April's Ag Advice by April Borders Alfalfa fields are starting to see some pressure from pests like the alfalfa weevil and the pea aphid. So like I've said before, you need to be out scouting your fields.
Damage from the alfalfa weevil is usually noted as small, circular holes in leaves at the growing tip of the plant; this is commonly referred to as "pinholing." As the larvae increases in size, feeding becomes more pronounced, with large areas of individual leaflets being removed. Larvae may feed on only the interveinal tissue, resulting in a skeletonized appearance to the leaves. Severely damaged fields typically take on a silvery appearance, much like a field which has suffered frost damage.
When scouting, examine 20 plants in each of at least five locations throughout the field. Look for leaf damage and the presence of larva. Samples should not be taken from the field margins. An economic threshold is reached if 30 percent of the plants show feeding damage and larvae are still present. If the alfalfa has reached the bud stage, an early harvest of the alfalfa is the recommended control strategy.
The pea aphid will often feed on the new growth and can be observed on the leaf tips of plants. If the feeding is intense, many plants will begin to yellow and display some wilting. Prolonged feeding under these conditions may produce stunted plants that have small leaves and spindly stems. Large infestations in early spring can significantly injure the first cutting and reduce the hardiness of future hay crops.
Pea aphid adults and immatures (nymphs) remove fluid from the alfalfa plants with their piercing and sucking mouthparts. The leaves, petioles, stems and flower buds are potential targets for injury.
The simplest and most accurate method of taking actual counts of pea aphids is to estimate numbers per stem. This is somewhat similar to stem sampling for monitoring tip injury due to alfalfa weevil. Six to 10 stems randomly selected from each of five field locations are shaken in a carton or over a tray and the average number of aphids/stem calculated. The stems must be very carefully handled since pea aphids readily fall from the plant. You can also monitor their presence by using a sweep net.
There are several insecticides that are available for controlling the alfalfa weevil and pea aphids. You will need to watch the pre-harvest intervals as they differ from chemical to chemical. A list of insecticides that are labeled for use against the alfalfa weevil larva is available from the Extension Office.
Other things to be on the look out for are cutworms, black and dingy, in corn. There have been reports in the Bon Homme and Yankton County areas of cutworm damage in fields. Watch for leaf feeding, cutting, wilting and missing plants.
With all the rain that we have had we will also need to be on the look out for water molds affecting our crops. This would include diseases like pythium (seed rots and seedling diseases) and phytophthora root rot.
With the rain that we have received we should be watching our fields for ponding and crusting. Too much water in low areas of the field may replace the oxygen in the soil. There may be crusting taking place that makes it difficult for the seedlings to emerge. If crusting is a problem, a rotary hoe works to break up the crust so that the seedlings can emerge more easily.
In the early stages of development (germination to V6 stage � when the leaf collar on the sixth leaf sheath becomes visible), corn can survive if underwater for about 48 hours. In cases where the weather is cool, then the plants' physiological process slows and it may survive under water longer. If soil and air temperatures are warm, 24 hours of flooding may cause the plant to die.
Producers need to dig up some of the plants and monitor the growing point. If the growing point is yellow to white in color, the plant is OK. If the growing point is brownish to black in color, the plant is dying. dying. Areas of the field with prolonged ponding may have dead areas. Depending on the severity of stand loss, growers may consider replanting.
A rule of thumb for corn stands is if 75 percent of the seeding population emerges, thesn it doesn't pay to replant. In areas of the field that have higher stand loss due to the ponding, replanting may be an option in those areas of the field.