Wheat as Cover Crop for Soil Conservation and Refugium of Beneficial Insects for Aphid Control

Winter Wheat as a Cover Crop

Using cover crops is a common practice across North America; however, national policies propose different management strategies to achieve optimal balance between conservation benefits of soil and water (Figure 1). As part of such sustainable plans, farmers from western Kentucky usually plant soft red winter wheat as cover crop from mid-September to early-December. Approximately, half-million acres of wheat are planted in Kentucky. This type of wheat is harvested for specialty products, such as cakes, cookies, and crackers. However, farmers can use this untreated and unsold wheat as an inexpensive cover crop aimed to prevent soil erosion, choke out weeds, add organic matter, and improve soil and water quality.

Figure 1. Crop cover termination map.(Obtained from USDA Cover Crop Guidelines, 2019).

Natural Biocontrol of Aphids by Local Predators

As temperatures rose this year, so did the populations of English grain aphids (Sitobion avenae) on wheat planted as cover crops in Lyon and Caldwell Counties (Figure 2). During a warm scouting day (approximately 75°F to 78°F) April 05, 2023, we scouted a commercial wheat cover crop field in Lyon County where we noticed large numbers of aphids, 45.8 ± 11.1 per row foot (Mean ± SEM). In this particular case, the farmer planted wheat to reduce soil erosion (Figure 3). However, as these aphid populations increase in unsprayed wheat fields, predators such as ladybugs also can increase. We observed two ladybug species actively feeding on English grain aphids: the seven-spot ladybeetle (Coccinella septempunctata) and the pink ladybug (Coleomegilla maculata) (Figure 4). The number of ladybugs was recorded on five 172-ft2 plots (13 ft x 13 ft) whereas the number of aphids was recorded per row foot. Overall, 4 ± 0.6 lady beetles were found per plot and the average number of both the seven-spot lady beetle (2.4 ± 0.7) and the pink ladybug (2.2 ± 0.8) were similar.

Despite the abundance of English grain aphids on wheat, the average number was below the economic threshold for commercial grain wheat (100 aphids/ row foot). In Missouri, the populations of English grain aphids rarely exceed the economic threshold on wheat. In Kentucky, English grain aphids are the second most abundant aphid identified over a 6-year scouting period, and it has not exceeded the economic threshold on commercial wheat grown for grain. Although no economic threshold exists for wheat as a cover crop, these fields may represent important habitats to maintain natural enemies (predators and parasitoids) and thus, helping to control aphids.

Figure 2.  Immature English grain aphids collected on wheat (Photo:  Armando Falcon-Brindis, UK).

How Does this Impact Wheat Production?

The numbers of English grain aphids found in these samples were below the economic threshold; this finding provides an idea of the important role natural enemies, such as ladybugs, syrphid flies, and parasitic wasps, are playing in suppressing aphid populations. It is important to recognize that these interactions were observed on cover crop wheat systems used for soil conservation to avoid soil erosion. However, we should look at the valuable services (biocontrol and pollination) that natural enemies can offer to agriculture, as well as the importance of these areas as cover crop planted for soil conservation and to serve as refugia for natural enemies. In this case, there are reports that showed that a single ladybug can eat up from 1700 to 5,000 aphids during its lifetime. These natural enemies often can keep pests at population levels that will not cause economic losses in wheat, but regular monitoring of pest and natural enemies levels is still recommended.

Figure 3. Soft red winter wheat as cover crop in Lyon County, western Kentucky. Notice the wheat was strategically planted in bands across the hills to prevent soil erosion (Photo: Armando Falcon-Brindis, UK).
Figure 4. Dorsal view of the seven-spot lady beetle (A) and the pink ladybug (B) (Photo: Armando Falcon-Brindis, UK).

Further readings

  • NRCS Cover Crop Termination Guidelines (USDA 2019) (Link)       

By Armando Falcon-Brindis, Entomology Research Associate, and Raul T. Villanueva, Entomology Extension Specialist

Posted in Grains
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