Spencer describes corn rootworms and their devastating effects on corn
“Corn rootworms are truly historic pests,” said Joseph Spencer of the Illinois Natural History Survey.
Spencer spoke about corn rootworm management during a webinar hosted by University of Nebraska-Lincoln Extension on Feb. 20.
Billion dollar insects
There are two species of corn rootworms that are the most prevalent across the U.S., including eastern Wyoming – the Western corn rootworm and the Green Northern corn rootworm.
“Together these two species are known as the billion dollar insects,” explained Spencer, “based on the cost of their management and yield losses that they can inflict.”
Both the Northern and Western corn rootworms lifecycle and biology are closely tied to its host plant – corn.
Eggs of the rootworms are laid in the moist soil cracks in cornfields during the summer. The lifespan of the rootworms is between five to six weeks.
“The rootworms are one millimeter in diameter and can fit inside the zero on the date of a U.S. penny,” described Spencer.
When the eggs are laid in the summer, they lay dormant over the winter and begin to hatch in May or June. The exact timing of the hatch depends on soil temperature.
“Mortality of the rootworms is high if there is flooding in the cornfields during the time the eggs are hatching,” stated Spencer.
The larvae that emerge go through three stages, and they can inflict significant feeding injury and expose disease to corn roots.
The larval neonates can survive for four to eight days without any food if they are unable to locate corn roots to feed on.
“Once severe damage is done to the corn roots, the stalks can tip over,” said Spencer.
In late June to early July, the rootworms begin to turn into beetles, and they begin to emerge from the soil. When they first emerge as beetles, they are a pale color teneral adult.
It takes about 12 to 24 hours for the adults to obtain a hardened cuticle.
The males emerge about a week prior to the females and are not sexually mature until five to seven days later.
The females are sexually mature upon their emergence of the soil. Females only mate once after emergence while males can mate multiple times.
“The female signals her readiness to mate by releasing a sex pheromone that will attract mate-seeking males,” said Spencer. “Both the Northern and Western corn rootworms use the same pheromone.”
After mating the female then begins to move about the cornfield for about a week.
A portion of the Western corn rootworms will leave the cornfields, flying great distances away.
“Approximately 15 to 24 percent of the young female mated Western corn rootworms are capable of engaging in sustained flight,” explained Spencer. “Some may travel up to 24 kilometers away in one sustained flight, although most will make several shorter multi-kilometer flights before settling down in another cornfield.”
The greatest transport of the Western corn rootworms, however, is made possible by thunderstorms.
“The updraft from approaching thunderstorms suck in flying rootworm beetles and carries them high into the sky,” described Spencer. “The storm later washes them out with the rain 10 to over 100 miles away.”
Northern corn rootworms may only disperse as far away as the next cornfield to begin depositing their eggs.
Egg laying of the Northern and Western corn rootworms begins six to 10 days after their emergence from the soil, and a female can produce up to 500 eggs in a field.
“The maximum egg production seen from a female is 1,800 eggs,” stated Spencer.
“Crop rotation seemed like an obvious solution to the corn rootworm problems,” stated Spencer. “Unfortunately an extended egg diapause can occur, resulting in the hatching of the eggs delayed until two winters had passed.”
“Effectively allowing these insects to circumvent crop rotation,” he added.
Producers have started to plant soybeans next to cornfields or instead of corn, but the rootworms have now become resistant.
“The rotational resistant rootworms now engage in much more inter-field movement between corn and soybeans,” said Spencer. “The rootworms will feed on the soybean foliage, but they will eventually move back to corn.”
With the spread of rotational resistant rootworms the use of soil insecticides has dramatically increased and spurred the argument to use more Bt-corn for corn rootworm management.
Bt-corn is a genetically modified variety that produces an insecticidal protein to help manage against rootworm pests.
“Learning about rootworm biology and applying that knowledge is key to using Bt-corn or other management tactics as part of an integrative approach to rootworm pest management in the Corn Belt,” explained Spencer.