ECOLOGY AND BEHAVIOR Nonlinear Degree-Day Models for Postdiapause Development of the Sunflower Stem (Coleoptera: )

1,2 3 4 1 SCOTT C. MERRILL, ASSEFA GEBRE-AMLAK, J. SCOTT ARMSTRONG, AND FRANK B. PEAIRS

J. Econ. Entomol. 103(2): 302Ð307 (2010); DOI: 10.1603/EC09297 ABSTRACT The sunßower stem weevil, Cylindrocopturus adspersus (LeConte) (Coleoptera: Cur- culionidae), has caused yield losses across much of the western Great Plains. Little is known about the Þeld biology of this pest. Simple prediction models, such as degree-day models, are an integral tool for development of C. adspersus management strategies. Using data collected in Colorado, Kansas, and Nebraska, we sought for predictable variation between C. adspersus pupation, adult eclosion, and emergence and accumulated degree-days Celsius (DD) by using a temperature threshold of 5ЊC.

Accurate phenological models can be used to time scouting efforts and pesticide applications. The Downloaded from relationship between phenological data and accumulated DD Þt nonlinear, Gaussian distributions better than uniform distributions. Phenological models were developed to describe these distributions for pupation, adult presence within the stalk and adult emergence. The pupation model predicts 50% pupation at 197 DD and 90% at 307 DD. Model results predict that 50% of adult eclosion within the stalks will have transpired at 396 DD and 90% at 529 DD. A model-averaged result from two data sets

predicts 5% adult emergence from stalks at 262 DD, 50% emergence at 540 DD, 75% emergence at 657 http://jee.oxfordjournals.org/ DD, and 90% at 777 DD. Scouting for adults thus can be initiated at 262 DD. Current chemical controls target adults to prevent oviposition. Thus, applications therefore should not be made before this point.

KEY WORDS Cylindrocopturus adspersus, degree-day model, predictive model, sunßower stem weevil

