Abstract

Various studies have been conducted on the mechanisms of host plant resistance (HPR) of cabbage against the diamondback moth (DBM). Understanding the role of wax-bloom has been a major focus of these studies. Our study distinguishes itself from previous research by focusing specifically on outer leaf angle and leaf hardness. The resistance mechanism of cabbage cultivar “Shinsei” against DBM was examined. Generally outer leaves of “Shinsei” cabbage have an erect position when compared with a common cultivar, “Kinkei 201.” Lethal effects of rainfall on II instar larvae on the outer leaves of “Shinsei” and “Kinkei 201” were investigated with an artificial rainfall system. The results showed that mortality of II instar larvae on the outer leaves of “Shinsei” was higher than that on outer leaves of “Kinkei 201.” Compared with II instar larvae on prone outer leaves, II instar larvae on erect outer leaves were killed more frequently by artificial rainfall. Tests showed that experimental results between erect and prone leaves were similar in both cultivars. First instar larvae took longer to mine into the outer leaves of “Shinsei” than into those of “Kinkei 201” and the probability of success in mining into the leaf was lower on “Shinsei” than on “Kinkei 201.” Artificial diets mixed with methanol extracts from outer leaves of each cultivar were fed to I instar larvae. However, there were no significant differences between mining rates on both artificial diets. On the other hand, outer leaves of “Shinsei” were harder than outer leaves of “Kinkei 201.” These results showed that the difference of leaf hardness was the cause of the difference in mining time and success rate. We concluded that the major HPR mechanisms of “Shinsei” were outer leaf angle and hardness.

Keywords

host plant resistance, HPR, leaf hardness, mining rate, rainfall

Introduction

Nemoto et al. (unpublished) have researched DBM densities in two common cultivars and four potentially resistant cultivars of cabbage within Saitama prefecture. When compared with the two common cultivars, “Shinsei,” a resistant cultivar, proved to have significantly lower densities of DBM. The current research has indicated the potential use of DBM resistant cultivars for control of DBM density.

Various studies have been conducted on the mechanisms of host plant resistance (HPR) of cabbage against the diamondback moth (DBM) and understanding the role of wax-bloom has been a major focus (e.g. Dickson & Eckenrode 1980, Lin et al. 1983, Eckenrode et al. 1986, Shelton et al. 1988, Dickson et al. 1990, Eigenbrode et al. 1990, Stoner 1990, Eigenbrode & Shelton 1990, Dickson et al. 1992, Verkerk & Wright 1994). The wax-bloom present on the cabbage leaves contains material which apparently repels DBM larvae from feeding. The wax-bloom has also been found to inhibit successful oviposition by DBM (Eigenbrode & Shelton 1992, Uematsu & Sakanoshita 1989).

Our study distinguishes itself from the previous research by focusing specifically on outer leaf angle and leaf hardness.

Materials and methods

The DBM used in the experiment were brought in from the Saitama Agriculture and Forestry Research Center in 1997 and reared in 16L:8D incubator conditions at 28ºC. The rearing methods used followed the protocol established by Nemoto (1991). The cabbage was cultivated by common methods and every 2–3 days the pests were removed manually by brush. No chemical pesticides were used.

Distribution (%) of eggs on different parts of cabbage plant

At the heading stage, 30 plants from each cultivar, “Shinsei” and “Kinkei 201,” were randomly selected and an egg count was done for each. Egg counts for the head and the whorl of each plant were kept separate. The different whorl structures were also used as a base for comparison.

Number of DBM II larvae dropped after the artificial rain treatment from leaves with different angles

The leaves for “Shinsei” and “Kinkei 201” were artificially set at 10° and 70° angles and had ten larvae placed on them before the artificial rain treatment. The experiments were repeated ten times for each cultivar and angle. A waiting period of one hour was used between inoculation and treatment. The artificial rainfall machine was set for 17.3 mm/h at a droplet size of 2.5 mm. After treatments, the remaining larvae were observed and counted.

Age-specific survival curves of DBM immatures (egg, larva, pupa) on headings of two cultivars

Ten eggs were placed on each of ten randomly selected leaves of both “Shinsei” and “Kinkei 201” cultivars at the heading stage. Survival rates were observed at 24 h intervals until pupation of the larvae. There were occasionally eggs that did not hatch and one week after egg placement, these eggs were removed and survival rates were then calculated based on the remaining number of eggs.

Mining rate of I instar larvae of DBM on whorl leaves of “Shinsei” and “Kinkei 201”

Ten whorl leaves from both “Shinsei” and “Kinkei 201” were selected and used to raise 30 I instar larvae per leaf. From the time the larvae were released on the leaves, mining rate observations were taken at 0.5, 1, 2, 4, 8, 12 and 24 h intervals.

Percentage of larval mining with artificial diet containing “Shinsei” and “Kinkei 201” MtOH extract

Methanol extracts from “Shinsei” and “Kinkei 201” whorl leaves were added to an artificial diet created by Miyasono et al. (1992). 30 larvae were released on each diet and after 24 hours' mining, rates for the “Shinsei” and “Kinkei 201” extract diets were calculated.

Leaf hardness for the “Shinsei” and “Kinkei 201” whorl leaves was calculated with a Williams (1954) type penetrometer (Figure 1). Various parts of the “Shinsei” whorl leaf were tested for hardness in the penetrometer. Different leaf sections were cut from the “Shinsei” whorl according to their different hardness. These sections were then fed to I instar larvae 30 minutes after hatching. Ten larvae were added to each 2 cm × 2 cm sample. Mining rates were observed after 3 hours.

