Description of the Pest
Key to identifying aphids
Several species of aphids may be found in corn, but corn leaf aphid and greenbug are the primary aphid species infesting corn in California. Corn leaf aphids are small to medium and bluish green in color and also infest small grains. The greenbug is a moderate-sized aphid. The color of the abdomen is light green with a darker stripe down the middle. Both winged and wingless forms of both aphids occur on corn plants.
Damage
Corn leaf aphid infestations usually start in the plant whorl. Heavy infestations may curl leaves and stunt the plant. Later infestations may completely cover the tassels and upper leaves. Corn leaf aphids excrete a sticky substance called honeydew, which accumulates on the plants. The honeydew eventually turns blackish as sooty molds grow on it. Heavy amounts of sooty mold may be more damaging to silage corn than to corn for grain.
Greenbugs and green peach aphids also infest corn, but usually do not build up to the high numbers of corn leaf aphids. Red lesions often form at the feeding sites of greenbugs. High numbers of greenbugs on small plants can kill the plants. All three species transmit maize dwarf mosaic virus to corn from nearby sources. Johnsongrass is one of the common weed hosts for this virus.
Management
Transmission of virus disease is the primary damage caused by aphids and the potential for this varies significantly from year-to-year and area-to-area. Insecticide sprays will not prevent virus transmission, but can reduce population levels. There are no established thresholds for aphids on field corn. Only on rare occasions do aphids reach damaging populations. Obtaining good coverage of the plant, which is essential for effective control, can be difficult when the plants are 5 feet tall or more and treatments may increase problems with mites by killing natural enemies.
Biological Control
Aphids can be kept below economic levels of feeding damage by the parasite Lysiphlebus testaceipes and by predators such as lacewings, lady beetles, and syrphid flies. However, biological control cannot prevent transmission of virus diseases.
Organically Acceptable Methods
Biological control and oil and soap sprays are acceptable for use on organically grown crops.
| Common name | Amount per acre** | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used. | ||||
| A. | DIMETHOATE 400 | 0.66–1 pt | 48 | Feed/graze: 14 |
| MODE OF ACTION GROUP NUMBER1: 1B | ||||
| COMMENTS: For field corn only. | ||||
| B. | ESFENVALERATE* | |||
| (Asana XL) | 5.8–9.6 fl oz | 12 | Seedcorn/Popcorn: 1 | |
| Field: 21 | ||||
| MODE OF ACTION GROUP NUMBER1: 3 | ||||
| COMMENTS: Do not exceed 0.25 lb a.i./acre/season on field corn and seed corn or 0.5 lb a.i./acre/season on popcorn. May cause mite outbreaks. | ||||
| C. | ENDOSULFAN* | |||
| (Thionex) 3EC | 1.33 qt | 17 days | 1 | |
| MODE OF ACTION GROUP NUMBER1: 2A | ||||
| COMMENTS: For fresh sweet corn only. Do not exceed 2 qt/acre/year. | ||||
| D. | NARROW RANGE OILS# | Label rates | 4 | 0 |
| MODE OF ACTION: Contact including smothering and barrier effect. | ||||
| ** | Mix with sufficient water to obtain full coverage. |
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. |
| * | Permit required from county agricultural commissioner for purchase or use. |
| # | Acceptable for use on organically grown produce. |
| 1 | Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). |