One of the most important row crops in North Amer- stalk. After hatch, the larvae feed on pith and con- by guest on March 16, 2016 ica is the cultivated sunßower, Helianthus annuus L. ductive tissue (Rogers and Jones 1979, Knodel and Bangsund and Leistritz (1995) estimated that the eco- Charlet 2002). Mature C. adspersus larvae overwinter nomic impact of the United States sunßower industry in the sunßower stalk below the Þrst node above the was Ͼ$2.5 billion annually. In 2008, Ϸ2.5 million acres ground and within the root crown below ground level was planted to sunßower (USDAÐNASS 2009), and (Rogers and Serda 1982, Charlet 1983a). Larvae con- the price of sunßower seed has increased by Ͼ10% tinue their development within the stalk, pupate and annually since 2002 (USDAÐNASS 2009). These indi- eventually emerge as adults in late spring and early cators demonstrate the growing value of this crop and summer of the following year (Rogers and Serda the growing need to protect this crop from pest dam- 1982). After emergence, adults feed on sunßower age. Numerous sunßower pests exist, many of which plants, mate, and oviposit. In addition to feeding dam- can dramatically reduce yields (Charlet 1999). One of age, C. adspersus also is associated with the stalk rot the most damaging of these pests is the sunßower stem diseases Phoma black stem (Phoma macdonaldii weevil, Cylindrocopturus adspersus (LeConte) (Co- Boerma), which contributes to premature ripening leoptera: Curculionidae), which has caused signiÞcant syndrome, and charcoal stem rot [Macrophomina yield losses in sunßower producing states west of the phaseolina (Tassi)] (Gaudet and Schulz 1981, Yang et Mississippi River (Rogers and Jones 1979, Armstrong al. 1983). 1996). Larval feeding within stem vascular tissues can Management tactics for C. adspersus include de- reduce sunßower yields and often causes plant lodging layed planting (Rogers and Jones 1979, Oseto et al. (Rogers and Jones 1979). Preharvest lodging has re- 1982, Rogers et al. 1983), stalk burial (Charlet 1994), sulted in 80% yield loss (Armstrong and Koch 1997, and the use of insecticides (Rogers and Jones 1979, Knodel and Charlet 2002). Females deposit eggs under Charlet and Oseto 1983, Rogers et al. 1983, Charlet et the epidermis in the basal region of the sunßower al. 1985). Natural enemies, including numerous spe- cies of Hymenoptera, have been found reducing C. 1 Department of Bioagricultural Sciences and Pest Management, adspersus populations (Rogers and Serda 1982, Char- Colorado State University, Fort Collins, CO 80523-1177. let 1983b, Charlet et al. 2002). However, efÞcient bi- 2 Corresponding author, e-mail: [email protected]. ological controls for C. adspersus do not currently exist 3 Colorado State University Extension, 525 15th Ave., Greeley, CO 80631-2049. (Charlet 1983b, Charlet et al. 2002). Compared with 4 USDAÐARS, 2413 E Business 83, Weslaco, TX 78596-8344. cultivated sunßower, some native sunßower species April 2010 MERRILL ET AL.: DEGREE DAY MODELS FOR SUNFLOWER STEM WEEVIL 303 have shown signiÞcantly less foliar feeding and sig- current sampling date. If weather stations failed to niÞcantly fewer larvae (Rogers and Seiler 1985, Barker collect temperature data, data were collected from the 1990). Their Þndings have implications for the devel- next closest Colorado State University weather station opment of resistant sunßower cultivars. Monitoring provided temperature readings. Therefore, weather for C. adspersus is made difÞcult by its size, coloration stations ranged from within meters to Ͼ10 km away. and behavior (Knodel and Charlet 2002). Current Distance from site to weather station could result in monitoring recommendations call for scouting from substantial noise in the data. However, temperature late June through mid-July, which is logistically difÞ- data for this region are relatively stable. cult and costly. An increased knowledge of the phe- We hypothesized that the relationship between DD nology of C. adspersus is needed to improve current and phenological stages were either 1) linear, with integrated pest management (IPM) practices (Char- uniform counts detected for a period during in each let 1987). Prediction models for the emergence of C. phenological stage; or 2) nonlinear, with an approxi- adspersus could improve timing of scouting efforts and mately bell-shaped curve (i.e., counts increasing with of insecticide treatments. Predictive models exist for increased DD to a peak and then decreasing as the predicting emergence of adults from the stalk but season progressed). Counts were transformed into were developed from data obtained over a limited cumulative fractions of yearly counts. Data were an- time period or from a limited geographical range. alyzed as a function relating the cumulative fraction of Charlet (1987) examined C. adspersus in North Dakota yearly counts to accumulated DD. The cumulative and found that initial emergence of adults from stalks fraction of each developmental stage of C. adspersus Downloaded from occurred at 420 degree-days Celsius (DD) and that was Þt to the accumulated DD data using regression 90% of emergence had occurred by 865 DD (using a models (Proc Reg, SAS 2002Ð2003) or logistic models temperature threshold of 5ЊC). In Colorado, Arm- (Proc Nlin, SAS 2002Ð2003). The linear model form strong (1996) observed Þrst emergence at 379 DD and was as follows: found 90% emergence had occurred at 651 DD (using Cumulative fraction of C. adspersus a temperature threshold of 5ЊC). To our knowledge, http://jee.oxfordjournals.org/ no DD models exist for determining timing of pupa- developmental stage ϭ ␣ * DD ϩ ␤ tion and adult eclosion within the stalk. Our goal was the development of a DD prediction model for C. where ␣ and ␤ are the slope and intercept, respec- adspersus pupation, adult eclosion within the stalks, tively. The nonlinear function was as follows: and adult emergence from the stalk. Cumulative fraction of C. adspersus

