Abstract

Food consumption and developmental duration of virus-infected DBM larvae were measured in the laboratory to determine whether Plutella xylostella granulosis virus (PxGV) can reduce DBM damage to cruciferous vegetables. The newly moulted larvae of each instar fed on vegetable leaves treated with a suspension of PxGV. The developmental duration of treated larvae was lengthened and the food consumption of virus-treated larvae of each instar increased slightly compared with the healthy larvae. However, the total leaf consumption of each instar was significantly reduced compared with untreated larvae. The proportion of decreasing food consumption from the I to IV instar larva was 93%, 77%, 53% and 46% compared with the corresponding healthy larval stage. The change of food consumption depends on two factors: feeding rates and time, either of which can result in change of food consumption of larval feeding.

Keywords

Feeding, development time, PxGV

Introduction

The diamondback moth, Plutella xylostella (L.), is one of the key insect pests of cruciferous vegetables and its feeding can seriously reduce quality and yield of vegetables. Feeding area and hole size by lepidopteran caterpillars have been taken as criteria for feeding consumption evaluation (Greene et al. 1969, Sears et al. 1983). Several researchers have developed action thresholds for control of Pieris rapae, Trichoplusia ni and P. xylostella with insect viruses in cabbage fields (Webb & Shelton 1991, Mailloux & Belloncik 1995). In this study, the relationship between numbers of DBM larvae and leaf loss or yield loss of flowering Chinese cabbage and Chinese cabbage was examined and the effects of food plants on feeding activity were investigated to provide basis for determining damage criteria.

Materials and methods

DBM and food resources

Test insects originated from pupae collected in the field. After emergence, the moths laid eggs on filter papers treated with extracted liquid of vegetable leaf. Newly hatched I larvae were obtained from eggs laid on filter paper, while the II, III and IV instar larvae tested in the experiments were raised on potted crucifer plants. Leaves of Chinese kale were provided as food for larvae.

Test procedures

The newly moulted larvae of each instar fed for 24 hours on leaves of Chinese kale, which had been dipped in a suspension of PxGV with one larval equivalent per litre (LE/L). They were then transferred to an uncontaminated leaf held in a clear Petri dish. Each of the 40 larvae of each instar per treatment was put into a Petri dish with a fresh leaf disc. The glass dishes were sealed with Parafilm^® and kept at 25±1˚C, 16L: 8D. The food consumption of each larva was evaluated every half-day by estimating leaf area consumed and development age was recorded until the larvae either died or pupated.

Statistical analysis

Data from the experiments were subjected to analysis of variance and means were separated by Duncan’s Multiple Range Test (P=0.05 level). The calculation was conducted using DPS software (Tang & Feng 1997).

Results and discussion

Effects of PxGV on larval developmental duration

First instar larvae treated with virus died before reaching III instar and the development time for these larvae was significantly longer than for the untreated group at both I and II instar. Second instar larvae treated with virus had an extended period of development at III instar and died before reaching IV instar (Table 1). Larvae fed virus at either III or IV instar had a significantly longer development period than the untreated larvae.

Table 1. Development duration and total feeding time of PxGV-treated DBM larvae

Means in the same column followed by the same letter are not significantly different (P=0.05), Duncan’s Multiple Range Test.

Variation in food consumption of virus-fed larvae

Significant reduction in initial food consumption was observed only for the IV instar larvae fed virus (Table 2). However, total food consumption was reduced by 93%, 76%, 53% and 47% respectively after treatment of I, II, II and IV instar larvae. This reflects the reduced longevity of virus-infected larvae. Older larvae (III and IV instar) were less sensitive to disease after inoculation with virus. Thus, their food consumption was higher than those of younger (I and II instar) larvae, but still much less than the untreated larvae.

Table 2. Food consumption of PxGV-fed larvae

Means in the same column followed by the same letter are not significantly different (P=0.05), Duncan’s Multiple Range Test.

Effects of granulosis virus on larval feeding rates

After virus feeding, feeding rates of all stages of larvae were significantly lower than those of the larvae in the control group (Table 3). This reflects the increased time of development of each larval stage post-infection.

Table 3. Feeding rates of PxGV-treated DBM larvae

Means in the same column followed by the same letter are not significantly different (P=0.05), Duncan’s Multiple Range Test.

Larval food consumption was determined by the two factors of feeding rate and duration. Larvae infected at the stage of II and IV instar initially developed at the same rate as the untreated larvae, but their rate of leaf consumption was also reduced. The feeding time of larvae infected at III instar was 2.6 days longer than that of the untreated control. The area eaten by a larva infected at either the III and IV instar was 0.072 and 0.325 mm² per day, respectively, less than the control group. Therefore, their total leaf consumption was decreased. These results are similar to reports in Pieris rapae, Ectropis oblique hypulina and Trichoplusia ni (Wang & Hu 1986, Hu et al. 1990, Harper 1973).

References

Greene GL, Genung WG, Workman RB & Kelsheimer EG. 1969. Cabbage looper control in Florida-A cooperative program. Journal of Economic Entomology 62, 798-800.

Hu C, Ye GY, Lin GM, Wu WL, Zhu JQ & Shang JN. 1990. Effect of the nuclear polyhedrosis virus from the tea geometrid (Ectropis obliqua) on feeding, growth and development of the tea geometrid larvae. Journal of Zhejiang Agricultural University 16, 113-118. [in Chinese with an English summary].