Association of host plant growth and weed occurrence with armyworm (Mythimna separata) damage in corn fields

ZHANG Kun-peng , YU Zhi-hao JIANG Shi-xiong SUN De-wen, HUI Jun-tao, ZHENG Yu-liang, LI Xiao-zhen, WANG Xing-yun, WU Jun-xiang

1 State Key Laboratory of Crop Stress Biology in Arid Areas/College of Plant Protection, Northwest A&F University, Yangling 712100, P.R.China

2 Anyang Institute of Technology, Anyang 455000, P.R.China

3 Plant Protection and Quarantine Station of Sanyuan County, Sanyuan 713800, P.R.China

4 Plant Protection and Quarantine Station of Lantian County, Lantian 710500, P.R.China

5 Plant Protection and Quarantine Station of Chang’an District, Chang’an 710100, P.R.China

6 Zhengping Plant Protection Technology Limited Company, Xi’an 710000, P.R.China

1. Introduction

Mythimna separata (Walker), commonly known as the armyworm, is a seasonal long-distance migratory pest and a major risk to grain crops in China and other Asian countries as well as Australia (Li et al. 1964; Oku and Koyama 1976; Sharma and Davies 1983; Ye 1993; Drake and Gatehouse 1995). The armyworm has been listed the second in the top ten pests to be controlled in China since 1958 (Jiang et al. 2014). The armyworm has historically been a threat to grain crops such as corn, wheat and rice(Li et al. 1964; Ye 1993). According to the National Bureau of Statistics (2016), the average annual planting area of corn in 2000–2010 reached 26.8 million ha, an increase of 17% from the 1991–1999 annual average. Even more recently, the corn planting area topped at 38.11 million ha in 2015. Despite the ongoing battle with the armyworm pest, food losses continue to increase and production levels decrease. The characteristically long distance and wide range migratory flight of the armyworm creates vast damage over a large geographical range. This, combined with the monoculture planting structure, allows for tremendous pest pressure on the crop. Once an armyworm outbreak occurs,it often causes complete crop devastation. However, if crop plantings varied within a region and planting patterns are manifold, armyworm occurrence can be inhibited to some extent. It is important to note that armyworm emergence is continuous in some areas and always present on a small scale, especially in Shaanxi Province, China (Zhang et al.2012; Zeng et al. 2013; Song et al. 2015; Cheng and Zhao 2016). In early August 2013, a corn armyworm outbreak occurred in the Yanliang District, Shaanxi Province, and nearly ruined more than 5.33 ha of corn; in late August 2014, 6.67 ha of corn in Xingping City, Shaanxi Province was subjected to armyworm damage leading to reduced corn yields (Song et al. 2015). Even more recently, armyworm occurrence was serious in individual fields in Tongguan,Huaying, Pucheng, Chang’an, Lantian, Gaoling, Jingyang,Sanyuan counties or districts, including 16 towns, covering an area of 733.3 ha (Cheng et al. 2016). In 2012, the armyworm-damaged area alarmingly reached nearly 350 million ha, the highest damage severity recorded in the last decade (Zhang et al. 2012; Zeng et al. 2013).

Farmland weeds are an important component in farmland ecosystems. However, weeds can seriously endanger agricultural production and are closely associated to insect pest occurrence. It was found that the corn yield loss rate was positively correlated with field weed density, and the corn yield loss rate increased rapidly with increased weed density (Li et al. 2013). Additionally, it has been shown that grasses of the Gramineae family and other weeds may be beneficial to armyworm population growth (Li 1961). In corn fields specifically, armyworm occurrence has been shown to be closely related to corn and weed growth(Jiang et al. 2014). This research also demonstrated that poor field management and high weed density allowed for a more serious armyworm occurrence; in contrast, better field management with weed removal led to significant decreases in armyworm populations. This phenomenon was more obvious when the two corn field management types were adjacently planted (Song et al. 2015). Further investigation found that armyworm damage was more serious in weeds found in dry millet and corn fields, and armyworm incidence was less in weed-less field plots(Li et al. 2013). Unfortunately, these aforementioned investigations regarding the relationship between armyworm occurrence and weeds lacked systematic and detailed survey data. These studies could not be well replicated and failed to offer conclusive findings. Therefore, this study developed a quantifiable method to measure the relationship between armyworm and field weed occurrence.

