2003 Sugarbeet
Research and Extension Reports. Volume 34, Page 216-218
Lygus Bug Control and
Sugarbeet Yield Impacts from
Tank-mixing Foliar
Insecticides and Fungicides
Mark A. Boetel,
Assistant Professor
Robert J. Dregseth,
Research Specialist
Allen J. Schroeder,
Research Specialist
Department of
Entomology
North Dakota State
University
Fargo, ND
Introduction:
The
Tarnished plant bug is one of several plant bugs belonging to the genus Lygus,
hence, the common name “Lygus bug.”
Lygus bug outbreaks have occurred sporadically in North Dakota and
Minnesota sugarbeet fields since 1998.
Both adults and nymphs (immatures) cause feeding injury to sugarbeet. They use piercing and sucking mouthparts to
penetrate the plant, inject a tissue-liquifying toxin, and ingest the resulting
liquid. Lygus bugs have only recently
been recognized as economic pests of sugarbeet in North America. Thus, only a few insecticide materials are
specifically labeled for their control in sugarbeet in the United States. In addition, little is known regarding the
effectiveness of registered insecticides or their compatibility with fungicides
used to control Cercospora leaf spot in sugarbeet. This study was carried out with the
following objectives: 1) to evaluate the performance of foliar insecticides at
controlling Lygus bugs in sugarbeet; and 2) to monitor for possible plant
injury and yield losses due to phytotoxic interactions between the insecticides
and two commonly used foliar fungicides.
Materials
& Methods:
This study was conducted in a commercial sugarbeet field site near Hoople, ND during the 2003 growing season. Whole-plant visual counts were taken on five plants in each plot 2 days before treatment applications, and indicated a pretreatment infestation level of about 1.8 Lygus bugs (adults and nymphs combined) per plant. The majority (70.5%) were nymphs at the time of spraying.
A total of 14 treatment combinations were arranged in a randomized complete block design with four replications. Individual plots were 25 ft long and 7 rows (22-inch spacing) wide. All insecticide products evaluated in the experiment were registered for use in sugarbeet, although the only insecticides that had a specific listing of Lygus bug control in the sugarbeet portion of their label were Dibrom 8 and Lorsban 4E. All insecticides were applied alone and as a tank mixture with Eminent 125SL fungicide to determine whether the combination would impact insecticide performance or sugarbeet yield parameters. An additional tank mixture of Lannate insecticide with SuperTin fungicide was also evaluated. In all cases, companion fungicide-only treatments were included as non-tank-mix controls, and all chemical treatments were compared with an untreated check. Teatments were broadcast-applied on 4 September using a C02 backpack cannister spray system with a 6-ft wide boom that was equipped with 4 Teejet 11002 nozzles. The system was calibrated to deliver a finished spray volume of 20 GPA while traveling at 3 mph. Assessments of control were carried out at 4 and 11 days after treatment using whole-plant visual counts on five plants per plot. The inner 2 rows of each plot were harvested on 30 September using a 2-row mechanical harvester, and subsamples were sent to the American Crystal sugarbeet quality laboratory in East Grand Forks, MN, for sugarbeet quality analyses.
Results & Discussion:
Evaluations at four days after spraying indicated that the following treatments provided significant control of Lygus when compared with the untreated controls: Dibrom 8 + Eminent 125SL, Lorsban 4E (1 pt/ac), Lorsban 4E + Eminent 125SL, Lannate LV + Eminent 125SL, Malathion 57EC at 1 pt/ac, and Malathion 57EC + Eminent 125SL. Good control was also provided by all insecticides and insecticide/fungicide combinations at 11 days after treatment, including the lower (1/2 pt/ac) rate of Lorsban 4E. Trends suggested slightly improved levels of control by tank-mixing the insecticides with fungicides, although the gains were not statistically significant. Interestingly, SuperTin fungicide alone also displayed a moderate level of insecticidal activity in the 11-day counts. This finding is not unusual since many fungicide materials also can be toxic to insects, although growers should not rely on fungicide applications to provide acceptable control of Lygus bugs. Excellent Lygus bug control was achieved at 11 days after treatment with all insecticide treatments. Those that resulted in the lowest numbers of surviving insects included Lannate LV + SuperTin, Lannate LV + Eminent 125SL, Lorsban 4E (both ½ and 1 pt/ac rates, and tank-mixed with Eminent 125SL), and Malathion 57EC (alone and tank-mixed with Eminent).
