With warm weather comes more insects for crops

Tuesday, April 17, 2012

University of Missouri Extension agronomy Specialist

The warmer weather is accelerating the development of insect populations. Warm weather in March lasting into early-April has resulted in earlier planting of corn and some fields have emerged in the area. We established traps for black cutworm in this area and began catching moths the first of April. As of present, there have not been any significant catches but the catches are increasing in number. Fields most at risk from black cutworm injury include those heavily infested with winter annual weeds. Favorite targets for egg-laying black cutworm moths include mouse-eared chickweed, bitter cress, shepherd's purse, yellow rocket and pepper grass.

If you planted a Bt hybrid don't become complacent. Under heavy infestations, control by some Bt hybrids may be insufficient. Corn in the one- to four-leaf stage of development is most susceptible to cutting by black cutworm larvae. As your corn reaches these stages you may need to look for early signs of leaf-feeding injury from black cutworm larvae.

Heavy alfalfa weevil and pea aphid populations have been noticed across Missouri. According to Wayne Bailey, a University of Missouri Extension Entomologist, alfalfa weevil defoliates the plant and if populations are high enough they can take all of the leaf material off the plant. Since most of the protein is in the leaves, the leftover stems don't provide much feed value for the herd.

Pea aphids are adding to that damage. The green insects, which are the size of a pinhead, puncture alfalfa with piercing/sucking mouthparts to drink its juice. With high populations of pea aphid you will see a yellowing of the field and a loss of quality to the alfalfa. Pea aphid is more of a spring problem and will not kill a 2-year old stand, they can cause a new stand of alfalfa to die.

Beneficial insects can keep smaller populations of pests under control. Ladybird beetles can eat both alfalfa weevils and aphids, consuming up to 50 aphids per day. Parasitic wasps will kill these pests by stinging them and laying eggs inside their abdomen.

When the eggs hatch, the emerging wasp larvae eat their hosts. If you have ladybird beetles in the field, Bailey recommends hesitating to spray because they often could bring an aphid population under the threshold level.

The economy pest threshold level is the number of insects where it becomes economical to spray pesticides. If you have 6-10 inch alfalfa, one alfalfa weevil or 70 pea aphids per stem puts you at the threshold where you need to treat your field.