In mid-August 2016, we investigated the relationship between the degree of armyworm damage and corn growth and weed occurrence in the Chang’an District, Lantian County and Sanyuan County, Shaanxi Province, where the armyworm occurrence was severe. Our aim was to provide a scientifically standardized guide to prevent armyworm occurrence and offer timely and effective control during heavy armyworm occurrence.

2. Materials and methods

2.1. Experimental site

Experimental fields were located in Shaanxi Province,China, where armyworm occurrence was severe in early August 2016. The fields were planted between June 9th and 12th with summer corn lines at a row spacing of 0.2–0.3 m×0.3–0.6 m. The corn sowing time had only 1–2 days difference within the same region. The previous crop in all fields was wheat. The survey was conducted in mid-August 2016, when corn plants were from silking to milk-ripe stage and had been heading. Experimental fields were in: 1)Manjianghong Village, Wangqu Town, Chang’an District(108°96´E, 34°06´N). The cultivated areas of corn were 88 ha in this village. Six fields were randomly surveyed,encompassing about 1.4 ha; 2) Xiaosi Village, Tangyu Town, Lantian County (109°16´E, 34°07´N). The corn planting areas were more than 40 ha. Four random fields were surveyed where armyworm infestations were serious.In total, this encompassed about 0.93 ha; 3) Qiyi Village,Chengguan Town, Sanyuan County (108°95´E, 34°61´N).Corn planting areas were more than 267 ha. Four random fields were surveyed where armyworm infestations were serious. In total, this encompassed about 1.13 ha. At the same time, we surveyed the amount of weeds. Overall,14 corn fields were surveyed in three different regions comprising approximately 3.46 ha.

2.2. Survey of corn damage

A total of ten sampling points were taken in each field,and five plants were sampled per two randomly selected rows. A total of 100 plants were investigated by recording plant damage, leaf numbers and severity in each field. We referred to a grading index for armyworm damage to record the percentage of the leaves fed on by armyworms, and subsequently, the plant damage severity was calculated(NATESC 2006).

2.3. Survey of corn growth

Ten sampling points were taken from each field, and one plant was sampled from two randomly selected rows. Five plants were investigated by recording plant height and stem diameter 0.8 m away from the ground. For each plant, vigor was expressed using the product of plant height and stem diameter (Yang et al. 2014).

2.4. Survey of field weed occurrence

Initially, weed species were identified and species identification was accomplished by referring to the most recent edition of Weeds in Tobacco Field for Shaanxi(Cheng and Wu 2014). A total of ten sampling points were taken in each field. In a 0.5 m×0.5 m sampling plot, weed coverage degree was recorded. Subsequently, all weeds were collected, dried to a constant weight and final weight was calculated.

2.5. Data analysis

All the measured parameters reflected the armyworm damage to corn. The damaged plants measurement reflected the armyworm-damaged corn in the cumulative field samples. The damaged leaves measurement reflected the armyworm-damaged corn for each individual corn plant. The severity of damaged plants reflected the armyworm-damaged corn in every sampled field.

Damaged plants (%)=Damaged plant numbers/Surveyed plant numbers×100 (1)

Damaged leaves (%)=Damaged leaf numbers/Total leaf numbers of surveyed plants×100 (2)

Severity of damaged plants (%)=Sum of each plant damage severity value/Surveyed plant numbers×100 (3)

All data were analyzed in Microsoft Excel 2010 (Microsoft Corporation, Redmond, WA, USA). Statistical analysis of data was performed using the SPSS17.0 Software.Significant differences were analyzed using Duncan’s new multiple range method. Different lowercase letters denote significant differences at 0.05 level.