|
Table 1. Lygus bug survival in sugarbeet
treated with foliar-applied registered and experimental insecticides, Hoople,
ND, 2003. |
|||
|
Treatment/form. |
Rate (lb product/ac) |
Live Lygus bugs / plant |
|
|
4 days after treatment |
11 days after treatment |
||
|
Eminent
125SL |
13 oz |
1.30 a |
0.70 ab |
|
SuperTin
|
5 oz |
1.10 ab |
0.45 bc |
|
Check |
-- |
1.10 ab |
0.95 a |
|
Dibrom
8 |
1 pt |
0.85 abc |
0.15 cd |
|
Lorsban
4E |
0.5 pt |
0.70 bc |
0.10 d |
|
Lannate
LV |
1 pt |
0.70 bc |
0.20 cd |
|
Lannate
LV + SuperTin |
1 pt 13 oz |
0.60 bc |
0.05 d |
|
Dibrom
8+ Lorsban
4E |
0.5 pt 0.5 pt |
0.60 bc |
0.15 cd |
|
Dibrom
8+ Eminent
125SL |
1 pt 13 oz |
0.55 c |
0.25 cd |
|
Lorsban
4E |
1 pt |
0.50 c |
0.05 d |
|
Lorsban
4E + Eminent
125SL |
1 pt 13 oz |
0.45 c |
0.05 d |
|
Lannate
LV + Eminent
125SL |
1 pt 13 oz |
0.40 c |
0.05 d |
|
Malathion
57EC |
1.5 pt |
0.40 c |
0.00 d |
|
Malathion
57EC + Eminent 125SL |
1.5 pt 13 oz |
0.40 c |
0.00 d |
|
LSD
(0.05) |
|
0.54 |
0.35 |
Yield data somewhat corresponded with post-spray counts of surviving Lygus bugs, but variability among samples minimized the detection of significant differences between treatments (Table 2). Therefore, yield data were analyzed using Fisher’s least significant difference (LSD) test at a significance level of 0.1 (i.e., with 90% confidence in treatment comparisons). Numerical yield increases were observed with several treatments, although no insecticide or insecticide/fungicide tank mixture gave a significant benefit when compared with the untreated check in statistical comparisons of either recoverable sucrose or sugarbeet root yield. This suggests that insecticide applications made later than September 1 for Lygus control may not provide economic benefit to the grower.
Treatment with Lannate LV + SuperTin, Dibrom 8 + Eminent 125SL, and Lorsban 4E (1 pt/ac) + Eminent resulted in significant losses in both root yield and recoverable sucrose. Gross economic return for plots treated with Lannate LV, Dibrom 8, and Lorsban 4E (all at 1 pt/acre and without fungicide) averaged $71, $43, and $14 above that from the untreated check plots, respectively; however, it is important to note these values are simply averages and not actual statistical parameters. More importantly, these results demonstrate that significant sugar yield reductions are likely to occur if these insecticide/fungicide tank mixes are applied to sugarbeet. Further research is needed to determine the following: 1) the effectiveness and crop safety of combining lower labeled rates of the insecticides with fungicides; and 2) how close to harvest growers can expect to achieve economic benefits from controlling late-season Lygus infestations in sugarbeet.
Acknowledgments:
Appreciation
is extended to the Sugarbeet Research and Education Board of MN and ND for
their provision of partial funding for this research. We also thank sugarbeet grower Robert Jackson for allowing us to
conduct this experiment on his farm.
The efforts of Charlie Hotvedt and his staff at the American Crystal
sugarbeet quality tare laboratory in East Grand Forks, MN, in processing
harvest samples for quality assessments are also greatly appreciated. Finally, we are grateful for the help from
Allan Cattanach (American Crystal Sugar Company) in spreading the word about
our need for a good research site, and for assistance from Nick Arends (also
with American Crystal) who participated in field scouting for Lygus
infestations.