Figure 1. Williams (1954) type penetrometer. A: Lateral view, B. Cross section

Results and discussion

Distribution (%) of eggs on different parts of cabbage plant

For the cultivar “Kinkei 201,” DBM egg distribution in the whorl was found to be 76.3%, while egg distribution in the whorl for “Shinsei” was determined at 91.0% (Table 1). There was a clear bias for egg distribution in the whorl of “Shinsei.” The whorl leaves of “Shinsei” are significantly more erect than those of “Kinkei 201.” The primary reason for whorl bias in larvae distribution was the degree of erectness of the whorl leaves.

Table 1. Distribution (%) of DBM eggs on different parts of heading stage cabbage plant

	Part of cabbage plant
Cultivar	Stem	Head	Whorl
Shinsei	0	9.0	91.0
Kinkei 201	0	23.7	76.3

Number of DBM II larvae dropped after the artificial rain treatment from leaves with different angles

The rate of fallen larvae in the erect (70°) whorl leaves was significantly higher than in the more prostrate (10°) leaves, however, the difference in rate of larvae fallen between the different cultivars was not found to be significant (Table 2). Whorl erectness decreased the ability of the larvae to hold on to the leaf surface.

Table 2. Number of DBM II instar larvae dropped after artificial rain treatment from leaves with different angles

		Whorl angle
Cultivar	n	70°	10°
Shinsei	10	8.8 ± 0.33a	5.7 ± 0.51b
Kinkei 201	10	8.8 ± 0.29a	6.2 ± 0.65b

Mean ± SE, Means followed by the same letter in a column are not significantly different (P<0.01, two-way ANOVA using √transformation value).

Age-specific survival curves of DBM immatures (egg, larva, pupa) on heads of two cultivars

When the whorl leaves of “Shinsei” were used as feeding material for larvae, there was a sharp drop in survival rate (Figure 2). This dramatic change in larval survival occurred between 3–5 days after egg placement at the I instar stage. Because of this result, it was realised there were characteristics of the “Shinsei” whorl, when used as feeding material for larvae, that were unfavourable to their survival and consequently the following research was focused on determining the reasons for this.

Figure 2. Age-specific survival curves of DBM immatures (egg, larva, pupa) on heading stage of two cabbage cultivars.

Mining rate of I instar larvae of DBM on whorl leaves of “Shinsei” and “Kinkei 201”

At the end of 24 h, the mining rate observed on the “Shinsei” whorl leaves was 43.6% and 60.3% for the “Kinkei 201” whorl leaves (Figure 3). It was also evident that the time elapsed before mining in the “Shinsei” whorl leaves took place was longer than for “Kinkei 201.” Evidently some component of the “Shinsei” whorl leaves caused a large number of I instar larvae to be unsuccessful in their mining attempts and thus produced a high mortality rate for larvae placed on the “Shinsei” whorl leaves. Two possible reasons for unsuccessful mining on “Shinsei” whorl leaves were suggested and experiments were conducted to confirm their validity.

1. Chemical components of the “Shinsei” leaf material
2. Physical aspects of the “Shinsei” leaf material

Figure 3. Mining rate of I instar larvae of DBM on whorl leaves of “Shinsei” and “Kinkei 201”.

Percentage of mining larval with artificial diet containing “Shinsei” and “Kinkei 201” MtOH extract

The mining rate for the “Shinsei” artificial diet was determined to be 52.0% and for the “Kinkei 201” artificial diet, 50.3% (Table 3). These differences were insignificant. In this experiment we were unable to find a chemical reason for decreased mining in the “Shinsei” whorl leaves. The used of MtOH to create the “Shinsei” and “Kinkei 201” diet extracts may have destroyed certain enzymatic components of the leaf material. Also, other chemical components may have evaporated with the MtOH, so chemical deterrence to larval mining can in no way yet be discounted.

Table 3. Percentage of mining DBM larvae with artificial diet containing “Shinsei” and “Kinkei 201” MtOH extract

Cultivar	Mining larvae (%)
Shinsei	52.0 ± 1.9 a
Kinkei 201	50.3 ± 2.0 a

Mean ± SE, Means followed by the same letter in a column are not significantly different (P>0.05, t-test)

Whorl leaf hardness of “Shinsei” and “Kinkei 201” measured by Williams type penetrometer

The average hardness of the “Shinsei” whorl leaves was 18.33 g and that of the “Kinkei 201” leaves was 14.38 g (Table 4). There was a distinctly higher degree of hardness in the “Shinsei” leaves. In the “Shinsei” whorl, as the leaf sections increased in hardness, the number of non-mining larvae increased proportionally (Figure 4). From these experiments we were able to conclude that leaf hardness plays a major role in larval leaf mining activity.

Table 4. Whorl leaf hardness of “Shinsei” and “Kinkei 201” measured by Williams type penetrometer

Cultivar	Threshold weight for piercing (g)
Shinsei	18.33 ± 0.70 a
Kinkei 201	14.38 ± 0.42 b

Mean ± SE, Means followed by the same letter in a column are not significantly different (P>0.05, t-test).

Figure 4. Regression of leaf hardness to mining rate of DBM. For regression analysis, data were transformed into angular values before calculation.

Conclusion

Due to the erect nature of the “Shinsei” whorl, the cabbage head is less exposed to DBM infestation. Because of this, a large proportion of DBM eggs is laid on the whorl of “Shinsei,” rather than the head. Comparing “Shinsei” whorl leaves to “Kinkei 201” whorl leaves, the erectness of the “Shinsei” whorl causes the larvae to be more susceptible to being physically removed from the leaf surface by rain. Also, due to the hardness of the “Shinsei” whorl leaves, the overall result is a higher mortality rate in “Shinsei” than in “Kinkei 201” cabbage. From the experimental evidence, the probability of a number of physical factors contributing to DBM resistance becomes high. While considering the influence of various physical factors such as leaf hardness and erectness, we would like to further ideas on how they might be used in pest management.

References