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Forecasting of root damage, plant lodging and yield loss caused by WCR larval feeding based on larval population density

Various authors in USA investigated the forecasting of root damage, plant lodging and yield loss caused by western corn rootworm (WCR) larvae. Depending on the year, WCR larvae are causing damages in Croatia and in Central Europe at different intensity. The total damage is the result of various factors out of which the most important are the WCR larval population and the weather conditions in given year at particular locality. For better understanding how larval population and weather conditions are influencing the damage, the investigations in the conditions of higher attack of WCR should be done. Overall increase of WCR population in Croatia and in Central Europe enabled us to conduct investigation under the condition of natural WCR infestation. The aim of the investigation was to develop a model for damage forecast based on established WCR larval population density. Therefore we investigated the relationship between WCR larval population which was established at the time of maximal larval attack and subsequent root damage, plant lodging and yield loss. The investigation was conducted in three regions of Croatia during three field seasons (2007-2009). In each season five continuous maize fields in each region (i.e., 15 fields per year) were involved. The WCR larval population was established in June at the Julian day (JD) 170 (in 2007), 163 (in 2008) and 167 (in 2009). Ten plants in each of four randomly selected rows in the field were dug (i.e., 40 plants per field) to assess the average number of WCR larvae per plant. By using the same methodology 40 roots per field were dug and scores on the Node Injury Scale (NIS, 0-3) were recorded at JD 203 (in 2007 and in 2009) and JD 206 (in 2008). Plant lodging was estimated at the JD 261 (in 2007), 267 (in 2008) and 265 (in 2009)). Lodging was measured using 100 plants in 5 randomly selected rows in each field (i.e., 500 plants per field). The plants were grouped as follows: upright, partially and fully lodged. In each fields from each grouping, three samples containing ten plants were harvested. The average yield was determined for each category and based on the ratio of each category in a particular field the average yield was calculated. The yield loss was calculated by comparing obtained yields with the yields of upright plants in each field. All relationships were provided by using linear regression model which involved interaction with year as an indicator of weather conditions. Significant interactions between WCR larval population and year for forecasting root damage rate were determined. The year resulting with the steepest slope was chosen for establishing damage threshold. The infestation with 1.08 WCR larvae per plant predicts an average of 0.75 node injury score. Significant interactions between the WCR larval population and year for relationship between WCR larval population and partially and fully lodged plants were not determined. Fully lodged plants are not possible to forecast according WCR larval population. An average of 10.9% of yield reduction could be forecasted if we established 1 WCR larva per plant, weather conditions did not affected the relationship significantly. A significant interactions between the year and upright plants in the field were determined if we used root damage rate as a forecast tool. An average of 10 % of partially lodged and an average of 10% fully lodged plants could be forecasted if the root damage rate of 0.62 and 2.14 were established, respectively. Weather had low impact on the relationship between root damage rate and yield, years had similar weather conditions. An average of 17.5% of yield reduction could be forecasted if the root damage rate of 0.75 was established. Results from this investigation are important for better understanding relationship between the final damage caused by WCR larval population and weather conditions. Therefore results could be used in creating guidelines for integrated maize production.

Croatia; interactions; larval infestation; plant lodging; root damage rate (NIS 0-3); yield loss; weather conditions; western corn rootworm

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