1. No spraying
2. Spraying every 2 weeks
3. Spraying at established economic threshold
4. Spraying 2 times over established economic threshold
PLH scouting will be completed on a weekly basis, along with 5 one-acre plus adjoining alfalfa variety plots. The same exact alfalfa plots used in 1995 were used in 1996 to also test if alfalfa damaged the year earlier by PLH will continue to adversely affect alfalfa yield and quality the following year. The plots were also sprayed identical to 1995.
1996 PLH populations as indicated in Table 1 started off much slower than the previous year. The 1996 Spring was very cool and wet which probably slowed PLH from establishing as quick. Also due to wet weather the first cutting was also delayed, which started our PLH scouting later than normal on July 2nd. At that time and actually the only time all year was PLH actually at threshold and we never reached 2x threshold in these plots the entire year. This factor became a mystery because many other alfalfa fields in the county reached much higher populations. We did continue our program by spraying the one set of plots every two weeks, and even without reaching threshold, we still measured height differences of over three inches in the sprayed plots, but these did not show similar differences in yield.
Yield results are indicated in Table 2. As expected, the stress from the previous year's unsprayed high PLH populations carried on into the next year showing a significant yield difference with P=.045. But as a result of much lower PLH populations, the 2nd cutting showed no significant difference with p=.80 and the 3rd cutting also showed no significant yield difference with p=.61.
Forage Quality data was also collected and comparisons made. Very little difference in quality was noted throughout the project except protein data from 2nd cutting which approached being significant. This data is available upon request.
Alfalfa stand counts were also taken starting in May and continued 4 times through the year to October. The stand counts ranged from 38 to 96 plants/sq. ft. Leaf analysis tissue data was also taken showing all nutrients as sufficient. Neither the stand count or leaf analysis data showed any significant differences and the data is available upon request.
2. Even in years of lower PLH populations, like 1996, 1st cutting alfalfa yields can still be reduced from PLH pressures the previous year.
3. After two years of this PLH research, alfalfa stand counts still show no significant differences between PLH populations.
4. This research definitely needs to be continued in 1997 using the same plots with the same treatments. It is highly probable that due to stress, the plots not sprayed could have thinner stands. Also it is also possible by cutting adjoining alfalfa later, that PLH populations could be driven into the PLH plots to better conduct the research.
Table 1. Observation of average PLH population based on sweep net sampling and height measurement.
| Potato Leafhopper Populations | |||||
|---|---|---|---|---|---|
| Adult | Nymph | ||||
| Week | Replication | Avg. | PLH Count/ | PLH Count/ | Notes |
| # | Height | 10 Sweeps | 10 Sweeps | ||
| Avg. | Avg. | ||||
| 7-2-96 | 1's | 7.75" | 5.25 | 0 | Plots 2's & 3's sprayed |
| 2's | 8.00" | 6.25 | 0 | 7-3-96 | |
| 3's | 8.25" | 4.75 | 0 | ||
| 4's | 8.25" | 5.75 | 0 | ||
| 7-10-96 | 1's | 16.50" | 5.50 | 0 | |
| 2's | 17.25" | .25 | 0 | ||
| 3's | 17.25" | 0 | 0 | ||
| 4's | 16.50" | 6.25 | 0 | ||
| 7-17-96 | 1's | 18.75" | 17.75 | 1.50 | Plot 2.s sprayed |
| 2's | 22.25" | 7.25 | 0 | 7-18-96 | |
| 3's | 22.75" | 5.00 | 0 | Field Cut 7-22-96 | |
| 4's | 19.50" | 13.75 | 0 | 3.5" height difference | |
| 7-31-96 | 1's | 4" | 0.75 | 0 | Plot 2's sprayed |
| 2's | 4" | 1.75 | 0 | 8-1-96 | |
| 3's | 4" | 0.25 | 0 | ||
| 4's | 4" | 1.75 | 0 | ||
| 8-7-96 | 1's | 11.75" | 5.00 | 0.25 | |
| 2's | 13.25" | .75 | 0 | ||
| 3's | 13.25" | 5.00 | 0 | ||
| 4's | 11.50" | 2.00 | 0.25 | ||
| 8-14-96 | 1's | 19.75" | 7.50 | 0 | Plot 2's sprayed |
| 2's | 23.50" | 4.25 | 0 | 8-15-96 | |
| 3's | 21.00" | 6.25 | 0.50 | 3.75" height difference | |
| 4's | 20.00" | 8.00 | 0 | ||
| 8-20-96 | 1's | 25.25" | 4.50 | 0 | First sign of natural |
| 2's | 27.75" | 1.25 | 0 | population decrease - | |
| 3's | 26.25" | 4.75 | 0.25 | 2.5" height difference | |
| 4's | 25.75" | 2.75 | 0.25 | Field Cut 8-26-96 | |
1's - No Spray
2's - Spray Every 2 Weeks
3's - Threshold
4's - 2 x Threshold
Table 2. Alfalfa yields under different leafhopper management strategies.
| Yield Results | |||||
|---|---|---|---|---|---|
| 1st Cut | 2nd Cut | 3rd Cut | |||
| Spray Treatment | # of | 6-25-96 | 7-22-96 | 8-26-96 | Total Yield |
| Replications | Ton/Acre | Ton/Acre | Ton/Acre | ||
| Every 2 Weeks | 4 | 2.7 | 1.4 | 1.1 | 5.2 |
| 2x Threshold | 4 | 2.1 | 1.3 | 1.1 | 4.5 |
| Threshold | 4 | 2.1 | 1.3 | 1.0 | 4.4 |
| No Spray | 4 | 2.2 | 1.4 | 1.1 | 4.7 |
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