Cereal Aphids Density Calculator

for control action decision support

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PURPOSE:  This calculator is designed to estimate aphid density and make recommendations regarding whether to spray insecticide to combat cereal aphids.

TIP:  Initial values are provided in the form below to get you started. Please customize the form fields to best represent your conditions.

ACKNOWLEDGEMENT: This work was supported by awards #2020-70006-32980 and #2021-67021-34253 from the National Institute of Food and Agriculture.

This calculator is designed to estimate the aphid density (aphids per plant) at which a control action should be taken to achieve break-even on a farm manager’s investment in control measures (purchasing and applying insecticide). This break-even point is called the Economic Injury Level (EIL) and can be calculated for a single pest based on the widely used and accepted formula (Pedigo et al. 1986):

EIL = C/(VYpDK)

Inputs

For aphids on wheat, the formula variables represent:

C = Control cost, the cost of purchasing and applying insecticide at the recommended rate, expressed as dollar amount per acre ($/acre).

V = Market value, the market value of wheat or the contracted price, expressed as the dollar amount per bushel ($/bushel).

Yp = Yield potential, the maximum yield per acre, expressed as bushels/acre and based on user estimates from experience can be used for this input.

K = Insecticide efficacy, the efficacy of the insecticide expressed as the proportion of the pest population killed, ranging from 0 to 1.0 at the recommended rate.

D = Damage per aphid, expressed as a proportion of crop yield lost per aphid sampled.

Multiple Aphid Species

The approach can be modified for use with multiple species of pests, like the several aphids that frequently simultaneously infest wheat in the PNW and elsewhere, each with a different per capita rate of injury (Dx) (for aphid species “x”). This replaces Dx in the Pedigo formula with Deff, the effective per capita aphid injury resulting from the combination of aphids present:

EIL = C/(VYpDeffK)

Deff = Effective damage per aphid, given a specific combination of aphid species present (aphid species can differ in their Dx).

Deff can be calculated based on abundances of each species (Nx) as a proportion of the total aphid abundance (px= Nx/Ntotal) multiplied by its unique relative Dx and then summed:

Deff = (D1 x p1) + (D2 x p2) + (D3 x p3) + …

For this calculator Dx has been measured through experiments at the University of Idaho or in published studies and standardized for use in the model. The species included are English grain aphid (Sitobion avenae), Rose grass aphid (Metopolophium dirhodum), Bird cherry-oat aphid (Rhopalosiphum padi), greenbug (Schizaphis graminum), Russian wheat aphid (Diuraphis noxia), and “Cereal grass aphid” (Metopolophium festucae cerealium) (MFC).

The units of the EIL are the same as the units used for sampling. We offer two: aphids/stem or aphids/25 sweeps.

The user must input the costs for purchase and application of insecticides, estimated potential yield in bushels/acre and value in $/bushel, and the density of aphids estimated using an established method (outlined below) to determine either the average number of aphids per wheat stem or the average number per 25 180° sweeps with a standard sweep net. The aphid data must be broken down by aphid species using the guides linked to below for identification. Accurate aphid identification is important—especially to distinguish phytotoxic species (e.g., D. noxia, S. graminum, M. festucae cerealium) from less damaging species. The form below contains an area for specifying average counts per stem of the key aphids in Idaho cereal crops. [Alternatively, the user can select “unknown” for the aphid species, which will generate a generalized recommendation that lacks precision]

The calculator should be used based on aphid samples taken during late tillering or stem elongation (jointing) stage of the crop (prior to booting). Only aphid densities assessed at these stages are useful for treatment decisions.

The calculator compares the sampling result with the Deff EIL and recommends treatment if sample data indicates it has been exceeded. In practice, a user may choose to estimate an Economic Threshold (ET), the pest density at which control is warranted to prevent the pest population from reaching the EIL. The EIL was based on realized yield after aphid populations were able to continue increasing after tillering and stem elongation.

This model does not include natural enemies in its calculations. However, if predators or parasitoids are observed during scouting, users may wish to adjust decisions toward more conservative (non-treatment) thresholds. For guidance, consult existing integrated pest management literature.

The model is built on a combination of our own data (primarily for Metopolophium festucae cerealium and Rhopalosiphum padi in the Pacific Northwest) and published studies from a range of geographic locations and years, including the northern Great Plains, Europe, and Egypt. No comprehensive studies cover all six aphid species evaluated here within the PNW. Estimates for other species (Diuraphis noxia, Schizaphis graminum, Sitobion avenae, Metopolophium dirhodum) are derived from disparate sources and expert interpretation.

