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Sugarcane borers attacking Louisiana grain crops

EDITOR'S NOTE – Sugarcane borers have been taking a heavy toll in recent weeks in corn, grain sorghum and rice fields in east-central Louisiana, which is not a traditional area for infestations of the pest. David Y. Lanclos and Jack Baldwin of the LSU AgCenter wrote the following article about this unusual problem for Delta Farm Press.

Over the past several weeks, sugarcane borers (Diatraea saccharalis) have shown up in rice and corn in many fields across Louisiana. Grain sorghum has also been severely affected in some parishes such as Catahoula and Concordia.

The sugarcane borer over winters in a late larval stage, and then pupates the following spring. In Louisiana, adults become active in the April and May, and the borer population continues to cycle until fall. Emergence and development time is highly variable, so there is usually considerable overlap in the generations. There is a potential for four to five generations to occur annually.

Young larvae feed for a few days on leaves or behind the leaf sheaths. Older larvae bore into the stalk and tunnel up and down the center of the stalk. Borer-infested stalks are smaller in diameter and may lodge before harvest. In the case of grain sorghum, damage from borers can cause the grain head to abort before it emerges or result in a blank grain head after it emerges. Injury by borers also makes the plants more susceptible to stalk rot pathogens.

Scouting for stalk borers requires careful examination of plants. Larvae enter the stalk by boring a small hole that is usually near a leaf collar. Once they are inside the plant, you have to split the stalk in order to find the larvae. Moths lay eggs in flat clusters of about 25 eggs, which may be found on the top or underside of leaves, depending on borer species.

Egg masses and small larvae must be detected and insecticides applied before larvae bore into stalks. However, the detection of small egg masses and newly hatched larvae is a difficult and tedious task.

From a crop management standpoint, once the borers are found boring in the plant tissue there is no chemical alternative that can be justified. There are some things that can be done, and that is to try and harvest with slightly higher moisture, which will lead to dockage at the elevator, however, some of the crop could be saved.

In the worse case scenarios, borer damage will “take the crop down” and make it unavailable to harvest. Another management tactic of planting crops earlier in the year is not a full proof method to deter borers.

If heavy infestations are found in rice, corn, or sorghum the best thing that can be done after harvest is to use flail mowers or bush hogs to destroy the crop residue as completely as possible. This is extremely important especially in rice and grain sorghum where the plants can produce another “second crop” if not destroyed. Leaving crop residue in the field only allows the borers to have enhanced opportunity to overwinter successfully.

The use of flail mowers would be more advantageous than bush hogs because crop residue is chopped into smaller pieces. Setting the implements as low as possible to get maximum crop residue cut or chopped also can help. After flailing or bush hogging, disking the fields at least twice will help destroy the borer population. Plowing to break stubble and bury crop residues soon after harvest destroys over-wintering larvae by exposing them to colder temperatures.

I realize that this management strategy does not fit into a no-till or even reduced till management system, but in heavily populated fields, there is no choice. Planting Bt corn can be effective against borers but since Bt is not available in rice or sorghum that is not an option in those crops.

Dr. Boris Castro, LSU AgCenter Extension entomology specialist has found sugarcane borers and southwestern corn borers infesting corn in north Louisiana since 1995. However, it was not until the 2000 corn crop that numbers of both borers increased dramatically, aided in part by the high over-wintering survival from mild winters.

Boris suggested that “taking action right now by destroying stalk residues is one of the best efforts to reduce future infestations, as long as this effort is carried out regionally not only in corn but in grain sorghum and rice where the borers have been detected as well.

“What we have in north and central Louisiana is a complex of southwestern corn borers and the sugarcane borer,” he says. “They both infest corn and sorghum, but the southwestern corn borer does not prefer rice because of rice’s narrower stalks. Therefore, it is particularly important to know whether we have only the sugarcane borer or the southwestern corn borer (or both) in a field because their over-wintering habits are different.”

He says the over-wintering stage in the sugarcane borer is dependent on environmental conditions and if more feeding material in the form of crop debris is available for the sugarcane borer to feed on, it will produce subsequent generations until temperatures drop enough to trigger the over-wintering mechanism.

“The southwestern corn borer has a clearer over-wintering behavioral pattern and has the habit of girdling the stalk at the base of the plant,” he notes.

Biology of the sugarcane borer on sugarcane has been intensively studied for the central and southern part of the state by Dr. Gene Reagan and others. However, information on the sugarcane borer in north Louisiana is limited. Both the southwestern corn borer and the sugarcane borer tend to stay within or feed on grass species.

Dr. Castro also said that “in corn and grain sorghum there is normally about eight to 10 days (after hatching) before the sugarcane borer and the southwestern corn borer larvae move inside the stalk.”

But in rice, they will enter within 3 to 5 days because of the shorter distance they need to feed through before reaching the stalks.

I have been to Catahoula Parish twice in the last week looking at sugarcane borer damage in grain sorghum. It is estimated from calls received from David Neal (Catahoula County Extension agent) that there could be 10,000 to 25,000 acres of grain sorghum that will not be harvested due to sugarcane borer damage. This is substantial considering NASS has reported that Louisiana has 200,000 acres planted this year.

Dr. Grady Coburn, a certified crop consultant from Pest Management Enterprises, Cheneyville, La., says “I have noticed sugarcane borer damage in areas of Rapides, Avoylles, St. Landry and Evangeline parishes.”

Roger Carter a certified crop consultant with Agricultural Management Service, Clayton, La., told me that he has seen borer damage in Tensas, Catahoula, and Concordia parishes. Also, Roger feels the acreage that is not going to be harvested could be as high as 22,000 acres in Catahoula parish alone.

Dr. Roger Leonard, LSU entomology professor, also has visited grain sorghum fields in Catahoula Parish and feels that the larvae numbers are high enough in the stalks that flailing would kill the majority of the insects in the field at this time.

Regrowth is another issue, according to Leonard. Complete residue destruction with tillage will likely be warranted in some cases (a double disking). Many of the larvae in sorghum are probably not overwintering, but represent an infestation and killing them now may reduce a later generation in volunteer plants.

Most of the larvae that he has seen in harvested corn fields are those which are ready to overwinter. Based on his observations, he does not feel that flail mowing will destroy enough of the borers and disking is warranted. However, Dr. Leonard stressed that what LSU is recommending may not be in accordance with conservation plans and many components of Best Management Practices for Louisiana crops.

Producers with conservation contracts should contact their local USDA Natural Resources Conservation Service office to make sure that there are no future problems which could result from these actions.

I have received numerous questions regarding why the borer damage is so high this year and do not have an exact answer. I can speculate that with the winters being mild over the past couple of years, coupled with more producers moving towards reduced tillage or no-till systems could partially explain this new development.

By moving to a reduced-tillage system, this is allowing more crop residue to act as overwintering material for the borer. Also, added sugarcane acreage in the state can be found in many non-traditional cane areas, and this could also be playing a role in increased borer numbers.

Right now, my recommendation to producers that have extremely high numbers of borers (if they decide to take the crop to harvest) is to get the stalks chopped immediately after harvest and disk them under as soon as you can. However, remember to check with the local NRCS office about the situation.

Also, encourage your neighbors to do the same if they have had a problem with the borers this year. If a concerted regionalized effort is made to destroy the population of overwintering larvae hopefully, we can have fewer numbers next year.

Lanclos is Extension soybean, corn and grain sorghum specialist and Baldwin is Extension entomologist for soybeans and grains with the LSU AgCenter.


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