Linda F. Benedict, Riley, Thomas J., Leonard, Billy R.
Evaluation of plant resistance, insecticide treatment and planting date
Boris A. Castro, Thomas J. Riley and B. Rogers Leonard
The sorghum midge is a key pest of grain sorghum in the United States and can cause serious yield losses in Louisiana. The adult midge is a small, red, gnat-like fly about 1.5 millimeters long (Figure 1). Damage is caused by the larva, which feeds inside developing sorghum seeds, eating the seed as it develops (Figure 2). Female midges lay eggs only in sorghum that is flowering (Figure 3). Although a female midge lives only one day, she can lay between 50 and 250 eggs. The sorghum midge spends the winter as a pre-pupa in sorghum crop residue or in johnsongrass, its natural wild-host plant (Figure 4).
Sorghum fields are susceptible to the sorghum midge only when the plants in the field are in flower. Flowering sorghum is recognized by the presence of fresh, yellow pollen on the sorghum head (Figure 3). Individual sorghum plants usually flower over three or four days, but sorghum fields can have plants in flower for several days to weeks, depending on the uniformity of field and environmental conditions that determine the rate of plant growth and development. Damage from the sorghum midge is not apparent until the seed heads mature. Sorghum heads heavily damaged by the sorghum midge are called “blasted” heads and have brown areas where no seeds have developed (Figure 5).
Sorghum midge integrated pest management (IPM) efforts in Louisiana rely on several practices. These include:
Timing of insecticide applications is critical to a successful sorghum midge IPM program. Insecticides must be applied to coincide with the flowering of sorghum fields. Differences in planting dates, hybrid maturity and environmental conditions, however, can cause many fields to flower non-uniformly, preventing insecticides and other recommended strategies from providing the level of control necessary for profitable sorghum yields.
Plant resistance techniques can be successfully used for sorghum midge management. Considerable progress has been made in the development of experimental sorghum hybrids that are biologically resistant or can tolerate infestation by the sorghum midge. These hybrids suffer less damage under equivalent infestation conditions and maintain acceptable yields. Although commercial availability of resistant sorghum hybrids is limited, our studies were conducted to evaluate how plant resistance technology can be integrated into local management strategies to increase the efficiency of sorghum midge IPM in Louisiana.
Solving the problem
Experiments were conducted in 1994 and 1995 at the Macon Ridge branch of the Northeast Research Station in Winnsboro. DeKalb DK60, a commercially available sorghum hybrid with resistance to the midge, and a commercial hybrid susceptible to the sorghum midge, Delta and Pine Land DPL1552, were compared at four planting dates in mid-March, mid-April, mid-May and mid-June. The April and May planting dates were chosen to represent the recommended range for planting grain sorghum in Louisiana.
Each planting date was split into two subplots. One subplot was treated with the insecticide chlorpyrifos at 0.5 pound per acre at three-day intervals while the subplot was in flower. No insecticide was applied to the other subplot at any time during the experiments.
At the time of these studies, DK60 was the only commercially available sorghum hybrid with some degree of sorghum midge resistance. It is a relatively tight-headed hybrid and may not be suitable to all parts of Louisiana. The research design included 16 treatment plots replicated four times. Plots were 12.2 meters long and consisted of 16 rows per main planting date, eight rows per insecticide treatment and four rows per hybrid.
Sorghum midge populations were measured twice each week when sorghum plants were flowering by counting the midges on each of 20 flowering sorghum heads per plot. Sorghum midge damage was assessed by estimating the percentage of blasted kernels on each of 50 sorghum heads selected at random from the two center rows of each plot. Yields were obtained by mechanically harvesting the two center rows of each plot.
WHAT WE FOUND
Sorghum midge numbers
Sorghum midge numbers varied significantly throughout the flowering period from June to August. When sorghum midge populations were low, more sorghum midges were found in susceptible DPL1552 than in the resistant hybrid DK60 (Figure 6). The highest numbers were observed during the first half of August and coincided with the period when sorghum plants from the mid-June planting date were flowering. When susceptible DPL1552 and resistant DK60 sorghum were compared over the entire growing season, the number of sorghum midges in the susceptible sorghum was generally higher than the number in the midge resistant sorghum (Figure 6).
Sorghum midge populations increased from June to August. In susceptible sorghum DPL1552, the economic threshold level of one sorghum midge per sorghum head was exceeded in early July 1994 and in late June 1995. In resistant sorghum DK60, the economic threshold level (ETL) of five midge per sorghum head was exceeded only at the end of the flowering period (Sept. 2) in 1994 and in early August 1995 (Figure 6). Under these conditions, resistant hybrid DK60 received less damage from the sorghum midge in 1994 and in 1995.
Sorghum midge damage
In our experiments, resistant hybrid DK60 received less damage from the sorghum midge in all four planting dates in 1994 and 1995. When considered with the number of sorghum midge found in each hybrid, our experiments suggest that the significant differences in damage in 1994 and 1995, and yields in 1994, resulted from the preference of female sorghum midges to lay eggs on the susceptible hybrid.
Resistant hybrid DK60 yielded significantly higher than susceptible DPL1552 during 1994, but not during 1995. In 1995, both hybrids yielded highest when planted in mid- April and mid-May. In 1994, yields for mid-April and mid-May also were high, and a slightly higher yield for the resistant DK60 occurred in the mid-June planting. Yields of susceptible hybrid DPL1552 were reduced when planted in mid-June. No yield differences were observed in resistant DK60 across all planting dates in 1994.
In 1995, the lowest yields were obtained in both hybrids when planted in mid-June. This yield reduction was because of severe drought stress during Insecticide effects sorghum grain fill that year. Hot weather and low moisture during grain fill are known to kill sorghum midge pupae. In our experiments, survival of developing midges was probably reduced by the hot, dry weather experienced during grain fill in 1995. This would explain why reduced sorghum midge damage was observed in the mid-June planted sorghum in 1995, even though it was exposed to numbers of ovipositing sorghum midges that exceeded economic thresholds (Figure 6).
Sorghum treated with insecticide when plants were blooming also had significantly less damage from the sorghum midge than untreated sorghum in 1994 and 1995. Sorghum yields were significantly improved with the use of an insecticide in 1994 and 1995. It is important to note that in our experiments untreated resistant DK60 experienced less damage and demonstrated the potential to produce yields comparable to the susceptible hybrid DPL1552 that was treated with insecticide to control the sorghum midge.
What it means to Louisiana sorghum producers
Our studies indicate that plant resistance to the sorghum midge has potential as an integrated pest management tool for Louisiana. As more sorghum hybrids become available with sorghum midge resistance, sorghum growers should be able to realize greater profitability by maintaining yields equivalent to those of locally adapted susceptible hybrids but with less cost because of reduced need for insecticides to control the sorghum midge.
We thank Dr. A. Bruce Maunder of DeKalb Plant Genetics at Lubbock, Texas, for providing seed of DeKalb DK60 and the Louisiana Soybean and Feed Grain, Research and Promotion Board for providing partial funding for this research.
Boris A. Castro, Research Associate, and Thomas J. Riley, Professor, Department of Entomology, LSU Agricultural Center, Baton Rouge, La.; and B. Rogers Leonard, Associate Professor, Macon Ridge Branch of the Northeast Research Station, Winnsboro, La.
(This article was published in the winter 1999 issue of Louisiana Agriculture.)