Lygus bug insecticide mortality bioassays: A preliminary look at 2021 results

Lygus bugs are starting to find their way from drying weeds on the hillsides to our beautiful blooming strawberry fields in the Santa Maria area about two weeks earlier than normal. Several fields have already been sprayed or are planning to be sprayed soon. As growers and PCAs start considering what insecticides to use, we wanted to share the data we collected from our mortality assays last summer (2021). We were only able to collect enough lygus bugs to test from six locations despite sampling in the Watsonville and Oxnard regions last year. The bioassays require ~300 lygus bugs for each location.

Note that the samples we took in 2021 started fairly late in the year. Testing samples earlier this season before much of the sprays have been deployed will be interesting to compare to later samples this year. We may see the mortality curves shift as the season progresses.

Figure 1. Map of Santa Maria locations tested.
Figure 2. Map of Nipomo locations tested.

Although you will see variability in mortality between locations sampled within the same chemical test, keep in mind that this isn’t necessarily resistance since, at the time of testing, we did not have a fully susceptible colony to compare to. This year we do have such a colony, and we look forward to comparing our known susceptible lygus bugs to our wild ones in each of the strawberry growing regions in CA. Recall that lygus bugs also are VERY mobile insects and can fly around between fields with ease so before anyone points fingers at neighbors, know that those lygus bugs we tested likely visited other types of crops in the area earlier in the year too.

Six sites were tested in 2021 in the Santa Maria growing region. Four insecticides were tested at the maximum field application rate. These include:
• Rimon at 12 Fl oz/a
• Beleaf at 2.8 Fl oz/a
• Sivanto at 14 Fl oz/a
• Actara at 4 Fl oz/a.
Brigade was also tested at a rate of 16 Fl oz/a.

Table 1. Location of each lygus bug collection site and date tested in 2021.

Of the locations tested, Rimon followed by Beleaf and Sivanto had the greatest mortality, and Actara and Brigade had the lowest mortality. More samples are needed across the strawberry growing regions of CA to understand lygus mortality and possible resistance to these insecticides.

Figure 3. The graphs show the simplified lygus bug percent mortality for each location and stage. Data shown have been corrected to account for the mortality in the untreated control. Missing bars indicate zero mortality.

Several other researchers have looked at lygus bug mortality in CA historically and we will show in an upcoming blog post how our 2021 data compare to the trends of the past… so stay tuned!

Authors: Dr. Sarah Zukoff and Kiley Jensen
Contact Sarah (805) 540-4876 for more information.