3. Results

3.1. Corn field damage caused by the armyworm

In each of the three different areas, the percent of damaged plants was 100% in all of the fields investigated (Table 1).The proportion of damaged leaves was 44.1–99.2% in Manjianghong Village. The Xiaosi Village had less leaf damage with proportions ranging from 30 to 70.9%. The Qiyi Village showed an intermediate proportion of leaf damage at 30.6–84.3%. The damage severity was 50.1–99.3% in Manjianghong Village, where the degree of damage was the most serious. The damage degree was lesser in Xiaosi Village at 30.7–81.2%, while the damage was intermediate at the other two locations in Qiyi Village ranging between 44.5–85.2%. The damage status reflected by both the damaged leaves and the damage severity showed that the corn damage degree was different in the three regions surveyed.

3.2. Relationship between the damage degree caused by the armyworm and corn vigor

In all fields, plant height was relatively short, stem diameter was relatively small, leaf blades were narrow, and the damage degree caused by the armyworms was serious.These factors all had a large influence on subsequent growth and yield formation. Statistical analysis showed that both the proportion of damaged leaves and the damage severity had a significant relationship with plant height,stem diameter and vigor, with very few exceptions. This was especially apparent in the correlation between either leaf damage proportion or damage severity and either the stem diameter or vigor (Fig. 1 and Table 2). The results were consistent with the relationship between the damage degree in the appearance of the corn and the plant vigor.In other words, fields with higher vigor were subjected to decreased losses, or on the contrary, poor vigor resulted in greater losses. Thus, reducing armyworm damage can promote the healthy growth of corn.

Table 1 Corn field damage caused by the armyworm

3.3. Relationship between the damage degree caused by armyworms and weed occurrence

Fig. 1 Corn growth in the fields investigated in this study. Values are means±SD.

Table 2 Relationship between damage severity caused by armyworms and corn growth

Survey results showed that weed species compositions were different among fields. This was apparent in the occurrence of the dominant weed species in the investigated fields. For example, the dominant weed species was Digitaria sanguinalis in Manjianghong Village, Wangqu Town, Chang’an District and Qiyi Village, Chengguan Town,Sanyuan County, whereas the dominant weed species was Chloris virgata in Xiaosi Village, Tangyu Town, and Lantian County. Both D. sanguinalis and C. virgata were dominant weed species in their respective regions in corn field ecosystems. Their coverage and biomass accounted for more than 96% of all weeds surveyed in their regions. A few other weed species including Setaria viridis, Chenopodium glaucum, Amaranthus ascendens, Convolvulus arvensis,Cirsum arvense var. integrifolium, and Abutilon theophrasti were seen sporadically.

Weed occurrence conditions were variable among the different corn fields (Table 3). Both weed coverage and biomass showed significant differences between different corn fields. Statistical analysis showed a significant correlation between the proportion of damaged leaves or damage severity and the weed coverage or biomass in corn fields (Table 4). In other words, the greater the grass coverage and biomass, the more serious the armyworm damage. The three different regions surveyed were analyzed cumulatively and the results showed that the correlation between the damaged leaf proportion or the damage severity and the grass coverage or biomass in corn fields was more significant than when fields were analyzed individually (Table 4). The general trend showed that clearing weeds in a timely manner plays an important role in reducing the occurrence of armyworm-induced corn destruction.

4. Discussion

The armyworm, M. separata (Walker), has historically been responsible for threatening the production of corn, wheat and rice globally (Li et al. 1964; Oku and Koyama 1976; Sharmaand Davies 1983; Ye et al. 1993; Drake and Gatehouse 1995). This pest is widespread, covering entire regions at high-density with banded distribution and the ensuing damage results in concentrated losses. In this study,we surveyed the degree of damage caused by the corn armyworm in fourteen corn fields in three different regions in Shaanxi Province. It was found that the damage degree caused by the corn armyworm in summer corn planted within the same regions showed significant differences between fields. This finding was consistent with Jiang et al. (2014).