͑␤ Ϫ ͒ Materials and Methods developmental stage ϭ 1/͑1 ϩ ␣ * e * DD ͒ Pupation and Appearance of Adults Within Stalks. where ␣ and ␤ are modiÞers to the logistic (sigmoid) by guest on March 16, 2016 Data were collected in 2003 and 2004 in Washington function. AkaikeÕs Information Criterion (AIC) (Akaike (eastern Colorado), Phillips (northeastern Colorado) 1981, Burnham and Anderson 2002) was used to select and Yuma (northeastern Colorado) counties in Col- the appropriate model (i.e., either linear or nonlinear) orado and Sherman county (northwestern Kansas) in for each developmental stage. The model with the lowest Kansas. Previous-year sunßower stalks were collected AIC was then used to describe the relationship between weekly starting 30 March of each year. Every week, 30 accumulated DD and each stage. stalks with their crowns and roots were removed from Adult Emergence From the Stalk. Methods for de- each of the four Þeld sites. In total, 1,560 stalks were termining C. adspersus emergence from stalks differed collected over 13 wk in 2003, and 1,440 stalks were primarily because additional data set were available collected over 12 wk in 2004. Stalks were dissected and for examining emergence. The Þrst emergence data the developmental stages of C. adspersus determined. set was developed as described above. To obtain C. Each C. adspersus developmental stage (larva, pupa, adspersus emergence numbers, we calculated the de- and adult) was recorded during dissection of the cline in the number of adult within the stalk stalks. Sampling continued until no C. adspersus were from 1 wk to the next. The decline in number of adults observed in the samples. present in the stalk was used as an indicator of C. Daily maximum and minimum temperatures were adspersus emergence from stalks. Thus, percentage obtained for these sites from the nearest Colorado decline in the number of adults was considered as State University weather stations. DD were tallied percentage emergence. Cumulative percentage of from 1 January of each year by using a temperature adult emergence from stalks was determined for each threshold of 5ЊC (Charlet 1987, Armstrong 1996, Arm- sampling date. These data were used to develop C. strong and Koch 1997). SpeciÞcally, DD were calcu- adspersus Emergence model 1. lated in Celsius as follows: Additional emergence data were collected in 1996 and 1997 at Thomas County, KS (northwestern Kan- ϭ DD (maximum temperature sas) (1996 only); Cheyenne County, KS (northwest- ϩ minimum temperature)/2 Ϫ 5ЊC ern Kansas); Scotts Bluff County, NE (western Ne- braska); Ellis County, KS (central Kansas); and For each sampling date, accumulated DD were cal- Washington County, CO (eastern Colorado). Each culated by summing DD from 1 January through the year at each site, six sets of 10 previous-crop sunßower 304 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 103, no. 2

Fig. 1. C. adspersus pupation. Logistic model of cumulative C. adspersus pupation as a function of accumulated DD (solid

line) with 95% CL (dashed lines). Black circles are observed data. Downloaded from stalks were collected in early April and placed under To quantify winter severity, we averaged tempera- screen cages in the Þeld. Cages were monitored every tures during the Þrst 90 Julian days of each year. The 2 to 3 d from mid-May through July for adult C. ad- coldest average temperatures were recorded in 1996 spersus. All adults were removed from each cage on with an average of Ϫ0.83ЊC, with the lowest site av- each sample date. The maximum and minimum tem- erage Ϫ2.13ЊC in Scotts Bluff County, NE. The warm- http://jee.oxfordjournals.org/ peratures were collected from the nearest National est average temperatures were recorded during 2004 Weather Service Station then used to calculate DD with an average temperature of 1.51ЊC across all sites and to develop C. adspersus emergence model 2. and the warmest site average 1.96ЊC in Sherman Model Fitting. C. adspersus emergence was assumed County, KS. to be either linear (uniformly distributed emergence) The earliest observations of C. adspersus pupae or nonlinear (Gaussian-distributed emergence). Data were on the 19 March 2004 sampling date at the Sher- were analyzed as the cumulative fraction of the total man County, KS, and Phillips County, CO, sites. The collected correlated to the number of accumulated Þrst adult occurrence within the stalk was on the 16

DD. Similar to the modeling techniques used for the April 2004 sampling date in Washington County, CO. by guest on March 16, 2016 pupal and adult eclosion stages, functions were Þt to The Þrst adult weevil emerged on 2 May 1997 at the C. adspersus emergence data using Proc Reg (SAS Washington County, CO, site at 410 DD. 2002Ð2003) and Proc Nlin (SAS 2002Ð2003). The forms The nonlinear, logistic model Þt the data better than of the linear and nonlinear functions were as de- the linear model for all phenological stages (pupation: scribed above. linear AIC ϭϪ296.6, nonlinear AIC ϭϪ415.0; adult: Because data collection methods differed, emer- linear AIC ϭϪ221.7, nonlinear AIC ϭϪ256.7; emer- gence model 1 and emergence model 2, were devel- gence model 1, linear AIC ϭϪ27.3, nonlinear AIC ϭ oped separately. That is, the lowest AIC values were Ϫ31.0; and emergence model 2: linear AIC ϭϪ1163.0, used to select either a linear or nonlinear model for nonlinear AIC ϭϪ1199.7). Because the nonlinear each data set independently (Akaike 1981, Burnham model AIC values were lower (i.e., better) in all cases, and Anderson 2002). Models selected from each data the nonlinear, logistic models were used to analyze set were then weighted based on the inverse of the results. mean squared error (MSE) between observed data Pupation and Adult Eclosion. The pupation of C. and model predictions. Weights were standardized to adspersus was suitably described using a Þtted logistic sum to 1. The two resulting weighted emergence mod- model (F-stat ϭ 2150.42; df ϭ 2, 93; P Ͻ 0.0001): els were then combined using a weighted average: Cumulative fraction (pupation) Combined emergence model ϭ 1/͑1 ϩ 49.6 * e(ϪDD * 0.0199)) ϭ ϩ EM1 * wEM1 EM2 * wEM2 The model predicts 5% pupation at 48 DD, 50% pu- where EM1 and EM2 are equal to emergence models pation at 197 DD, 75% pupation at 252 DD, and 90% at 307 DD (Fig. 1). 1 and 2, respectively. wEM1 and wEM2 are equal to the weights for emergence models 1 and 2, respectively. Adult C. adspersus eclosion was suitably described using a Þtted logistic model (F-stat ϭ 873.02; df ϭ 2, 59; P Ͻ 0.0001): Results Cumulative fraction (adult) Winter severity likely affects overwintering success and timing of C. adspersus phenology (Charlet 1989). ϭ 1/͑1 ϩ 701.2 * e(ϪDD * 0.0165)) April 2010 MERRILL ET AL.: DEGREE DAY MODELS FOR SUNFLOWER STEM WEEVIL 305

