Why is there a need for insect resistance in soybean?
Impact of insecticides
The photo to the left is from a farmer’s field in South Georgia where insecticide ran
out mid-application.
Plants on the right were destroyed by velvetbean caterpillar.
Promise of genetic resistance
A trial of hills of different lines under heavy Mexican bean beetle pressure.
Though no insecticides have been applied, the resistant line Sodendaizu (PI 229358) suffers no insect damage while other lines in the trial are decimated.
Some of the major pests of soybean
Velvetbean caterpillar, Anticarsia gemnatalis
Corn earworm, Helicoverpa zea
Soybean looper, Pseudoplusia includens
Kudzu bug, Megacopta cribraria
Mexican bean beetle, Epilachna varivestis
Issues with resistance sources
Many of the early sources of resistance to insects suffered from poor agronomic traits that prevented incorporation of resistance into cultivars without carrying negative traits as well. Lodging is an issue with some types (yellow box) as well as small mottled seed (inset).
Bioassays and QTL mapping facilitate breeding for
insect resistance
Bioassays
Resistance was mapped as two traits: antibiosis and antixenosis.
- Antibiosis measures the impacts of plant genetics on development of the insect.
- Antibiosis is observed using a detached leaf assay, comparing larval weight between plant genotypes. Resistant plants hamper insect growth.
- Antixenosis measures insect preference between plant genotypes.
When plants are arranged together in either mesh cages or a flooded metal tray and infested equally, insects migrate to plants they prefer and damage susceptible plants first. - Antixenosis is measured by percent defoliation of all leaves on a plant.
QTL mapping
Biparental mapping
QTLs have been validated from three resistant soybean lines at UGA:
PI 229358, PI 227687, and Boggs.
PI 227687
PI 227687 carries QTL-E for antibiosis and antixenosis which commonly segregates with the Pb gene for sharp trichomes.
PI 229358
PI 229358 carries QTLs M, G, and H. QTL-M is the most effective single QTL to date and provides antibiosis and antixenosis-based resistance. QTL-G is for antibiosis only and is only detectable when QTL-M is present in the background. QTL-H has a similar requirement for QTL-M but only provides antixenosis-based resistance.
Boggs
QTLs N, O, and F were mapped in Boggs, a cultivar released from the UGA
breeding program.
Quantitative resistance to insects and pathogens are
located at alleles for seed isoflavonoid content
Resistance to chewing insects is quantitative, and QTLs have been detected in similar genomic positions for multiple insect pests. QTLs for quantitative resistance to insects also notably collocate with seed isoflavone QTLs and QTLs
for quantitative resistance to pathogens. While this association was noticed first by another group (Zhao et al. 2015) it is considerably more extensive than previously documented.
Native QTLs can be combined with Bt to get better resistance:
QTL validation and combinations
QTLs M, G, H, and E have been tested in combinations to determine a suitable pyramid for breeding. The pyramid of QTLs M and E provides the most resistance with the least resistance QTLs crossed in and is the most efficient for breeding. The combination of QTLs M and E can delay the economic threshold of insect damage by almost a week under field conditions (blue shading). Testing of combinations of QTLs N and O with QTLs M and E is underway.
