Beating Bugs With HPR
By Larry Charlet
For growers used to managing their sunflower insect problems through insecticide applications, the idea of “host-plant resistance” (HPR) may seem simultaneously (1) attractive and (2) unrealistic.
It certainly is attractive from a financial standpoint. If a sunflower field can tolerate pressure from one or more problem insects without economic yield loss or seed quality issues, insecticides may become an unnecessary input. Also, the grower may not have to go to the time and expense of monitoring his fields closely — and then having to decide when to “pull the trigger” and call his aerial applicator. Plus, of course, there is the environmentally friendly aspect of reducing pesticide applications.
As to the notion that host-plant resistance reflects a “pie-in-the-sky” mentality, yes, finding and then incorporating good HPR traits into commercial cultivars is far from easy.
But there actually are numerous examples of host-plant resistance being utilized in cultivated crops. One of the earliest cases dates back to the 1920s, when a Kansas State University breeder developed the first wheat variety with resistance to the Hessian fly. More-recent examples of host-plant resistance would be in sorghum, i.e., resistance to the sorghum midge and greenbug.
What is host-plant resistance? The term basically refers to heritable traits in a plant that mitigate the degree of damage done to it by a given insect. The plant, in effect, uses its own built-in defenses to ward off harm from the insect population.
There are three types of tolerance/ resistance mechanisms. Plants may also possess a combination of two or three of these mechanisms:
• Antibiosis — This mechanism encompasses all of the adverse effects exerted by the plant on an insect’s biology to prevent the insect from developing normally or surviving to the adult stage.
• Antixenosis — Also referred to as “nonpreference,” here the plant comes across to the insect as a “bad host.” As a result, the insect either does not choose that plant to feed on, or it lays eggs while searching for another, more-suitable host.
• Tolerance — Under this mechanism, the plant is able to survive and produce even in the presence of significant insect numbers — numbers that would severely damage a susceptible plant.
Within sunflower, we have been working for several years on the development of host-plant resistance as a strategy to reduce damage from these major insect pests: banded sunflower moth, sunflower (head) moth, sunflower stem weevil, red sunflower seed weevil and the longhorned beetle (Dectes). We also have evaluated current commercial hybrids for tolerance to larval feeding by the sunflower midge.
Work with all these insects (except the midge) has focused on screening genetic materials — accessions, interspecific crosses and breeding lines — for reduced damage or reduced larval numbers. The premise is that by discovering materials with lower insect damage, we can eventually provide commercial sunflower breeders with germplasm that can be incorporated into hybrids that are targeted to geographic areas where these insects are economically important.
USDA-ARS and university sunflower specialists are involved in this effort, which is spread across four locations: Colby, Kan. (sunflower stem weevil and sunflower moth); Highmore, S.D. (red sunflower seed weevil); and Prosper, N.D. (banded moth).
Why these particular locations? We can’t always count on the same degree of insect pressure from year to year; nor, as is the case in weed or disease research, can we “insert” an appropriate level of pests into our plots. So we must rely upon natural populations in traditional “hot spots.” Our trial locations consistently tend to have significant pressure from the insect(s) noted.
What have we discovered thus far? Provided below are brief summaries from our 2007 trials at the various locations. (The 2008 data are still being compiled and analyzed.) At each site, three groups of sunflower material were evaluated:
#1 — Selected accessions and interspecific crosses.
#2 — “S1s” trial. This group consists of initial crosses made by now-retired ARS sunflower geneticist Jerry Miller.
#3 — “New” crosses. This group is comprised of newer materials crossed (by Miller) with previously analyzed lines.
• Stem Weevil Resistance Trial / Colby, Kan. — With group #1, a total of 20 lines were tested, with the number of larvae per stalk ranging from just five up to 53. With the S1s group, 37 lines (including checks) were evaluated. The larval count, per stalk, ranged from six to 64. Twenty-one lines had fewer than 16 larvae per stalk. With the third group, of the 83 lines tested, the per-stalk larval count ranged from two up to 40. Thirteen had fewer than 10 larvae per stalk.
Longhorned beetle (Dectes) infestation also was reduced in some lines in all three of these groups. Dectes is a stalk-girdling insect, and there is only one larva per stalk. So rating the various breeding lines for Dectes tolerance, as opposed to stem weevil tolerance, requires a different manner of evaluation.
• Sunflower (Head) Moth Trial / Colby, Kan. — Twenty-one lines in group #1 were evaluated for percent damaged seeds, with the range going from about 9% all the way up to 82%. With the S1s group, 29 lines (including checks) were evaluated. Percent damaged seeds ranged from 2% up to 100%, with four lines showing less than 25% damaged seeds. Of the 65 lines in group #3, the percentage of damaged seeds ranged from 3% up to 91%. Fourteen rated lower than 20%, and six had less than 10% damaged seeds.
• Red Sunflower Seed Weevil Trial / Highmore, S.D. — Twenty-two lines in group #1 were evaluated for percent damaged seeds, with the range going from about 2.5% up to 37%. With the S1s group, 36 lines (including checks) were evaluated. The percent damaged seeds ranged from 1% up to 25%, with 20 lines having less than 10% damaged seeds. With the third group, of the 45 lines tested, the percentage of damaged seeds ranged from just 0.4% up to 21%. Thirty-one lines rated lower than 10%, and 13 were below 5% damaged seeds.
• Banded Sunflower Moth Trial / Prosper, N.D. — Thirteen lines in group #1 were tested for percent damaged seeds, with a range from 4% all the way up to about 67%. With the S1s group, 32 lines (including checks) were evaluated. Percent damaged seed ranged from 3% up to 62%, with eight lines having less than 15% damaged seeds. With the third group, of the 66 lines tested, the percent damaged seeds ranged from 2% up to 72%. Forty-three lines rated lower than 20%, and 16 lines were below 10% damaged seeds.
• In the 2008 sunflower midge trial, conducted at Mapleton, N.D., we evaluated 76 commercial hybrids. Most of these hybrids actually had been provided to USDA-ARS plant pathologist Tom Gulya for Sclerotinia resistance testing. We planted and assessed them at a location that has been used for over 20 years for sunflower midge studies.
The basis for rating the hybrids was a necrosis index of 0-5. (Necrosis is the scarring caused by midge larval feeding.) The necrosis rating ranged from 0.3 to 2.7, with a trial average of 1.57. Nineteen of the hybrids were below 1.0.
So what is the take-home message from the 2007 trial findings and those of prior years?
Results have been very encouraging. The range of tolerance to these important insect pests shows that there is, indeed, genetic material available that can provide the foundation for future insect-tolerant or -resistant commercial hybrids.
Following analysis of the 2008 results, in 2009 we will test the most-promising materials and make further crosses. We hope to begin preliminary hybrid evaluation by as early as 2010. Our ARS breeding program will incorporate these tolerance traits into advanced germplasm, which will then be refined until it is ready for release to commercial sunflower breeders for use in their hybrid development programs.
The entire process obviously is lengthy and complicated. But we believe the eventual outcome will be commercial hybrids with notably better tolerance to key insect pests — and that’s very good news for both growers and the sunflower industry in general.
Larry Charlet is research entomologist with the USDA Agricultural Research Service at Fargo, N.D. Among his key site cooperators on this project are Rob Aiken of Kansas State University, Colby, and Kathy Grady, South Dakota State University.
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