Planting Date and Susceptibility to Pink Bollworm

Transcription

1 Planting Date and Susceptibility to Pink Bollworm P. Brown, J. Silvertooth and L Moore Abstract The susceptibility of cotton to spring emergence of pink bollworm (PBW) was evaluated for a variety of planting dates in Pinal, Maricopa, LaPaz and Yuma counties using historical climate records and heat - unit -based models that predict PBW emergence and cotton development. Early planted cotton proved most susceptible to the PBW emergence, however, springtime weather conditions also played an important role in determining overall susceptibility. Growers wishing to incorporate planting date as one aspect of PBW management should keep abreast of early season weather conditions. Introduction Control of pink bollworm (PBW) (Pectinophora gossypiella (Saunders)) is most successfully accomplished using a combination of cultural and chemical control. One aspect of cultural control involves making adjustments in planting date to minimize the survival of early season PBW larvae. Low survival in this initial larval population reduces the size of subsequent generations, making control of PBW less difficult and costly. The PBW overwinters in a dormant, larval form. As weather conditions warm in the late winter and early spring, overwintering PBWs complete their life cycle and emerge as adults. Upon emergence, the female moths lay eggs which produce the next generation of PBW larvae. The survival of this new generation is heavily dependent on whether the young larvae find cotton (Gossypium spp.) that has developed squares 1/3 of full size or larger (defined as the susceptible square stage). Younger cotton provides inadequate food and shelter, resulting in poor survival of larvae. Planting date affects when cotton reaches susceptible square and therefore can have a major impact on the survival of early season PBW larvae. To conduct a quantitative assessment of how planting date impacts the susceptibility of cotton to early season PBW larvae, one must carefully examine early season weather conditions, since both PBW emergence and early season cotton development are dependent on temperature. One such assessment conducted by Brown et al. (3) revealed that planting date, springtime weather conditions and cotton type (short vs. long staple) all played a role in determining overall susceptibility of cotton to early season PBW larvae. Early planted cotton proved more susceptible than late planted cotton as expected. However, springtime weather conditions also impacted overall susceptibility. For a given planting date, cotton proved far less susceptible to spring PBW emergence when springtime weather conditions were hot. Susceptibility was highest in cool springs. Another conclusion of the work by Brown et al. was that for a given planting date long staple cotton was less susceptible to spring PBW emergence than short staple cotton. This conclusion was reached based on preliminary information from Huber that indicated long staple cotton required more time (heat units) to reach susceptible square than short staple cotton. Recent work by Silvertooth et al. (4) revealed that Pima S -6, Deltapine 9, 5 and 51 all reach susceptible square after approximately 9 heat units have accumulated after planting. This is due primarily to the similarity of these varieties to initiate fruiting at similar node positions. Because no significant difference in heat unit requirement to susceptible square was observed for Pima and common short staple varieties, both types 126

