Validation of a model simulating the feeding effects of the potato leafhopper (Empoasca fabae) on potato growth

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Validation of a model simulating the feeding effects of the potato leafhopper (Empoascafabae) on potato growth K. B. Johnson* and E. B. Radcliffe ~ *Department of Botany and Plant Pathology, Oregon State University, Corvallis, 97331-2902, and t Department of Entomology, University of Minnesota, St Paul, 55108, USA

Abstract

Keywords

A model that simulates feeding effects of potato leafhopper nymphs on potato growth and yield was modified and then validated with independent field data. The principal modification was to implement a response function to estimate accurately the feeding intensity of nymphs within individual leafage-classes from a count of nymph populations on leaves located in the middle of the canopy. The response function was developed from field observations and included both linear and quadratic terms for leaf and crop physiological age as well as an interaction term. Growth, yield and hopperburn responses of the model to high and intermediate nymphal infestations were similar to responses observed in the field. Analysis with the model indicated that potatoes are most sensitive to potato leafhopper-induced yield reductions from full bloom until 2 weeks before harvest, but are relatively insensitive for the first month after plant emergence. Potato leafhopper; crop damage; crop growth simulation; population dynamics

Introduction The potato leaf hopper, Empoascafizbae(Harris) (PLH), is a serious pest of potato in the north central United States (Radcliffe, 1982). While feeding, this insect injects a toxin into leaves that dramatically reduces net photosynthesis (Ladd and Rawlins, 1965). Prolonged feeding results in a marginal necrosis of leaves, termed hopperburn (Peterson and Granovsky, 1950). The nymphal stage of the potato leaf hopper is the most damaging (Prasad, 1961; Ladd and Rawlins, 1965) and can be sampled efficiently to determine the need for insecticide applications (Cancelado and Radcliffe, 1979; Walgenbach and Wyman, 1985). Potato leaf hopper adults show an ovipositional preference for leaf tissue of intermediate age and, consequently, nymphs are concentrated on that tissue (Miller and Hibbs, 1963). Several field studies have correlated yield loss with the number of nymphs on leaves sampled from the middle of the potato canopy (Cancelado and Radcliffe, 1979; Walgenbach and Wyman, 1984). Counts of PLH nymphs on midplant leaves have proved to be a practical method for determining their relative abundance within a potato crop. However, because action threshold densities for this pest are very low, it has been difficult to establish field experiments to ascertain the dynamics of the yield-loss function as influenced by PLH infestation size, crop age and environment (Walgenbach and Wyman, 1985). As an alternative to field experiments, an approach to this problem was developed that uses a dynamic potato growth simulation model (Johnson, Teng *To whom correspondenceshould be addressed 0261-2194•91/05/0416- 07 1991 13utterworth-HeinemannLtd

and Radcliffe, 1987). This model requires daily inputs of weather and densities of PLH nymphs on midplant leaves. Within the model, density of nymphs on midplant leaves is multiplied by a leaf-age-dependent distribution to provide an estimate of nymphal feeding intensity on all age-classes of leaf tissue. Nymphal feeding within a leaf age-class quantitatively reduces the net photosynthetic rate of that age-class. Hopperburn develops after a threshold of feeding activity has been surpassed, reducing the amount of green leaf area at a rate proportional to the size of a leaf age-class and the feeding intensity. Model output includes temporal estimates o f leaf area index, tuber yield and the percentage of leaf area with hopperburn. Comparison with field data showed that this model provided realistic yieldloss estimates over a relatively wide range of PLH infestations (Johnson, Teng and Radcliffe, 1987). Analysis of the model indicated that the yield-toss function was influenced by stage of crop development when the insects were feeding and by environmental conditions (e.g. moisture stress). The initial model (Johnson, Teng and Radcliffe, 1987) made the assumption that the relative feeding intensity oF PLH nymphs on different age-classes of potato leaf tissue could be estimated from a fixed distribution that had a maximum at an intermediate leafage and two minima that occurred on the youngest and oldest leaf age-classes. Subsequently, it was confirmed that nymphs are concentrated on leaves of intermediate age (Johnson, Watrin and Radcliffe, 1988a), but that the ratio o f overall,nymph density per potato leaf to nymph density per midplant leaf increased linearly with crop age. The purpose of this study was threefold: first, to describe

Potato leafhopper damage model: K. B. Johnson and E. B. Radcliffe 417 more accurately the influence of potato leaf size, and ofleaf and crop age, on relative PLH nymph-feeding intensity; second, to incorporate this distribution of relative nymphfeeding intensity into the potato growth model and to validate the output of the modified model against independent field data; third, to use the model to investigate the dynamics of PLH nymph-induced yield loss as influenced by age of the crop when the insects are present.

