**4. Future directions in breeding for Hemipteran resistance in soybean**

While much of current research has focused on traditional and classical breeding and screening methods for host-plant resistance, research is emerging which incorporates newer genomic and molecular technology. Likely, a combined approach will be necessary to improve the durability and decrease development time of Hemiptera resistant soybean. Here we list several considerations for the future of HPR in soybean:

**1.** Tolerant cultivars must be emphasized in breeding programs. As mentioned earlier, tolerant cultivars have the ability to withstand or recover from damage caused by insect populations equal to those on susceptible cultivars. Thus, unlike antixenosis and antibiosis, tolerance comprises of plant features which are not involved in plant-insect interaction. Though breeding for tolerant cultivars is more difficult and time consum‐ ing (as the crop has to be grown till maturity in the infested condition) tolerance based resistant cultivars nonetheless offer two major advantages. 1) New virulent biotypes are less likely to emerge in a cropping system based on tolerant cultivars. While feeding upon tolerant plants, infesting insect populations are not reduced as they are on plants exhibiting antibiosis and antixenosis. As a result, there is no selection pressure and frequency of novel virulence trait remains lower. This is directly in contrast to what is observed in insects feeding on plants showing antixenosis and antibiosis, as various physical and chemical factors in these plants allows for selection of virulent individu‐ als. Thus, the chances for development of biotypes that can overcome resistance genes are significantly reduced through the use of tolerant cultivars. Russian wheat aphid populations were not able to overcome tolerant plants but can break antibiosis based resistance in wheat [46]. 2) Tolerant cultivars are highly compatible with biological control measures, thus can be combined in IPM program. The allelochemical based toxins in plants exhibiting antibiosis and antixenosis could be detrimental for natural ene‐ mies of insect pests [45]. On the other hand, tolerance based cultivars do not have any known adverse impact on beneficial insects. In North-America, biological control employing natural enemies makes up an important component forIPM of soybean aphid. Thus, tolerant cultivars provide an excellent opportunity for integrating HPR and biological control in IPM.

significantly changed the pest scenario in these regions. Thus, for effective HPR based IPM program, there is a need to identify novel germplasm sources that are resistant against more than one insect. Further, the known resistant sources against a single pest can be explored for their response to other insect pests e.g. in breeding programs for pest management in north central US, soybean aphid resistant (*Rag* containing soybean) PIs can be screened for response against brown marmorated stink bug. Previously, several soybean PIs have shown resistance against multiple insect pests e.g. PI171444 which originally identified for resistance against stink bug complex also showed resistance against bean leaf beetle and banded cucumber beetle [44,118]. Thus, there is strong potential for soybean PIs having multiple pest resistance and to be incorporated into the

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**4.** Bt soybean potential against Hemipterans: The development of transgenic (e.g. Bt) resistance against the Hemipteran insects has not succeeded so far. Pyramiding of Bt genes with HPR genes may be an useful strategy. In Lepidoteran insects, the highest level of resistance to Lepidopteran insects obtained through MAS using the native soybean genes was 70% reduction in feeding [119]. However, when the soybean insect resistance loci were pyramided with a *cr*y1Ac transgene from *Bacillus thuringiensis* (Bt) the level of feeding damage was reduced to 90% compared to susceptible checks [119]. Such native crop gene and transgene pyramids may be useful in several aspects of insect resistance. First, the Cry protein from a single Bt transgene may only protect the host plant from one or at best two classes of insects. For instance, the Cry1Ac toxin provides resistance against many Lepidopteran pests, but not to Coleopteran pests. A combina‐ tion of native insect resistance gene with resistance to beetle (e.g, insect resistance loci on chromosome 7) with the Bt transgene could broaden resistance of plants to in‐ clude Coleopteran pests that are insensitive to Cry1Ac toxins. Second, several insect pests have demonstrated the ability to develop resistance to Cry toxins, so effective strategies are needed to manage resistance to Bt [120]. Some populations of the diamondback moth [*Plutella xylostella* (L.)] have developed resistance to Bt toxins in different parts of the world where Bt are routinely used on cruciferous crops [121]. Soybean lines carrying the PI 229358 allele at the insect resistance locus on chromo‐ some 7 in addition to a Cry1Ac transgene were more protected against defoliation by corn earworm and soybean looper than related transgenic lines lacking the PI 229358 allele [119]. Studies to investigate weight gain of tobacco budworm larvae from Cry1Acresistant and Cry1Ac-sensitive strains demonstrated that larvae fed on leaves of plants with both a Cry1Ac transgene and the native insect resistance allele on chromosome 7 gained weight more slowly than larvae fed on leaves from transgenic plants lacking the