The value of K or “insecticide efficacy” potentially differs among cereal aphid species (Gong et al. 2021). The insecticides included in the calculator are taken from PNW Pest Management Handbook for 2025. Efficacy data are from various published studies, but all tend to be greater than 85% or higher. Without specific data on our PNW populations, however, we use a common K value for each insecticide and rate (see insecticide information table) for all six aphid species based on the literature (see references tab, below). 

Despite these limitations, to our knowledge, this is the only tool currently available that offers species-specific or multi-species aphid guidance in wheat. Where estimates conflict, we prioritize local data gathered under PNW conditions.

  • Susceptibility to aphid damage is similar across cultivars.
  • The Deff is estimated assuming aphid species ratios input is stable and correct.
  • The impact or contribution of each pest species to Deff is additive with the others (pest species do not amplify or damp one another’s per capita impacts on yield).
  • The EIL omits the effects of viruses that can be transmitted by some of the aphid species. RWA and MFC are not documented vectors of barley yellow dwarf viruses, but the three other species are. When the virus and vectors are prevalent the Deff and EIL is invalid.
  • The model is solely developed based on data and publications concerning wheat. Applicability for other grains, including barley is limited.
  • Damage relationships are calibrated to two general growth stages: Vegetative, which includes late tillering and stem elongation or jointing prior to booting; and Reproductive, booting through heading and flowering. Later stages (grain filling and ripening) are omitted because mature wheat exhibits greater tolerance to aphid feeding, and benefits of controlling aphids in those stages are minimal.

We welcome user feedback. Please contact us with comments or suggestions after using the tool. Comments can be addressed to Sanford Eigenbrode.

References

Bakry, M.M., Mohiy, M., El-Zoghby, I.R. (2020) Negative effects of the infestation by Schizaphis graminum on the vegetative growth measurements, yield and its components of bread wheat cultivar (Giza 171) in Luxor governorate, Egypt. International Journal of Entomology Research 5, 91-96.

Gong, P., Chen, D., Wang, C., Li, M., Li, X., Zhang, Y., Li, X., Zhu, X. (2021) Susceptibility of four species of aphids in wheat to seven insecticides and its relationship to detoxifying enzymes. Frontiers in Physiology 11, :623612. doi: 623610.623389/fphys.622020.623612.

Kieckhefer, R.W., Kantack, B.H. (1998) Yield losses in winter grains caused by cereal aphids (Homoptera: Aphididae) in South Dakota. Journal of Economic Entomology 81, 317-321.

Niehoff, B., Stablein, J. (1998) Vergleichende Untersuchungen zum Schadpotential der GetreidebIattIausarten Metopolophiurn dirhodurn ( WIk.) und Sitobion avenae (F.) in Winterweizen (Investigations on the damage potential of cereal aphids (Metopolophiurn divhodum [Wlk.], Sitobion avenue [F.]) in winter wheat. Journal of Applied Entomology 122, 223-229.

Pedigo, L.P., Hutchins, S.H., Higley, L.G. (1986) Economic injury levels in theory and practice. Annual Reviews of Entomology 31, 341-368.

Rabbinge, R., Sinke, C., & Mantel, W.P. (1983). Yield loss due to cereal aphids and powdery mildew in winter wheat. Landbouwwetenschappen, Rijksarchief Gent, 1159–1168.

Crop Information

Crop Stage:  Vegetative    Reproductive stage

Vegetative includes late tillering and stem elongation or jointing prior to booting. Reproductive includes booting through heading and flowering.

Crop Income (potential value)

Maximum Yield:  bushels per acre

Crop Market Value:  $ per bushel

Potential Crop Income:  $  per acre

Spray Cost

Insecticide & amount: 

Insecticide Cost:  $ per gallon

Application Cost:  $ per acre

Product Efficacy:   % effectiveness
Total Spray Cost:  $  per acre

Estimated Aphid Density

Collect average counts for each of these aphid species per stem:

Cereal Grass Aphid:  see photo, learn more
Russian Wheat Aphid:  see photo, learn more
Bird Cherry-Oat Aphid:  see photo, learn more
English Grain Aphid:  see photo, learn more
Rose-Grain Aphid:  see photo, learn more
Greenbug:  see photo, learn more

Analysis & Recommendation

Economic Injury Level (EIL) or Break-Even Aphid Density:

Economic Injury Level (aphids/stem) Aphids per stem equivalent
   

Recommendation
Based on your selections and estimated aphid density:

no spray
Spraying is not recommended.