Materials and methods

Experimental design Field measurements of potato growth and potato leafhopper density were made within plots of potato, cv. Russet Burbank, established on 22 May 1986. Plots were located on a Waukegan silt loam soil at the University of Minnesota's Rosemount Agricultural Experiment Station. Experimental design was a randomized complete block with four replications of four PLH infestations: early, intermediate, late and control. Individual plots measured 10.4m long by six 1.0m rows wide. Alfalfa, Medicago sativa L., was interplanted in 10m strips between the potato plots to enhance PLH populations (Cancelado and Radcliffe, 1979). Within the potatoes, the four PLH infestations were created by differentially varying the rate and date of application of the insecticide methoxychlor. Details of the methoxychlor applications as well as description of cultural practices have been published previously (Johnson et aL, 1988a,b). Nymph population size PLH nymphal populations were monitored in each plot every 3-13 days (usually once a week) by removing 30 midplant leaves and counting the number of nymphs on these leaves. For days without nymph counts, midplant leaf populations were estimated by linear interpolation of the observed values. Nymph density per 30 midplant leaves was retained as the basic PLH unit because this sample size is commonly used by pest management scouts. Relative nymph feeding intensity Relative nymph feeding intensity was defined as the ratio of relative nymph density on a leaf to relative leaf size. The relative feeding intensity of PLH nymphs on different age-classes of potato leaves was determined on five dates during the growing season: 16 and 31 July, I 1 and 27 August and 12 September. These dates corresponded to the cumulative crop physiological ages (PA) [i.e. P-days (Sands, Hackett and Nix, 1979)] of 290, 401,492, 627 and 727, respectively. Relative nymph density on leaves at different positions along potato stems was calculated from data on the vertical distributions of PLH nymphs collected from within these same field plots (Johnson et al., 1988a). For each PLH infestation and each sampling date, a relative nymph density for each leaf position was determined by dividing the mean nymph density at a leaf

position by the maximum nymph density observed. Relative leaf size for each position along the potato stem was determined by dividing the mean dry weight ofeach leaf by the largest mean leaf dry weight observed across all sampling dates. Leaves for this purpose were sampled only from control plots. Dry weights of mainstem leaves and branch leaves were recorded separately. Sample size comprised 12 mainstems and 12 branches on each date (three from each plot of the control infestation). The PA values of individual leaves were monitored by tagging weekly the day's date on the top leaf of four plants in each control plot. At the same time, the position on the stem of previously tagged leaves was noted. Potato growth data Plants were sampled every 2 weeks throughout the season to determine crop biomass. On each sample date, four hills were harvested from the second row of each plot and separated into tubers, stems, and leaves that were either wholly or partially green. Roots and chlorotic leaves were discarded. Fresh weights of tubers were recorded immediately and dry weights of leaves and stems were recorded after desiccation at 600C for 1-2 weeks. On 16 September, final tuber yields were measured by harvesting the fourth and fifth row of each plot. Estimates of leaf biomass were transformed to leaf area index and tuber dry weights were estimated from the fresh-weight values with procedures previously described (Johnson et al., 1988b). Plant growth data were not collected from the late PLH infestation. Estimates of percentage of leaf area with hopperburn were made in each plot on 4 and 18 August and 2 and 10 September with the aid of standard area diagrams (Granovsky and Peterson, 1954). These estimates were used to adjust the green-leaf biomass samples to the amount actually green. Model validation Forward and backward stepwise multiple regression ('REG Procedure, SAS Institute Inc., Cary, NC) was used to compute a polynomial response function for relative nymph-feeding intensity dependent on individual leaf and cumulative crop PA. The chosen response function was included in the growth model and the output compared with the non-modified model and 1985 field data previously published (Johnson, Teng and Radcliffe, 1987). During this process, the model parameter that determines the threshold of nymph-feeding activity at which hopperburn begins to develop (i.e. kth) was increased from 3.0 to 4.0. The other parameters, which regulate effects of nymphal feeding on the net crop growth rate (kpr) and the rate of hopperburn development (khb), retained their original values of 0.0009 and 0.03, respectively. The modified model was then compared in graphical arrays with the 1986 potato growth and hopperburn data described above. Analysis of variance (ANOVA Procedure, SAS Institute Inc., Cary, NC) was used to determine if significant differences (p = 0.05) occurred among responses from the observed (field) PLH infestations.