**5.** RNAi and other genomic approaches: Given issues with ineffectiveness of Bt on Hemi‐ pterans, RNA interference (RNAi)- mediated control presents an attractive avenue for management of these pests. RNAi results in sequence specific knockdown of gene expression at the post-transcriptional level as introduced dsRNA causes the degradation of identical mRNAs [122]. Crops based on RNAi-mediated pest protection are expected

breeding programs.

native resistance allele [119].


Traditionally, lepidopteran foliage feeders and 3 species of stink bugs (*N. viridula, A. hilare, E servus*) were major insect pests of soybean in southern us. However, as mentioned earlier, recent expansions of red-banded stink bug in southern US, brown-marmorated stink bugs in eastern, central and southern US, and soybean aphid in north central us have significantly changed the pest scenario in these regions. Thus, for effective HPR based IPM program, there is a need to identify novel germplasm sources that are resistant against more than one insect. Further, the known resistant sources against a single pest can be explored for their response to other insect pests e.g. in breeding programs for pest management in north central US, soybean aphid resistant (*Rag* containing soybean) PIs can be screened for response against brown marmorated stink bug. Previously, several soybean PIs have shown resistance against multiple insect pests e.g. PI171444 which originally identified for resistance against stink bug complex also showed resistance against bean leaf beetle and banded cucumber beetle [44,118]. Thus, there is strong potential for soybean PIs having multiple pest resistance and to be incorporated into the breeding programs.

populations equal to those on susceptible cultivars. Thus, unlike antixenosis and antibiosis, tolerance comprises of plant features which are not involved in plant-insect interaction. Though breeding for tolerant cultivars is more difficult and time consum‐ ing (as the crop has to be grown till maturity in the infested condition) tolerance based resistant cultivars nonetheless offer two major advantages. 1) New virulent biotypes are less likely to emerge in a cropping system based on tolerant cultivars. While feeding upon tolerant plants, infesting insect populations are not reduced as they are on plants exhibiting antibiosis and antixenosis. As a result, there is no selection pressure and frequency of novel virulence trait remains lower. This is directly in contrast to what is observed in insects feeding on plants showing antixenosis and antibiosis, as various physical and chemical factors in these plants allows for selection of virulent individu‐ als. Thus, the chances for development of biotypes that can overcome resistance genes are significantly reduced through the use of tolerant cultivars. Russian wheat aphid populations were not able to overcome tolerant plants but can break antibiosis based resistance in wheat [46]. 2) Tolerant cultivars are highly compatible with biological control measures, thus can be combined in IPM program. The allelochemical based toxins in plants exhibiting antibiosis and antixenosis could be detrimental for natural ene‐ mies of insect pests [45]. On the other hand, tolerance based cultivars do not have any known adverse impact on beneficial insects. In North-America, biological control employing natural enemies makes up an important component forIPM of soybean aphid. Thus, tolerant cultivars provide an excellent opportunity for integrating HPR and

**2.** Marker assisted breeding will facilitate faster and more efficient development of resistant cultivars. Markers will also be useful for pyramiding major genes as well as quantitative trait loci (QTL) for multigeneic defense against the insect as found in tolerant soybeans. Development of closely linked markers for known resistant genes in soybean will enhance

**3.** Exploration for new sources of HPR: In South America especially Brazil, extensive research on HPR against stink bugs has been conducted. But in US especially, the southern states, lepidopteran foliage feeders have been the focus of HPR research [68]. Further, due to the preference for insecticide based control, soybean germplasm has not been extensively explored for resistance against stink bugs. No cultivar of soybean showing resistance against stink bugs has been released so far in US. There is a need to identify native sources of resistance against stink bugs. Though the selection for resistance against stink bugs is relatively time consuming and labor intensive, novel sources that offer wider pool of traits such as pest resistance, yield, etc. should be

Traditionally, lepidopteran foliage feeders and 3 species of stink bugs (*N. viridula, A. hilare, E servus*) were major insect pests of soybean in southern us. However, as mentioned earlier, recent expansions of red-banded stink bug in southern US, brown-marmorated stink bugs in eastern, central and southern US, and soybean aphid in north central us have

biological control in IPM.

