Crop Report – Week of July 3rd, 2023

Dimethoate Resistant Two-Spotted Spider Mite Populations in Ontario Soybeans

Results from bioassays conducted on two-spotted spider mite (TSSM) populations collected in soybean fields across Ontario show the prevalence of dimethoate (Cygon or Lagon) resistance is high in some areas. Through a collaborative project with Dr. Vojislava Grbic, Professor at Western University, Dr. Ian Scott at AAFC-London and OMAFRA, Drs. Joseane Nascimento and Vladimir Zhurov at Western University analyzed mite populations collected in 2021 and 2022 before or after failed dimethoate applications.

Out of 31 fields tested, 18 (58% of sites) were found to have resistant TSSM populations (Figure 1). Fields with dark green dots had TSSM populations that were still susceptible to dimethoate (100% kill), while fields with yellow (50% killed), orange or red (0% killed) dots had populations that were only partially killed or were fully resistant. Soybean fields close to one another tend to have similar levels of resistance.

Figure 1. Levels of dimethoate effectiveness in two-spotted spider mite populations collected in 31 soybean fields in 2021 and 2022.

As part of the study, other miticides currently registered on horticultural crops were tested against these TSSM populations as potential candidates for registration on soybeans. These include the active ingredients abamectin, bifenazate, cyflumetofen, etoxazole, pyridaben and spiromesifen. Spiromesifen (as Oberon) is the only miticide registered on field crops (corn and dry beans) in Canada. The heatmap see (Figure 2.) shows the mortality rates of the bioassays performed by location and active ingredient. In general, TSSM populations collected on soybeans were susceptible to these miticides, except for a few localized populations that show tolerance to spiromesifen (Wardsville and West Elgin) or abamectin (Niagara). This isn’t entirely surprising, given TSSM are pests of almost all horticultural crops and are easily carried by wind from neighbouring fields where these products are already in use. Some active ingredients are showing promise as good candidates for registration of soybeans.

Figure 2. Summary heatmap showing results of bioassays with seven active ingredients on spider mite samples collected from soybean fields in southwestern Ontario in 2021-2022.

Fingers crossed for frequent rains to keep TSSM infestations low this year. If the season turns hot and dry, we are at risk of significant yield loss if dimethoate applications fail. Until new cost-effective products are registered on soybeans, we may need to explore other Integrated Pest Management (IPM) strategies to manage or reduce the risk of TSSM infestations. Preliminary research from Texas using low-cost releases of predatory mites looks promising as a biocontrol option against TSSM in field crops.

OMAFRA is looking for more TSSM populations in 2023 to have tested at Western. If you find any spider mite populations in soybeans or dry beans before a dimethoate application (preferable), or after an application that was not effective, please contact Tracey Baute at 519-360-7817 so that collections can be made. Thank you to all the crop consultants and growers who have participated so far in providing spider mite samples. Funding for this project is provided by The Grain Farmers of Ontario (GFO), Ontario Greenhouse Vegetable Growers and MITACS, and the Ontario Research Fund.

Weather Data – June 26 – July 2, 2023

Location	Year	Highest Temp (°C)	Lowest Temp (°C)	Rain (mm)	Rain (mm) April 1st	GDD 0C April 1st	GDD 5C April 1st	CHU May 1st
Harrow	2023	27.4	13.9	48.7	203	1323	869	1216
	2022	31.1	9.2	0.9	209	1388	950	1399
	10 YR Avg. (2011-20)	28.6	16.5	33.3	319	1483	991	1521
Ridgetown	2023	29.3	11.9	95.8	282	1253	807	1127
	2022	31.2	6.6	2.4	177	1293	864	1275
	10 YR Avg. (2011-20)	29.4	14.3	24.2	260	1389	902	1411
London	2023	29.0	8.1	26.0	272	1251	807	1132
	2022	29.8	6.6	8.6	190	1247	827	1225
	10 YR Avg. (2011-20)	29.1	14.7	22.6	288	1366	882	1390
Brantford	2023	29.7	9.4	37.2	224	1242	795	1128
	2022	31.4	7.4	4.2	180	1249	818	1189
Welland	2023	27.7	10.7	59.9	240	1242	790	1111
	2022	29.3	10.7	1.2	191	1293	853	1266
	10 YR Avg. (2011-20)	28.6	15.5	9.0	277	1368	883	1391
Elora	2023	27.5	7.4	25.5	188	1143	714	1033
	2022	28.9	7.0	3.7	167	1143	724	1086
	10 YR Avg. (2011-20)	28.1	12.3	21.8	284	1217	746	1227
Mount Forest	2023	27.0	7.6	21.3	233	1139	715	1029
	2022	29.5	7.1	8.4	188	1143	731	1105
	10 YR Avg. (2011-20)	27.8	12.6	13.1	274	1199	736	1221
Peterborough	2023	28.4	8.5	12.2	203	1152	711	1014
	2022	27.6	6.7	0.6	212	1154	721	1122
	10 YR Avg. (2011-20)	28.8	12.4	13.0	256	1233	762	1234
Kemptville	2023	29.5	13.4	1.9	245	1217	781	1082
	2022	27.7	10.9	5.2	305	1243	790	1215
	10 YR Avg. (2011-20)	29.8	14.5	12.7	275	1294	824	1312
Earlton	2023	29.2	5.4	10.7	204	1028	639	973
	2022	25.1	6.6	11.3	191	1019	636	1049
	10 YR Avg. (2011-20)	28.4	11.0	16.5	230	995	609	1053
Sudbury	2023	28.9	7.3	30.8	314	1042	654	977
	2022	23.8	8.0	6.2	195	1015	623	1027
	10 YR Avg. (2011-20)	28.2	12.1	17.8	254	1078	668	1131
Thunder Bay	2023	28.3	9.2	19.5	176	906	539	890
	2022	30.6	3.6	3.6	344	814	458	793
	10 YR Avg. (2011-20)	27.8	10.1	22.3	257	916	517	922
Fort Frances	2023	29.7	7.7	18.0	154	1089	715	1120
	2022	26.3	2.4	4.0	377	868	515	895
	10 YR Avg. (2011-20)	27.9	9.8	23.7	246	1034	617	1071