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Yield sensitivity

Sensitivity of final tuber yield to increasing PLH nymph infestations that started at different stages of crop development was evaluated in a simulation analysis. Nymphal infestations were initiated every 50 physiological age-units from crop PA 150 to 650. Initial populations were either one, two or three nymphs per 30 midplant leaves and increased daily by the number of nymphs present on the first day of the infestation. The populations increased until final tuber yields were reduced by either 1 or 2% of the non-infested control. The terminal population of nymphs just before the yield-loss criteria were achieved were recorded as the dependent variables. Environmental data from 1986 were used in the analysis. Approximately 900 simulation runs were completed. Results

Relative nymph feeding intensity

Relative nymph density and leafdry weight by leafposition on the first and fourth sampling dates (16 July and 27 August, respectively) summarize shifts made by these variables during the growing season (Figure 1). On 16 July, maximum relative nymph density occurred on the leaf

positioned tenth from the top of the plant and very few nymphs were found near the top of the canopy (leaf positions 1-5). The largest mean leaf dry weight was observed on this date. Mainstem leaves, which wer(; larger than branch leaves, made up about one-half of the crop canopy (Johnson et aL, 1988a). On 27 August, maximum relative nymph density occured at leaf position 6 or 7, and compared with 16 July, a greater proportion of nymphs were located near the top of the canopy. Mainstem leaves averaged less than half the weight of leaves sampled on 16 July and the ratio of mainstem leaves to branch leaves had declined to 0.26. Maximum values of relative nymph feeding intensity (i.e. the ratio of relative nymph density to relative leaf size) increased with each sampling date (Figure 2). Regression of relative nymph feeding intensity on individual leaf and cumulative crop PA resulted in significant (p < 0.05) main effects and interaction term (Table1, Figure3). The nonsignificant quadratic term for leaf PA was included in the response function because, in separate analyses, this term was significant on 16 and 31 July. The final response function also included the non-significant quadratic term for crop PA because its omission resulted in peak values of relative nymph feeding intensity substantially < 1.0 in the crop PA range of 100-250.

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Compared with the initial model (Johnson et al., 1987), modifying the distribution of nymph feeding intensity delayed the onset of hopperburn about 50PA units (Figure4a). The modification also altered temporal development of hopperburn to a pattern very similar to that observed in the 1985 field plots. Similarly, the modified distribution delayed the initial decline in the rate of tuber bulking compared with the unmodified model, but the reduction of tuber growth rate was greater at later stages of crop development (Figure 4b ). These responses Of the modified model were more consistent with the timing and magnitude of differences observed between the PLHinfested and uninfested field plots. The final tuber-yield reduction predicted by the modified version of the model was greater than that of the unmodified version (0.204 and 0.180kgm -2, respectively) but slightly less thar~ the observed field response [0.210 k g m - 2 4-standard error (s.e.) of 0.046]. Modification of the distribution of nymph feeding intensity did not greatly alter the modelled stem dry-weight accumulation and leaf dry-weight accumulation and attrition. In 1986, the methoxychlor applications resulted in significant differences in nymph populations between PLH infestations (Figure 5a). Observed green leaf area, stem dry weight and tuber dry weight showed late-season reductions consistent with differences in nymph density among PLH treatments (Figure 5b,c,d). Final measures ofleafarea index and tuber dry weight all differed significantly among treatments. Hopperburn developed to a maximum of 28% in the early infestation, 18% in the intermediate infestation, and did not develop in the control plots (FigureSe); differences between these percentages were significant (p = 0.01). The timing and magnitude of model responses to the 1986 P L H nymph populations' were generally similar to the field observations. Differences in modelled green leaf area, stem dry weight and tuber dry

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Table 1. Stepwise multiple regression a of the calculated relative potato leafhopper nymph feeding intensity on individual potato leaves (cv. Russet Burbank) on the independent variables of leaf and crop physiological age Variable