Emergence model 2 (F-stat ϭ 2557.59; df ϭ 2, 331; P Ͻ 0.0001) was suitably described by the logistic formula: Emergence model 2: cumulative fraction ϭ 1/͑1 ϩ 226.1 * e(ϪDD * 0.0091)) Emergence model 2 predicts 5% adult eclosion at 272 DD, 50% at 594 DD, 75% at 715 DD, and 90% at 835 DD (Fig. 3). Both models had similar MSE and therefore similar weights (emergence model 1Õs weight was 0.56 and emergence model 2Õs weight was 0.44). Using these weights, models were averaged. Combined model re- sults were as follows: 5% emergence occurred after 262 DD, 50% after 540 DD, 75% after 657 DD, and 90% emergence occurred after 777 DD. Fig. 2. C. adspersus eclosion. Logistic model of cumula- tive emergence of C. adspersus adults from pupae as a func- tion of accumulated DD (solid line) with 95% CL (dashed Discussion and Conclusions Downloaded from lines). Black circles are observed data. Understanding C. adspersus phenology in relation to weather will enhance IPM practices by enabling more The model predicts 5% adult eclosion at 218 DD, efÞcient scouting and better timed insecticide appli- 50% at 396 DD, 75% at 463 DD, and 90% at 529 DD cations. In addition, adult emergence prediction mod-

(Fig. 2). els will help us better understand the consequences of http://jee.oxfordjournals.org/ Adult Emergence From Stalks. Adult emergence altering sunßower planting dates. Prediction of other from stalks was modeled using Þtted logistic models. life stages of this pest may allow us to target potentially Emergence model 1 (F-stat ϭ 207.42; df ϭ 2, 20; P Ͻ vulnerable life stages with control tactics. 0.0001) was suitably described by the logistic for- It is a concern that emergence models 1 and 2 mula: estimated different peak emergence. Differences in methodology in data collection may explain some of Emergence model 1: cumulative fraction the differences in values obtained between these mod- Ϫ els. That is, Rogers and Serda (1982) reported that C. ϭ 1/͑1 ϩ 459.4 * e( DD * 0.0125)) adspersus adults chewed exit holes but then might by guest on March 16, 2016 Emergence model 1 predicts 5% adult eclosion at 255 remain in the stalk for several days or weeks until DD, 50% at 490 DD, 75% at 578 DD, and 90% at 666 DD emergence was prompted by a signiÞcant rainfall (Fig. 3). event. Differences may be due to differences in data