Table 3 Weed occurrence in the investigated corn fields

Table 4 Relationship between damage severity caused by armyworms and weed occurrence

Both the proportion of damaged leaves and the damage severity showed a relationship with plant height, stem diameter and vigor. This was especially seen in the relationship between stem diameter and vigor. However,whether corn vigor was high or low or whether weeds were present or not did not determine armyworm occurrence,but conversely, the degree of armyworm damage did have an impact on corn growth or weed frequency. These research results were not in line with previously reported conclusions suggesting that close plantings, excess fertilizer, high irrigation and highly vigorous growth may allow for severe armyworm damage (Wu 2011). There was also a significant relationship between the proportion of damaged leaves or damage severity and weed coverage or biomass in the corn fields. That is, the greater the weed coverage or biomass, the more serious damage the corn withstood from the armyworm. It follows that corn vigor and weed occurrence were the main factors affecting armyworm damage severity. However, it was seen that the correlation between weed occurrence and the degree of corn armyworm damage was larger than when compared to the corn vigor. Previous studies have also shown that the farmland microclimate has a large impact on armyworm occurrence. In a 2014 survey, it was found that one of the common characteristics of serious armyworm occurrence in the Yanliang District area of Shaanxi Province and in the local area of Xingping City is high weed density in the fields,especially the incidence of grass family weeds such as D. sanguinalis (Song et al. 2015). Another recent survey in Hebei Province from 2012 to 2013 found that high weed density in corn fields are associated with local outbreaks of the armyworm (Zhang et al. 2014). This meant that the higher density of D. sanguinalis in corn fields, the heavier damage to corn plants by the armyworms. Farmland weeds also provide rich feeding conditions for the armyworm(Cheng et al. 2016). High weed vegetation strongly selects for armyworm oviposition, where the mature insects tend to oviposit in yellow curled plant leaves (Yin et al. 2007) and on grass weeds (Kanda Ken-ichi 1986; Jiang et al. 2014).It was found that the hatching larvae are mostly distributed in field weeds and poorly managed plots in protected spots where they are more hidden and difficult to see. When late instar larvae enter the overeating period, they transfer to the corn and begin eating leaves and spikes (Zhang et al. 2012).With cultivation system adjustments aiming to maximize production and field utility, corn planting areas are growing and close-planted varieties are becoming extensively cultivated. However, the abundant precipitation in China during July allows the crops and weeds to grow well, but form a protected field canopy that provides adequate food and shelter for armyworm larvae (Zhang et al. 2012).

Armyworm outbreak characteristics include vast occurrence ranges, high-density weeds regions and severe damage to corn. With the numerous recent reports of severe armyworm damage, control measures are key in corn production. However, armyworm prevention and control are a systematic and complex procedure that must take into account multiple parameters. We explored some associations between armyworm damage severity and field conditions. This study elucidated the correlation between armyworm occurrence and corn growth and weed abundance. When corn growth was stunted, the armyworm damage was heavy. Oppositely, when corn grew well, armyworm harm was the minimal. We also found a relationship between the armyworm and the weed occurrence through a quantifiable measurement of factors in the corn field reflecting armyworm damage. Corn growth and weed frequency had an impact on the degree of armyworm damage. Thus, these measures offer a guide to the appropriate amount of weed control in corn fields and demonstrate an easily reproducible approach to calculating armyworm damage.

5. Conclusion

In this study, we clarified the association between armyworm(Mythimna separata) damage level and the corn growth and weed occurrence. Significant correlations were found between corn plant height, stem diameter and vigor as well as weed coverage and biomass analyzed. In a word, at stunted corn growth and presence of plenty of weeds, the armyworm damage tended to be heavy; oppositely, when corn grew well and weed density were low, armyworm harm was the minimal. Corn growing status and weed density can significantly affect armyworm damage level. Therefore,promoting corn growth and timely removal of weeds are conducive to reducing armyworm occurrence.

Acknowledgements

This work was supported by the National Public Welfare Industry (Agriculture) Scientific Research of China(201403031), the National Key Research and Development Program of China (2017YFD0201807) and the Project of Agricultural Science and Technology Innovation Transformation in Shaanxi Province, China.

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