34 Soybean - Pest Resistance

the selection efficiency.

incorporated into breeding programs.


to achieve the same level of success as Bt-based transgenic crops [123]. Though there are various categories of insect genes that could be silenced through RNAi to achieve the desired results, targeting of genes encoding for effector proteins in salivary glands of Hemipteran insects has been promising. At the start of feeding, Hemipteran insects inject the saliva produced by salivary glands into plant tissues. Hemipteran saliva contains various chemical substances such as digestive enzymes that facilitate feeding. Important‐ ly, the saliva also contains the effector proteins that are determinants of virulence for these insects. RNAi knockdown of *coo2*, which is an effector protein of pea aphid secreted into the fava beans leaves during feeding, significantly reduced the survival of this insect [124,125]. In addition to pea aphid, successful RNAi studies in Hemipterans insects like peach aphid (*Myzus persicae*), Brown plant hopper (*Nilaparvata lugens*) have also been reported [126-129]. To develop soybean employing RNAi-based management of Hemi‐ pteran pests, there is a need to generate significant amount of molecular resources for these insects. To date, a whole genome sequence is only known for 1 Hemipteran insect, the pea aphid, *Acyrthosiphon pisum* [130]. Besides RNAi, there are other novel approaches such as the transgenic plant resistance against Hemipteran pests. The management of Hemipteran pests by use of transgenic plants expressing lectins and protease inhibitors has been recently reviewed [131], thus not discussed in this chapter.

**Author details**

Ohio State University, USA

20 June 2012).

Management 2012;3: E1-E10.

Society of America; 1994.

2012;7(2): e32152.

ment Center, The Ohio State University, USA

, Tae-Hwan Jun1,2, M. A. R. Mian2,3 and Andy P. Michel1

3 USDA-ARS Corn and Soybean Research Unit, Madison Ave., Wooster, OH, USA

Association. http://www.soystats.com/ (accessed 20 June 2012).

1 Department of Entomology, Ohio Agricultural Research and Development Center, The

Developing Host-Plant Resistance for Hemipteran Soybean Pests: Lessons from Soybean Aphid and Stink Bugs

http://dx.doi.org/10.5772/54597

37

2 Department of Horticulture and Crop Sciences, Ohio Agricultural Research and Develop‐

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[2] USDA-National Agriculture Statistical Service. http://www.nass.usda.gov/ (accessed

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[4] Ragsdale DW, Landis DA, Brodeur J, Heimpel GE, Desneux N. Ecology and manage‐ ment of the soybean aphid in North America. Annu Rev Entomol 2011;56: 375-399.

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Raman Bansal1

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### **5. Conclusions**

Although there has been some success with HPR for Hemipteran pests, for example the glandular hairs for potato leafhopper, there are many opportunities for expanding this important pest management tool. Research has already resulted in the commerical availability of HPR against the soybean aphid, with many more varieties to come. However, HPR research for the other major Hemipteran pests of soybean continues to lag behind. More molecular and genomic techniques increase the feasibility of finding HPR loci and improve the ability to combine both traditional HPR approaches and newer RNAi methodologies. This includes not only developing resistance to multiple insect pests, but potentially other pathogens that they may interact with to impact soybean [113]. However, these new varieties will need to be studies and balanced in terms of the other aspects of integrated pest management (i.e. chemical and biological control) to both limit non-target impacts and extend durability in the face of insect adaptation.

## **Acknowledgements**

We would like to thank the members of the Michel and Mian Laboratories for assistance in Hemipteran soybean pest research, specifically L. Orantes, J. Wenger, C. Wallace, R. Mian, W. Zhang, J. Todd, T. Mendiola, K. Freewalt. Funding for this work was provided by the Ohio Soybean Council, the North Central Soybean Research Project, OARDC, The Ohio State University and USDA-ARS.