Fig. 3. C. adspersus emergence. Logistic models of cumulative emergence of C. adspersus adults from sunßower stalks as a function of accumulated DD depicting two models from different data sets (dashed lines) and the model-averaged values (solid line). Black circles are observed data. 306 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 103, no. 2 development methods. SpeciÞcally, data for emer- Armstrong, J. S. 1996. Development of a degree-day based gence model 1 were obtained by removing stalks prediction model for adult sunßower stem weevil, Cylin- from the Þeld and dissecting them, whereas emer- drocopturus adspersus, emergence, pp. 49Ð51. In Proceed- gence model 2 data were established by collection ings of the 18th Sunßower research Workshop, National of emerged (i.e., adults with exit holes cut Sunßower Association, 11Ð12 January 1996, Fargo, ND. but remaining within stalks were not considered Armstrong, J. S., and M. D. Koch. 1997. Predicting Sun- ßower stem weevil emergence on the Central Great emerged.) Disturbance caused by removal of the Plains using degree-days, pp. 107Ð109. In Proceedings of stalk (emergence model 1) from the Þeld may have the 19th Sunßower Research Workshop. National Sun- caused earlier emergence than would have been ßower Association, Bismarck, ND. observed otherwise. Therefore, emergence model 1 Bangsund, D. A., and F. L. Leistritz. 1995. Economic con- data may be slightly conservative. tribution of the United States sunßower industry, pp. Charlet (1987) estimated 90% emergence at 865 1Ð11. North Dakota State University Agricultural Eco- DD. Armstrong (1996) estimated 90% emergence at nomics Report 327-S. North Dakota State, Fargo, ND. 651 DD. A simple average of results from Charlet Barker, J. F. 1990. Sunßower trichome defenses avoided by (1987) and Armstrong (1996) suggests that 90% emer- a sunßower stem weevil, Cylindrocopturus adspersus Le- gence will occur when Ϸ758 DD have accumulated, conte (Coleoptera, Curculionidae). J. Kans. Entomol. Soc. 63: 638Ð641. which compares with the model-averaged result from Burnham, K. P., and D. R. Anderson. 2002. Model selection this study of 777 DD. Data within our models, as well and multimodel inference: the practical information the- as the work of Charlet (1987) and Armstrong (1996) oretic approach. Springer, New York. Downloaded from deal with populations from different locations (i.e., Charlet, L. D. 1983a. Distribution and abundance of a stem they may be distinct populations), which likely caused weevil, Cylindrocopturus adspersus (Coleoptera, Curcu- additional noise in the data. However, the relative lionidae), in cultivated sunßower in the Northern Plains. concurrence of these Þgures increases conÞdence in Environ. Entomol. 12: 1526Ð1528. our Þndings. Charlet, L. D. 1983b. Parasitiods of a stem weevil, Cylin- drocopturus adspersus (Coleoptera, Curculionidae), in Also, accurate emergence prediction models would http://jee.oxfordjournals.org/ have signiÞcant value in areas where it is possible to sunßower: incidence and parasitization rates in the Northern Great Plains. Environ. Entomol. 12: 888Ð890. manipulate planting date (Rogers and Serda 1982). Charlet, L. D. 1987. Emergence of the Sunßower stem wee- That is, timing the planting of the sunßower crop so vil, Cylindrocopturus adspersus (Coleoptera, Curculion- that plants are too small to be attractive for oviposition idae), relative to calendar date and degree-days in the when adults are active. In addition, in areas where Northern Great-Plains. J. Kans. Entomol. Soc. 60: 426Ð manipulation of planting times was not a viable alter- 432. native, predictions could be used to time scouting and Charlet, L. D. 1989. Overwintering survival of the Sun- insecticide applications. ßower stem weevil (Coleoptera, Curculionidae) and 2 of Scouting for C. adspersus should be initiated after its parasitoids in the northern Great Plains. J. Econ. En- by guest on March 16, 2016 262 DD have accumulated so that current chemical tomol. 82: 130Ð133. controls can target C. adspersus in the narrow window Charlet, L. D. 1994. Effect of stalk burial on emergence of sunßower stem weevil adults (Coleoptera, Curculion- between emergence of adults from stalks and before idae). J. Kans. Entomol. Soc. 67: 288Ð290. oviposition. Charlet, L. D. 1999. Biological control of sunßower pests: Future work should concentrate on development of searching for new parasitoids in native HelianthusÑchal- an increased understanding of the ecological factors lenges, constraints, and potential, pp. 91Ð112. In L. D. impacting the timing of C. adspersus oviposition. More Charlet and G. J. Brewer [eds.], Biological control of precise understanding of the factors inßuencing ovi- native or indigenous pests: challenges, constraints position timing will allow for improved timing of pes- and potential. Thomas Say Publications in Entomology, ticide applications. In addition, research toward the Entomological Society of America, Lanham, MD. development of more effective biological control, host Charlet, L. D., and C. Y. Oseto. 1983. Toxicity of insecticides plant resistance and improved planting techniques on a stem weevil, Cylindrocopturus adspersus (Co- could be highly beneÞcial toward controlling this leoptera, Curculionidae), and its parasitoids in sunßower. Environ. Entomol. 12: 959Ð960. damaging pest species. Charlet, L. D., C. Y. Oseto, and T. J. Gulya. 1985. Applica- tion of systemic insecticides at planting: effects on sun- ßower stem weevil (Coleoptera, Curculionidae) larvae Acknowledgments numbers, plant lodging, and seed yield in North Dakota. 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