TAM 110 Wheat Variety and Greenbug Resistance
August, 2002
Greenbugs in wheat. This problem often arises in Southern High Plains wheat, sometimes in the fall, more often in late winter/spring. In the fall greenbug populations might move out of other crops such as grain sorghum, and they can hit a young stand of wheat hard. Much of the damage often occurs, however, in late winter and into spring when the plants are still small. Greenbugs numbers per foot of row that are generally considered damaging range from a low end of 25 to 50 on very young plants to 300 to 800 on plants 6 to 16 inches high. For further information on greenbugs in wheat and other small grains consult Texas Cooperative Extension’s Managing Insect and Mite Pests of Texas Small Grains, B-1251 (available from http://agpublications.tamu.edu/)
This note will focus on TAM 110 wheat and greenbugs. TAM 110 is essentially a TAM 107 variety that is resistant to greenbug biotypes E, I, and K. TAM 110 has performed well in the Texas High Plains and is among several recommended varieties, year in and year out, for irrigated and dryland grain production. The history of TAM 107 includes tolerance to wheat curl mite which transmits wheat streak mosaic. Also TAM 107, and by extension TAM 110, has a long coleoptile, which allows it to be planted deeper, and it responds to high seeding rates. Under the right conditions TAM 110 can mature early, especially if planted early and winter is warm. Grazing potential for TAM 107/110 is believed to be average, and it very susceptible to leaf rust.
Carl Patrick, Texas Cooperative Extension entomologist, Amarillo, has seen about half the numbers of greenbug on TAM 110 compared to other varieties in variety trials. Brent Bean, Texas Cooperative Extension agronomist, Amarillo, concurs that they have seen greenbugs in TAM 110 fields, but usually not as bad as in other varieties. Brent reported a Panhandle example in 1999-2000 where a farmer planting TAM 107 ran out of seed and finished with TAM 110. A heavy greenbug infestation occurred, but the TAM 110 survived as 107 was lost.
Greenbug Resistance Does Not Mean Immunity
In general, growers should understand that TAM 110 is resistant to greenbug but not immune. Mark Lazar, Texas A&M wheat breeder, Amarillo, notes that resistance rarely (possibly never) means immunity, especially when talking about greenbugs. In a field situation, growers can expect greenbugs to populate TAM 110, but that aphid reproduction should be slower and impact on plant health will develop much more slowly than for susceptible varieties at the same infestation rate. In practice, the most likely way that much damage can occur is if large aphid inflights occur when the wheat is still small. This is probably what happened in 2002. Other than a new biotype, the main reason for “loss” of resistance would be mixing TAM 110 seed with other varieties.
Many farmers tend to think of resistance as immunity but that is an error that farmers, consultants, seed dealers and Extension personnel may make. We have never tried to give the impression that resistance does mean immunity. Regardless of one’s source of information we must be careful about leaping to the conclusion about “immunity,” whether we are talking about greenbugs, other insects, disease, freeze injury, etc. in wheat.
As a result of numerous reports of greenbugs in fairly large numbers on TAM 110 in winter 2001-2002, Mark Lazar obtained two aphid collections from TAM 110 fields and tested them against their standards. Field samples performed exactly as would be expected for greenbug biotypes E, I, and K. Under ideal temperature (for the aphids), at fairly high infestation rates (about 10 aphids per plant), they kill TAM 107 seedlings in about 10-14 days. At that time, TAM 110 appears unaffected, though if the aphids are left with only TAM 110 to feed on, they will do so, and will eventually kill it. This has always been the result, i.e. the resistance is strong, but does not confer immunity. Efforts will continue to examine greenbugs to ensure that we are not dealing with a new greenbug biotype, but we have not seen evidence of it so far.
Development of TAM 110 Into New Herbicide-Tolerant Varieties
As a side note, TAM 110 was chosen by the Texas A&M wheat breeding program at Amarillo for development of the Clearfield wheat system, which will allow the wheat to be sprayed with Beyond herbicide (BASF label forthcoming, the same herbicide compound as Raptor for soybeans). Colorado State University has joined Texas A&M as a developer of the Clearfield system, and technology has been licensed to AgriPro as well. Early releases include Above (Texas A&M and CSU) and AP502CL (Texas A&M and AgriPro), which are now in the A&M variety testing program. Texas A&M further hopes to release an additional line that should have better yield potential for Texas than either of these two initial varieties. (For a brief description on Clearfield wheats, visit http://www.colostate.edu/Depts/CoopExt/GPA/clrwheat.htm) Indications are that the same greenbug resistance found in TAM 110 is also present in Above and AP502CL.
Consultants, county extension staff or others may see or hear of TAM 110 fields that are strongly affected by greenbugs. The Texas A&M Center at Amarillo would like to receive representative samples of both the aphids and the seed for testing (any remnant planting seed and seed harvested from the field). Growers must be sure planted TAM 110. Damage could be due to high greenbug numbers, not necessarily a failure of the varietal tolerance.
This extension update was compiled by Calvin Trostle (extension agronomist, Texas A&M—Lubbock, 806.746.6101, c-trostle@tamu.edu), Mark Lazar, Carl Patrick, and Brent Bean (August 2002).