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Physiologicalage of crop (units) Figure4. a, Comparison of predicted foliar hopperburn development by modified (. . . . ) and unmodified ( . . . . . ) versions of a model that simulates the feeding effects of potato leafhopper nymphs on potato growth to actual 1985 field observations (AI; I , observations from the control plots, b, Comparison of predicted tuber dry-matter accumulation by modified (. . . . ) and unmodified ( . . . . . ) versions of a model that simulates the feeding effects of potato leafhopper nymphs on potato growth to actual 1985 field observations ( A ) ; - - , modelled controls; I , observed controls

weight began to develop at a cumulative crop P A of 350. Between the early PLH and control infestations, modelled green leaf area index was reduced by about 0.7 after midseason (Figure5c). Modelled stern dry weight was reduced by a maximum of 10 g m - 2, which was slightly less than was consistently observed in the field plot (Figure5d). Final modelled tuber dry weight was reduced by 0.198 and 0.406kgm-2 in the intermediate and early infestations, respectively. For the intermediate infestation, this compares with observed reductions (+s.e.) of

The number ofnymphs required to reduce final tuber yield by either 1 or 2% varied according to the stage of crop development when the PLH infestation began, and to how rapidly an infestation increased (Figure 6). Maximum yield losses occurred during the period of crop growth between PA values 400 and 600. When PLH populations increased by one nymph per day, a terminal population of eight nymphs per 30 midplant leaves was sufficient to reduce yield by 1% at the crop PA of 442. Faster rates ofincrease in the nymphal population slightly increased the number of nymphs required to meet the .yield loss criteria. Infestations of PLH that began at early stages of crop development (crop PA range of 100-250) had little effect on final tuber yield (Figure 6). Between crop PA values of 350 and 400, however, yield loss sensitivity increased dramatically. At late stages of crop development (crop PA values> 700), yield loss sensitivity to PLH feeding diminished.

Discussion

The major biological factors affecting the distribution of relative nymph feeding intensity within age-classes of potato leaf tissue were the ovipositional preference shown by PLH adults for leaves of intermediate age (Miller and Hibbs, 1963), differential leaf size at various positions within the canopy (Figure1), diminished leaf size with increasing crop age (Figure 1), and differential rates of leaf area expansion at different times of the season (Johnson et aL, 1988a). For all sampling dates, maximum leaf size was observed on leaves of intermediate age. On a per unit dry weight basis, this observation slightly diminishes the importance of the site preference of PLH adults for

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oviposition. Rate of leaf area expansion regulates the proportion of the canopy that is older than the incubation period required for PLH eggs to hatch [about 10-14 days (Simonet and Pienkowski, 1980)]. Early in the season (crop Pa range of 100-300), potato leaves emerge at a rate of 4-5 per stem or branch per week, but, by the time the crop is two-thirds mature (crop Pa range of 500-600), this rate declines to < 1. Thus, a declining rate of new leaf emergence, combined with interactions involving leaf size, broadens the distribution of nymphs within the canopy and increases PLH nymph feeding intensity in all leaf age-classes as the crop season progresses. Inclusion of the response function for relative PLH nymph feeding intensity into the potato growth model resulted in improved descriptions of the timing and magnitude of tuber yield reductions and hopperburn development (Figure4). In the initial model (Johnson et at., 1987), it was hypothesized that imprecise prediction of hopperburn could have resulted from an inaccurate description of nymphal distribution on foliage or from physiological interactions between the insect toxin and crop canopy age. Although such physiological interactions may still occur, the present results suggest that the effects of these interactions are relatively small compared with nymphal distribution. The data set used for validation was of high quality and generally indicated that the model provides reasonable

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Figure& a, Observed potato leafhopper nymph populations; modelled (lines) and observed (points) values of (b) leaf area index, (c) stem dry weight (d), tuber dry weight, and (e) percentage hopperburn, from an early leafhopper infestation ( . . . . . ; O), an intermediate infestation ( - - - - ; A ) , and uninfested ( - - ; I ) cv. Russet Burbank potatoes grown in 1986. Vertical lines two standard errors of the m e a n in length were drawn through points when aH observations on the same date differed significantly (p<0.05)

