their seeds. Given these apparent weaknesses in its reproductive capacity, we are interested in finding out why giant ragweed is such a persistent and growing problem. Since the time between seed shed and seed burial seems to represent the weakest link in giant ragweed’s life cycle, we have concentrated much of our research on investigating what happens to the seeds during this critical period.

Current Research: The Role of Earthworms in Giant Ragweed Spread and Survival

The common nightcrawler (Lumbricus terrestris) is known throughout most of the U.S. as a popular fishing bait and beneficial inhabitant of agricultural soils. It lives in permanent, vertical burrows and feeds on plant litter, which it collects from the soil surface and stores inside its burrow. As the plant litter softens and decays inside the burrow, it becomes palatable to the earthworms. The earthworms build a ‘midden” over the top of the burrow, a mound of castings and organic debris that seals the opening. Middens are usually easily distinguished from the surrounding soil and indicate the presence of nightcrawlers.  

Nightcrawlers are widely distributed in agricultural soils of the central and eastern U.S., where it improves soil physical and chemical properties for crop growth. It is not native to the United States and was probably introduced from Europe with the settlers. Nightcrawler populations increase in the absence of tillage and can be as high as 15 earthworms per square foot in some no-tillage fields.

We have found that nightcrawlers actively gather and store giant ragweed seeds in its burrow by grasping the seed with its mouthparts and burying it inside its burrow or middens. In field experiments in Ohio, over two-thirds of the giant ragweed seedlings that emerged in spring came directly from nightcrawler burrows. These findings suggest that the earthworm-giant ragweed association is an important factor in the survival and spread of giant ragweed. This seed foraging behavior by nightcrawlers and its effect on giant ragweed seedling distribution have been confirmed in controlled experiments and surveys ofgiant ragweed populations.  Nightcrawlers behave as a seed dispersal agent for giant ragweed and this behavior may protect the seeds from predation by rodents, insects, and birds. The earthworms do not forage exclusively for giant ragweed seeds, but also collect and store other seeds in the same size range.

A summary of research results thus far:

•     In plots excluding vertebrate seed predators, nightcrawlers began collecting and burying giant ragweed seeds immediately after their dispersal thus shortening the exposure time of seeds on the soil surface to as little as 1 or 2 days. By 20 days after dispersal, the earthworms had collected and buried 95% of giant ragweed seed in their burrows and middens.

•     Earthworms buried seeds from 0.2 to 8 inches deep. Most of the seeds were buried in the upper 4 inches of the soil, which represents the depth limit from which giant ragweed seedlings can emerge, but some seeds were buried below 4 inches, where giant ragweed seeds tend to remain dormant.

Earthworms buried over two-thirds of giant ragweed seeds dispersed naturally by a giant ragweed stand. An earthworm burrow contained an average of 127 seeds, and earthworms buried an estimated total of about 500 seeds per square foot.

Giant ragweed seedlings were aggregated in nightcrawler burrows in no-tillage corn and soybean fields in Ohio, Indiana, and Illinois, indicating that when nightcrawler and giant ragweed occur together, the earthworm is likely to bury giant ragweed seeds and thereby influence seed survival and seedling distribution.

•    Nightcrawlers collected and buried seeds of 10 other weed and crop species by the same behavior, but exhibited preferences among them. Giant ragweed, bur cucumber, and sunflower seeds were preferred over common cocklebur and tall morningglory seeds.

Summary and Conclusions

Our research has shown that nightcrawlers forage actively for giant ragweed seeds, that it is capable of burying the majority of the seed produced by a stand of giant ragweed, and that it distributes seeds over a range of depths thus influencing seed dormancy and seedling emergence. It also suggests that seed burial by nightcrawlers protects seeds from predation and exerts a strong influence over giant ragweed populations in subsequent years. With the introduction of nightcrawlers in the U.S., it appears that giant ragweed has acquired a new mechanism of spreading and survival, which may help explain its persistence and expansion as a major weed problem in the U.S.