Update:
Objective 1a. Bioassays for pyrethroid resistance.
UPDATE:We collected aphids in three locations in Iowa and one in Minnesota this summer. Aphids were screened for insecticide resistance using a glass-vial bioassay treated with lambda-cyhalothrin and bifenthrin.

What it means for farmers: Insecticide resistant aphids are found in Iowa, and have been found every year since 2016. The amount of these aphids within a population is high enough to suggest that these traits are fixed and will not go away when insecticides are not used.

Objective 1b. Maintain soybean aphid colonies.
UPDATE:Four pyrethroid-resistant colonies are being maintained in our laboratory since 2018. In addition, aphids collected this summer from the same locations specified on objective 1a are being kept in mini-colonies for further studies.

What it means for farmers: The Soybean Entomology Laboratory maintains a resource that is a value to agribusiness and scientist exploring the threat of insecticide resistance in the soybean aphid. We will continue to explore these colonies to determine the mechanism and means to manage soybean aphids as pyrethroid resistance spreads. We have shared these resources with agribusiness when requested.

Objective 2a. Screening aphid susceptibility to alternative insecticides.
UPDATE:Leaf dip bioassays were performed using the soybean aphid colony most resistant to lambda-cyhalothrin and a susceptible population to lambda-cyhalothrin. The two colonies were compared for susceptibility to bifenthrin, flonicamid, flupyradifurone, sulfoxaflor, spirotetramat, and chlorpyriphos.

What it means for farmers: We have confirmed that lambda-cyhalothrin resistant aphids in Iowa. We are working to determine if these aphids are also resistant to other insecticides.

Objective 2b. Evaluating potential mechanisms for pyrethroid resistance.
UPDATE:The most resistant to lambda-cyhalothrin was used to evaluate for rapid metabolism/detoxication as a mechanism of pyrethroid resistance in soybean aphid using synergistics (Piperonyl butoxide (PBO), S,S,S-Tributyltrithiophosphate (DEF) and triphenyl phosphate (TPP)). The results obtained for the resistant population were compared to those from the lab-susceptible population.

What it means for farmers:There is the potential to manage pyrethroid-resistant soybean aphids with pyrethroids if a synergist is added. Such an addition could allow farmers to prevent outbreaks without having to switch to a different (any potentially more expensive) active ingredient. Our findings revealed that adding a synergist to a pyrethroid did not improve the efficacy when used against pyrethroid resistant soybean aphids. Because the synergist didn't work, metabolic detoxification is not a part of pyrethroid resistance in the soybean aphid. This suggests that cross-resistance is unlikely to other insecticides with different modes of action. Therefore, based on our data to date, we do not anticipate pyrethroid-resistant soybean aphids to be more resistant to insecticides like chlorpyrifos than susceptible aphids.


Objective 3a. Determine gene mutations.
UPDATE:Given the resistance to lambda-cyhalothrin identified in Objective 2, we sought to determine any genetic differences among these populations. We used a candidate gene approach to determine if previously described mutations in the voltage-gated sodium channel (VGSC), specifically, if knockdown(kdr) mutations were present in individual soybean aphids from each subpopulation. These kdr mutations change the amino acid sequence and are highly correlated with pyrethroid resistance in other insect species.

What it means for farmers: We have found evidence in the genome of the soybean aphid that a mutation is responsible for pyrethroid resistance. Such a mutation can help explain how aphids are resistant and be used as a marker to identify insecticide-resistant aphids in the field.

3b. Develop a diagnostic tool that identifies pyrethroid resistant aphids.
UPDATE:We furthermore predicted that the C to T mutation between putative resistant and susceptible alleles could be differentiated by digestion of a PCR product with the restriction enzyme BstEII (recognition sequence 5’-GGTNACC-3’; i.e. susceptible gDNA digested but resistant gDNA not digested). To test this, the PCR product from each individual was digested with BstEII overnight and run on a 2-3.5% agarose gel to detect differences between lab susceptible (SUS) and resistant field-collected (RES) populations. Digestion of the PCR product with BstEII yielded fragments of size 154bp and 285bp in the lab (SUS) populations. In contrast the RES populations were heterozygous and produced bands of size 154bp, 285bp (susceptible allele) and 439bp (resistant allele). These results confirmed success of the PCR-Restriction Fragment Length Polymorphism (PCR-RFLP) marker. Due to presumed dominance of the 1024F encoding resistant allele, clonal heterozygotes from the RES population would be resistant.

What it means for farmers: Preliminary evidence suggests that the kdr gene can be used a marker to more quickly identifying insecticide-resistant soybean aphids. This will speed id-work from days to hours.

View uploaded report

Realized Stakeholder Deliverables and Outcomes:Matt O’Neal and Erin Hodgson organized a resistance management field day in August 2017 to raise awareness of pyrethroid resistance. Outcomes from the field day:
• Understanding SBA life cycle and biology had a 1,100% percent increase.
• Knowing SBA host plant resistance options had a 900% percent increase.
• Awareness of SBA insecticide resistance had a 700% percent increase.
• Implementing integrated SBA management had a 1,000% percent increase.
• Participants were asked about their perception of value, and 100% said it was a “very valuable” workshop. There were notable behavior changes indicated in the survey summary: 58% said they will consult with ISU regarding future pest management decisions and 44% said they will scout and use economic thresholds for SBA.
• In general, respondents indicated the four speakers contributed to a successful workshop. For example, 93% of people attending the workshop said Erin Hodgson had “excellent” energy, and 100% of people said Erin had “excellent” engagement with the group and were experts in their respective fields.