Linda F. Benedict, Riley, Thomas J., Leonard, Billy R.
Because the sorghum midge depends on flowering sorghum to lay its eggs, understanding how the length of the flowering period affects damage can aid in developing more effective programs to manage this key insect pest.
In experiments conducted during 1994 and 1995 at the Macon Ridge Branch of the Northeast Research Station near Winnsboro, higher sorghum midge damage occurred with extended blooming periods. We used two hybrids: DeKalb DK60, a commercially available sorghum hybrid with tolerance to the midge, and a commercial sorghum midge susceptible hybrid, Delta and Pine Land DPL1552. Both sorghum hybrids were planted at four planting dates: early (mid-March), two planting dates within the recommended range for Louisiana (mid-April and mid-May) and a late planting date (mid-June).
Within each hybrid, 1,600 plants were randomly selected and labeled. The number of days each plant was in the yellowbloom stage was determined by counting the days from the beginning of yellow bloom until the last day that yellow pollen was present on the sorghum head. Also, the number of days to reach half-bloom, which is the time when one-half of the plants in a field are in the yellow-bloom stage, was calculated from the date of planting to the date that half of the selected plants were in yellow-bloom. Midge population density and midge damage to sorghum panicles were determined also.
Half-bloom in sorghum planted in mid-June was reached in significantly fewer days compared to the other planting dates. Also, the late-planted sorghum experienced a significantly shorter blooming period compared to all other planting dates. Unexpectedly, sorghum midge damage in both years was highest in sorghum planted in mid-March.
Our research suggests the prolonged flowering period in early-planted sorghum caused this to occur. Sorghum from the mid-March planting flowered under low sorghum midge pressure, but it was exposed and available for midge oviposition 1.4 times longer than sorghum in the mid-April and mid-May plantings, and 2.1 times longer than sorghum planted in mid- June. Consequently, damage from the sorghum midge was higher and seed yields were lower in early-planted (mid-March) sorghum.
Traditional recommendations for planting sorghum have been to plant early to avoid the high sorghum midge populations that occur later in the season, but planting too early or an unusually cool spring and early summer could prolong flowering, exposing sorghum to the midge over an extended period.
The length of the flowering period observed in the resistant (DK60) hybrid was significantly shorter (12 days) than flowering period in the susceptible (DPL1552) hybrid (15 days). The shorter flowering period in the resistant hybrid may have conferred additional protection from ovipositing midges compared to the susceptible hybrid.
Our results stress the importance of the flowering period to an integrated pest management program (IPM) for the sorghum midge. The length of time that plants in a sorghum field are in flower affects the field’s vulnerability to sorghum midge. Fields with lengthy flowering periods may require extending spraying with insecticides to protect against midge damage. Hybrids with consistently short, uniform flowering periods will be less susceptible to egg laying and subsequent damage from sorghum midge. This information helps in understanding the characteristics that make sorghum plants susceptible to the sorghum midge and is useful for implementing an IPM program for grain sorghum in Louisiana.
Boris A. Castro, Research Associate, and Thomas J. Riley, Professor, Department of Entomology, LSU Agricultural Center, Baton Rouge, La.; and B. Roger 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.)