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Insect pest management in corn and sorghum

Insect pest management in corn and sorghum
Little insect activity reported in Gulf coast corn. Consider height of sorghum, leaf damage in determining treatments. Monitor insect populations.  

Except for chinch bug infestations to the north and inland from the coast, little insect activity has been reported in corn. However, that could change as corn earworm moth numbers are relatively high as indicated in our pheromone traps for the past several weeks. Their numbers are much greater than for the same time period for the last several years.

I suspect these numbers will reflect, more-or-less, what will end up in the whorl and/or ears of corn. We may see fall armyworm in corn, but pheromone trap numbers have been low with an average of 6 per trap for last week.

Yellow sugarcane aphid and greenbug infestations have been generally low but nevertheless present in some field throughout the Gulf Coast region. In the case of the yellow sugarcane aphid, higher numbers can be sustained as plants increase in size. In many fields, we may be past significant damage from the yellow sugarcane aphid.

The following discussion is directed to the thought process I use in determining whether to apply insecticide for yellow sugarcane aphid in sorghum a foot or more in size. This aphid colonizes the underside of the lower leaves. For sorghum over a foot tall the loss of the very smallest lower leaves can be tolerated without yield loss. However, larger lower leaves which sustain the aphid population for a period of time could result in enough damage for yield loss.

Where yellow sugarcane aphid numbers are high enough for a long enough period, the leaves will first turn a purple color followed by yellowing and finally death. Generally a treatment is needed before the aphid kills more than one “normal sized” leaf on 20 percent of the plants.

Colonies of more than 40 aphids on these leaves over a period of time can cause leaf death. Therefore, a general procedure would be to find out if at least 20 percent of the plants have yellow sugarcane colonies on lower leaf surfaces and then estimate if their numbers average more than 40 on these leaves. Once these criteria are met or exceeded wait 48 to 72 hours to examine the field a second time to determine whether the population is increasing and if additional leaves are dying.

Monitor populations

If the populations and infestations continue to increase, treatment most likely would be warranted. Dimethoate 4E (8 to12 ounces per acre) applied with hollow cone nozzles has provided control where volume rates of 7 to10 gallons per acre were utilized. It is critical to use hollow cone spray nozzles.

We are likely to seecorn leaf aphid in the whorls of sorghum. Plants can sustain much greater numbers of this aphid since it does not inject a toxin as do greenbug and yellow sugarcane aphid. In fact, even under very heavy corn leaf aphid numbers with abundant honeydew and seemingly a lot of damage I have not been able to show a yield increase through insecticidal control.

It would also be advisable to watch for increasing numbers of greenbug as plants increase in size. This aphid also has a toxin which affects the leaves. The aphid can cause economic damage up to hard dough. Consult B-1220, Managing Insect and Mite Pests of Texas Sorghum dated May 2007 for a thorough discussion.

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