Aphid population outbreaks are largely dependent on weather and planting date of the grain. Mild temperatures during November and December increase the possibility of serious fall infestations. Wheat and barley planted by October 15 are also more susceptible to aphid attack.
These fields should be checked weekly during mild fall weather to determine the presence of economically damaging populations of aphids and barley yellow dwarf virus. To sample, examine all plants in a linear foot of row in at least 5 locations, and estimate the number of aphids per row-foot.
The field should also be examined for the presence of small yellowing patches of stunted plants (reddened areas in wheat and oats). When sampling for aphids, be careful not to take samples from the field edges where populations are usually highest.
The side of a field that is downwind of a tree line often has more aphids than other parts of the field. Fall insecticide applications are generally needed only when average aphid populations (combining all four aphid species) approach 15 to 25 per row-foot.
Continued mild weather during the winter makes spring outbreaks more likely. The beneficial insects that attack aphids reproduce slowly at temperatures below 65°F, whereas aphids can rapidly increase when temperatures exceed 50° F. Fields should be examined for the presence of aphids just prior to top dressing grain with nitrogen.
Many insecticides can be mixed with nitrogen and applied at top-dress; however, this practice should be limited to only those fields where aphid populations approach 15 to 25 per row-foot if top dressing is to be applied before plants have begun rapid spring growth, or 100 to 300 per row-foot if top dressing is to be applied after plants have resumed vigorous growth.
Regular sampling should continue through April and May. If aphids begin to move into the grain heads, examine at least 50 heads from throughout the field, and determine the average number per head. Treatment should be considered if an average of 20 to 25 or more aphids are found per head.
Natural Enemies:
There are a number of aphid predators and parasites that, if left untreated, can effectively keep aphid populations below economic thresholds. Among the most common are lady beetles (both adults and larvae), syrphid fly (hover fly) larvae, common lacewing larvae, and a group of small parasitic wasps.
Lady beetle adults are small, oval, hard-shelled, and often brightly colored. Larvae are up to 1/3 inch long, somewhat flattened, bluish-gray with orange markings, and with legs stretched outward on the sides.
Syrphid fly larvae are green or brown maggots that have tapered front and back ends. They have no head capsule or legs and hold their hind ends in place while moving their heads about when searching for aphids.
They are commonly found in grain heads and are often mistaken for pests. Lacewing larvae are light brown, mottled with brownish- red markings, about 1/4 inch long, and resemble tiny alligators. They have relatively large sickle-like mandibles, or jaws, which they use to spear and suck body fluids from aphids and other prey.
Adult lacewing females attack small white oval-shaped eggs to tiny upright strands of silk. Perhaps the most effective natural enemies are the tiny wasps that parasitize aphids. You may not see the wasps, but “mummies,” or aphid remains, can be easily seen when wasps are active. Adult female wasps insert eggs into aphid bodies.
Upon hatching from eggs, the tiny wasp larvae eat out aphid bodies and emerge as adults through round hatch-door like exit holes, leaving papery brown empty shells or “mummies.” Parasitic wasps can virtually eliminate aphid infestations, especially when temperatures are warm in early fall or late spring.
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