Small Aphid Populations Observed in Wheat

May 18, 2021

By Adam Varenhorst

This week we received the first report of aphids in wheat for the 2021 season. At this point in time, the populations are well below the economic thresholds (Table 1). The populations won’t be increasing rapidly until the daily temperatures warm up a little, but it is still a good idea to scout fields and determine if aphids are present.

Aphid Identification

BIRD CHERRY OAT APHIDS

Bird cherry oat aphids can vary in color from olive to dark-green and can be identified by the characteristic burnt red-orange patch that is present on the end of the abdomen near the cornicles or “tailpipes” (Figure 1).

Small Aphid Populations Observed in Wheat

Figure 1. Bird cherry oat aphids.

ENGLISH GRAIN APHIDS

English grain aphids can vary in color from light or dark green to brown, and they can be identified by characteristic black antennae, cornicles and leg joints (Figure 2).

Small Aphid Populations Observed in Wheat

Figure 2. English grain aphid.

GREENBUGS

Greenbugs are a species of aphids that are light-green in color and can be identified by the dark-green stripe present on their back. Feeding by this species causes yellow discoloration and red spots on the leaves due to a toxin present in its saliva (Figure 3).

Small Aphid Populations Observed in Wheat

Figure 3. Greenbug.

Scouting Wheat for Aphids

The easiest way to scout for wheat aphid populations is to start at one side of the field and walk in a “W” or zig-zag pattern. While walking, randomly choose 20 plants from each leg of the pattern and examine them for aphids. During the spring, the aphids are most commonly observed on the leaves and stems of the plants. Table 1 contains the economic thresholds for the three aphid species. If populations exceed the thresholds, please refer to the most-current edition of the South Dakota Pest Management Guide - Wheat.

TABLE 1. ECONOMIC THRESHOLDS FOR APHID PESTS OF WHEAT.

Pest	Number of Aphids Per Plant
	Seedling Stage	Boot to Heading Stage	Flowering Stage	Milky Ripe Stage	Milk to Medium Dough Stage
Bird Cherry Oat Aphid	20	30	>5	10	>10
English Grain Aphid	30	50	5	10	>10
Greenbug	5–15	25	>25	>25	>25

Source : sdstate.edu

Video: Turning Plant Defense Into a Management Strategy

Turning Plant Defense Into a Management Strategy

Understanding how a plant responds to stress is one thing.

Using that knowledge to make better management decisions is another.

Systemic acquired resistance, or SAR, is the plant's more direct defense response. When stress or infection occurs, the plant begins signaling throughout its system and preparing defensive compounds.

But if we know that response exists, can we help prepare the plant before the stress arrives?

The answer starts with understanding what triggers the response and what the plant needs to carry it out.

The Trigger and the Fuel

Salicylic acid plays an important role in triggering the SAR pathway.

Think of it as turning the truck on.

The engine may be running, but it still needs fuel to do the work.

In this case, manganese plays an important role in supporting the enzyme systems involved in the plant's defensive response.

This makes manganese status an important part of the conversation. Whether a producer is using tissue testing, sap analysis, or simply scouting for visible deficiency symptoms, the goal is to make sure the plant has adequate manganese available.

Manganese is required in relatively small amounts, but that does not make its role small.

If the plant receives a signal to defend itself but lacks the nutrition needed to support that response, it may struggle to carry out the process efficiently.

The trigger matters.

The fuel matters too.

Prepare Before the Stress Arrives

The best time to think about stress management is before the plant is overwhelmed.

Once a crop is already struggling, management can quickly turn into a game of catch-up.

This is similar to nitrogen management. Once a plant becomes severely deficient, correcting the problem does not necessarily erase the time and yield potential already lost.

Plant defense can work the same way.

Low-rate, targeted approaches designed to support the SAR pathway may fit best ahead of an expected stress event rather than after significant damage has already occurred.

That requires producers to think about predictable stress.

We may not know exactly what the weather will do tomorrow, but we generally know summer heat is coming. We know certain field conditions increase disease pressure. We know a herbicide application can temporarily stress a crop as the plant processes the chemistry.

Even a properly timed and labeled herbicide application can create a temporary response in the plant.

That does not mean the herbicide is bad.

Weeds can create significantly more yield loss than the temporary stress caused by controlling them.

The question is not whether we should eliminate every stressor.

The question is whether we can better prepare the plant to manage necessary and predictable stress.

Not All Stress Is Bad

Stress is a normal part of plant growth.

A perfectly stress-free environment does not exist in the field.

In fact, some stress is necessary for normal plant development. A plant responds to wind, temperature, moisture, sunlight, and countless other environmental signals throughout the season.

The environment is stress.

The plant's job is to manage it.

Problems begin when the stress load becomes greater than the plant's ability to respond.

Extreme heat, drought, high salt concentrations, disease pressure, and even certain management practices can add to that load.

This is where understanding SAR becomes useful.

Instead of waiting until the plant is visibly struggling, producers can begin identifying periods when stress is likely and make management decisions around those windows.

ISR Starts With the Soil

While SAR is a more direct defense response, induced systemic resistance, or ISR, brings the conversation back to soil health.

The longevity of a farm is closely connected to the health of its soil.

Carbon plays a major role because it supports biological life within the soil. Bacteria, fungi, and other organisms interact with plant roots and influence how the plant grows, accesses nutrients, and prepares for stress.

This is why soil health cannot be reduced to one product or one application.

It is a system.

Keeping living plants in the field longer can support biological activity. Cover crops may fit some operations. Better water management can improve soil conditions.