2 of cotton are equally susceptible to PBW spring emergence: the previous work by Brown et al. is therefore in need of revision. In this report we present a revised assessment of the how planting date impacts the susceptibility of cotton to PBW spring emergence in Pinal, Maricopa, LaPaz and Yuma counties. Methods Historical climate records for the major cotton growing areas of Pinal, Maricopa, LaPaz and Yuma counties were obtained from the Office of the State Climatologist. Length of the historical records varied from 24 to 3 years depending on location. Annual accumulations of heat units (temperature thresholds = 86F and 55F) were computed for each year of record for all locations. The warmest and coldest springs were selected for each location and designated as the historical hot and cold years respectively. The mean annual accumulation of heat units was also computed for each location to serve as the normal year. Springtime PBW emergence curves (Figure 1) were then generated for the hot, cold and normal years for all locations using the equation presented by Huber (1). Seven planting dates encompassing the first six weeks of the cotton planting season were then evaluated for susceptibility to emerging PBWs during hot, cold and normal years. For each planting date the heat unit accumulation since 1 Jan. was determined for the cold, normal and hot years. Next, the heat unit accumulation (since 1 Jan.) at susceptible square (for both short and long staple cotton) was determined for each planting date by adding 9 heat units to the heat unit accumulation at planting. The heat unit accumulations at susceptible square were then transferred to the correct PBW emergence curves to determine the percentage of PBW emergence remaining after susceptible square. Results and Discussion The results of the planting date /PBW susceptibility analysis are presented for each location in Table 1. The numbers presented represent the percentage of PBW spring emergence remaining after cotton has reached susceptible square. Larger numbers in Table 1 indicate higher levels of susceptibility to PBW spring emergence. Average countywide susceptibility is depicted graphically in Figures 2-5. Planting date and springtime weather conditions both play a role in determining the susceptibility of cotton to PBW spring emergence. Earlier planting dates are more susceptible to PBW emergence (Figures 2-5) with the highest susceptibility occurring in western Yuma Co. where the legal first planting date is i 5 Feb. (Figure 5). The average percentage of PBWs emerging after susceptible square in cotton planted on the legal first date in a normal year in LaPaz, Maricopa, Pinal and Yuma counties was 38 %, 45 %, 49% and 63% respectively. Planting 3 weeks after the legal first date dropped this percentage to 16 %, 23 %, 28% and 45% in LaPaz, Maricopa, Pinal and Yuma counties respectively. Susceptibility of cotton to PBW also proved quite sensitive to springtime weather conditions. For a given planting date, cotton was found to be most susceptible in cold springs and least susceptible in hot springs (Figures 2-5). In many cases the effect of springtime weather on susceptibility was quite large. Cotton planted 29 Mar. in Buckeye offers an excellent example of this large weather effect. Susceptibility to PBW ranged from 47% for the cold spring to only 11% during the hot spring (Table 1). Summary Adjustments in planting date can significantly lower the susceptibility of cotton to PBW spring emergence. However, springtime weather conditions play a major role in determining the overall susceptibility of a given planting date. Growers wishing to incorporate planting date into their PBW control program should keep abreast of early season weather conditions. Weather information specifically tailored to the planting 127

3 date /PBW susceptibility question is available from the Arizona Meteorological Network operated by Arizona Cooperative Extension (2). References 1. Huber, R.T Heat units and population prediction. In Proc Beltwide Cotton Production Mechanization Conf. 6-7 Jan Las Vegas, NV. 2. Brown, P.W Accessing the Arizona Meteorological Network by Computer. Extension Rpt Univ. of Arizona, College of Agric., Tucson, AZ. 3. Brown, P., R. Huber and L. Moore Planting date and susceptibility to pink bollworm. In J. Silvertooth and M. Bantlin (ed) Cotton: A College of Agriculture Report. Series P -81 College of Agric., Univ. of Arizona. Tucson, AZ. p Silvertooth, J., P. Brown and J. Malcuitt Basic cotton crop development patterns. In J. Silvertooth and M. Bantlin (ed) Cotton: A College of Agriculture Report (This issue). College of Agric., Univ. of Arizona, Tucson, AZ. Acknowledgement The authors wish to acknowledge the Office of the State Climatologist for providing the historical weather records used in this analysis. Also the funding provided by the Az. Commission of Agriculture and Horticulture in support of this study is greatly appreciated. 128

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5 SPRING EMERGENCE OF PINK BOLLWORM HEAT UNIT TOTAL SINCE JAN. 1, degree -days Figure 1. Spring emergence of PBW as a function of the annual accumulation of heat units (after Huber (1)) HOT - NORMAL /15 3/22 3/29 4/5 4/12 4/19 4/26 Figure 2. function of planting date and weather conditions for Pinal County. Countywide mean values were obtained by averaging the location-specific values presented in Table 1. 13

6 HOT -- NORMAL /15 3/22 3/29 4/5 4/12 4/19 4/26 Figure 3. function of planting date and weather conditions for Maricopa County. Countywide mean values were obtained by averaging the location- specific values presented in Table 1. Note, Aguila not included in countywide averages due to significant differences in temperature relative to other Maricopa County locations HOT - NORMAL. 3 2 \ 1-3/15 3/22 3/29 4/5 4/12 4/19 4/26 Figure 4. function of planting date and weather conditions for LaPaz County. Countywide mean values were obtained by averaging the location -specific values presented in Table

7 HOT - NORMAL `NN 3. N NN /15 2/22 3/1 3/8 3/15 3/22 3/29 Figure 5. function of planting date and weather conditions for Yuma County. Countywide mean values were obtained by averaging the location -specific values presented in Table