Incorporating Native Plants in Insectary Strips to Promote Insect Diversity and Belowground Beneficial Microbes

We established a randomized block experimental design  to test the multiple benefits of insectary strips at a certified organic vegetable farm, in Edinburg, Texas. On October 2, 2020, 3 feet wide insectary strips were created between every 4 rows of cash crop that were made 5 feet wide from center to center. We transplanted 2 rows of broccoli 18” apart in a staggered manner on December 10, 2020. A total of 3 treatments of insectary strips were replicated 6 times on the 3,2670 square foot research plot. This three insectary strip treatments include:   

1. Native wildflower mix, 19 species 
2. Sunn hemp, a nonnative hedge row species commonly used 
3. Control, no management  

Sunn hemp seeds were purchased from Johnny’s Selected Seeds Co. and the native wildflower cocktail was purchased from Douglas King Seeds in San Antonio, Texas. The seed mix, ‘Feed the Bees Pollinator Mix’, contains 18 species: Texas Bluebonnet, Cuero Purple Prairie Clover, Eldorado Engelmann Daisy, South Texas Gaillardia, Venado Awnless Bush Sunflower, Zapata Rio Grande Clammyweed, Partridge Peas, Plains Coreopsis, Purple Coneflower, Clasping Leaf Coneflower, Black-Eyed Susan, Mexican Hat, Lanceleaf Coreopsis, Lemon Mint, Standing Cypress, Drummond Phlox. 

Sunn hemp was planted at a recommended rate of 28 grams per square meter, and the native seed mix was planted at 10 grams per square meter.  Both Sunn hemp and the native wildflower mix were broadcasted by hand October 2nd, 2020. Seeds were slightly covered with soil and padded down with a garden hoe. Drip irrigation was applied once a week for 4 hours during the study period. Sunn hemp will be replanted in June or July before planting onion for the next round of cash crop. These strips will be maintained during the summer and again in the fall of 2021. 

Insect community dynamics 

To determine if the native wildflower mix attracts more biodiversity compared to Sunn hemp and control, I collected insect samples 4 different times: before cash crop planting, during cash crop establishment, before cash crop harvest, and after cash crop harvest. I set up sticky traps and pitfall traps in three different locations: in the middle of the second cash crop row to the north of the insectary strips, on the edge of the insectary strips, and in the middle of the insectary strips (n=54) (Figure 3). Pollinator traps were installed at the middle of each insectary strips (n=18). The pitfall trap was prepared in 250 ml clear plastic cups filled with 60 ml of water and micro-90 solution. The pollinator traps were also filled with the water micro-90 solution. The insect samples in all traps were collected after 48 hours and transported to the lab for analysis. The sticky traps were stored in the freezer, and the pitfall and pollinator traps were cleaned on a 120-mesh sieve immediately after collection and transferred in test tubes with 70% ethanol. Insects collected from these traps will be grouped as herbivores, predators, parasitoids, and pollinators and analyzed to determine the insect diversity, species richness, species evenness, and beneficial to pest ratio across the three treatments. Insect trap layout and timing will again be repeated in the fall of 2021. 

 Cash crop yield and pest damage 

Ninety days after planting, when the broccoli heads were ready to harvest, 5 random broccoli plants per treatment were photographed to evaluate pest damage. Each leaf on the plant was turned over to inspect for insects. Each plant was rated on a scale of 0-5; 0 meaning no insect damage and 5 indicating severe damage.  Insects on the leaves, when visible, were photographed for record. On the same day, yield measurements were taken for 10 randomly selected plants. The broccoli plants suffered mainly from frost bite from the freeze.   

 Soil health 

In the fall of 2020, we collected 6 soil cores to make one composite sample, each 0-20 cm deep, from each subplot in the same locations where the insect samples were collected. We maintained even spacing between each of these cores and were considerate of the distances of these cores from vine or plant trunks and irrigation lines. Holes were refilled to prevent a “swizz cheese” effect. Tools and gloves were disinfected with 90% isopropyl alcohol using a clean paper towel between each plot. Cores were mixed to make a composite sample by touching the outside of the bag and shaking well. A portion of soil samples were then transferred to 50 mL centrifuge tubes that were sealed and placed into a cooler with ice and transported to the lab. The soil samples in the 50ml tubes were divided into 2 subsamples, ~30 g for PFLA and ~20 g for DNA extraction and stored in a freezer until analysis. Soil samples for PLFA were placed on dry ice and shipped overnight to the Water Management Research Unit. For the soil microbial community analysis, we currently extracting the DNA from 72 samples (samples from planting and harvest times) using the MP Fast DNA Spin Kit. These DNA samples will then be sent to the UT Austin DNA Sequencing Facility for the 16s sequencing. This will again be repeated for Fall 2021 planting season.  Additionally, during the fall season, we will collect root samples in both the cash crop and insectary strips to determine the total mycorrhizal colonization in the roots, nematode community, and the prevalence of plant parasitic nematodes. We will measure soil respiration biweekly with a 6400-09 Soil CO2 Flux Chamber.