422

Potato leafhopper damage model: K. B. Johnson and E. B. Radcliffe

predictions of the effects of PLH feeding on potato growth (Figttre5). Oversensitivity of modelled yield to very high PLH populations was initially a problem (Johnson et al., 1987), and based on the results presented here from the early infestation, may require some readjustment of the model's parameters. Nevertheless, the objective of this model was to predict dynamics of PLH-induced yield losses caused by relatively low PLH populations that are near the economic threshold. In this regard, the authors are of the opinion that the results obtained with intermediate PLH populations are sufficiently precise to provide reasonable confidence in the extrapolative simulations that this objective requires. The yield loss criteria of 1 and 2% chosen for the simulation analysis roughly represent current economic thresholds for PLH within Minnesota-grown irrigated and dryland potatoes, respectively. Published PLH action thresholds developed from field experiments range from 3 to 20 nymphs per 30 midplant leaves (Cancelado and Radcliffe, 1979; Walgenbach and Wyman, 1984). The results reported here generally support these previous observations, although recognizing that other factors which were not examined (temperature, size of the adult population, plant nutrition) can influence the exact action threshold population. The most important result ofthis study is the finding that potatoes show a relatively constant sensitivity to PLH damage in the period of development from about full bloom to 2 weeks before harvest. Walgenbach and Wyman (1985) demonstrated a similar period of sensitivity to PLH feeding, by measurement of photosynthesis within caged potato field plots. The timing of maximum yield loss sensitivity to PLH occurs at a later stage ofgrowth and tbr a longer period of time than yield reductions caused by defoliation (Shields and Wyman, 1984; Johnson et al., 1988b). The crop- and leaf-age dependent distribution of relative nymph feeding intensity is the reason for this difference.

Granovsky, A. A. and Peterson, A. G. (1954) Evaluation of potato leaf injury caused by leafhoppers, flea beetles, and early blight. J. Econ. EntomoL 47, 894-902 Johnson, K. B., Teng, P. S. and Radcliffe, E. B. (1987) Coupling the feeding effects of potato leafhopper, Empoascafabae, nymphs to model of potato growth. Enrir. EntomoL 16, 250-258 Johnson, K. B., Watrln, C. G. and Radcliffe, E. B. (1988a) Vertical distributions of potato leafhopper nymphs on potatoes relative to leaf position, plant age, and population size. J. Econ. EntomoL 81,304-309 Johnson, K. B., Conlon, R. L., Adams, S. S., Nelson, D. C., Rouse, D. I. and Teng, P. S. (1988b) Validation of a simple potato growth model in the north central United States. Am. Potato J. 65, 27-44 Ladd, T. L. and Ranlins, W. A. (1965) The effects of the feeding of the potato leafhopper on photosynthesis and respiration in the potato plant. J. Econ. Entonwl. 58, 623-628 Miller, R. L. and Hibbs, E. T. (1963) Distribution of eggs of the potato leafhopper, Empoasca fabae, on Solalmm plants. Ann. Ent. Soc. Ant. 56, 737-740 Peterson, A. G. and Granovsky, A. A. (1950) Relation of Empoascafabae to hopperburn and yields of potatoes. J. Econ. EntomoL 43, 484-487 Prasad, S. K. (196 I) Quantitative estimation of damage to potato caused by potato leafhopper, Empoascafabae. hzdian Potato J. 3, 105-107 Radcliffe, E. B. (1982) Insect pests of potato. A. Rev. EntomoL 27, 173-204 Sands, P. J., llackett, C. and Nix, H. A. (1979) A model of the development and bulking of potatoes. I. Derivation from well-managed field crops. Field Crops Res. 2, 309-331 Shields, E. J. and Wyman, J. A. (1984) Effect of defoliation at specific growth stages on potato yields. J. Econ. EntomoL 77, 1194-1199 Simonet, D. E. and Pienkonski, R. L. (1980) Temperature effect on development and morphometrics of the potato leafhopper. Envir. Entomol. 9, 798-800 Walgenbach, J. F. and Wyman, J. A. (1984) Dynamic action threshold levels for the potato leafhopper on potatoes in Wisconsin. J. Econ. Entomol. 77, 1335-1340 Walgenhach, J. F. and Wyman, J. A. (1985) Potato leafhopper feeding damage at various potato growlh stages. J. Econ. EntomoL 78, 671-675

References Cancelado, R. E. and Radcliffe, E. B. (1979) Action thresholds for potato leafhopper on potatoes in Minnesota. J. Econ. EntomoL 72, 566-569

Received 7 July 1990 Revised 28 February 1991 Accepted 1! March 1991