18. After_effect of Insecticide Application on Population of Spodoptera litura F. in Soybean Field

Arifin, M., A. Naito, and A. Kikuchi. 1989. After_effect of insecticide application on population of Spodoptera litura F. in soybean field, pp. 379-384. Dalam S. Hardjosumadi et al. (Eds.). Seminar Hasil Penelitian Tanaman Pangan Balittan Bogor, 5-6 Januari 1988.

Muhammad Arifin¹, Atsushi Naito², and Atsushi Kikuchi²

¹Bogor Research Institute for Food Crops, Indonesia

²National Agriculture Research Center, Tsukuba, Japan

ABSTRAK

Pengaruh Setelah Aplikasi lnsektisida di Pertanaman Kedelai terhadap Populasi Spodoptera litura F. Suatu seri tabel kehidupan (life table) ulat grayak, Spodoptera litura F. (Lepidoptera, Noctuidae) dikembangkan di pertanaman kedelai dalam hubungannya dengan pengaruh aplikasi insektisida terhadap populasi ulat grayak. Janis maupun populasi musuh alami menurun setelah aplikasi insektisida; konsekuensinya, laju daya tahan hidup (suwival rate) atau populasi ulat grayak meningkat. Beberapa jenis predator dan parasit dilaporkan dan efektifitasnya terhadap ulat grayak dievaluasi di laboratorium.

The common cutworm, Spodoptera litura F. (Lepidoptera, Noctuidae) is one of the most important pests in some Asian countries, especially in Indonesia (3, 5). Usually farmers spray their soybean crops with insecticide one a week or more at 20 to 65 days after sowing to control this insect pest (4).

In Indonesia, it has been known that the resurgence of S. litura occationally occurs in soybean fields after spraying with insecticide (2). It is assumed one cause of the insect resurgence is due to the decreased population of natural enemies by spraying with insecticide. Investigation was done on the age-specific life table to clarify the effect of insecticide application against the living organism related to the lawal population of S. litura in a soybean field.

MATERIALS AND METHODS

The experiment was carried out from June to September 1987 at the National Agriculture Research Center, Tsukuba, Ibaraki, Japan. Soybean seeds were sown on June 9, 1987 in a field at spacing of 10 cm within each row and 70 cm between the rows, and one plant per hill. The experimental field was plotted into six 5 m x 5 m plots, consisting of 2 treatments and 3 replications. One of the treatments was insecticide application (Fenvaralate 10% + MEP 30%) and the other was no insecticide application. Three fixed suwey sites (each consists of 4 soybean plants) were selected in each plot for obsewation of the life table.

S. litura eggs used in the experiments were reared on an artificial diet. Egg masses laid on the paper during the previous night were selected and the number of eggs was counted under a microscope. In each suwey site about 500 eggs (just before hatching) were infested in underside of soybean leaves one week after spraying with insecticide. After the infestation, number of the unhatched eggs or infertile was counted. The number of the first instar larvae was calculated based on from the number of the hatched eggs. Actual number of the larvae after second instar stage was counted in each stage of development. Life table was constructed and factors affecting the growth of the larvae were investigated.

An experiment was also conducted to determine various kinds of S. litura eggs and larva parasites, and to study the feeding ability of some predators attacking the larvae.

The newly laid egg masses S. litura were infested in underside of soybean leaves in the field to identify the egg parasites. Two days after infestation, the egg masses were collected and the emerging parasites from the egg were identified. The first to fourth instar larvae of S. litura were also infested into the soybean field to identify parasites of the larva. Two days after infestation, the larvae were collected and the emerging parasite were identified.

Effectiveness of 2 species of predators to young larvae of S. litura namely Orius sp. and Crysopa sp. were evaluated in the laboratory. Nymphs or adults of Orius sp. (Hemiptera, Anthocoridae) were placed in petridishes (2 cm depth and 9 cm diameter) with different numbers (10, 25, and 50, respectively) of the first and second instar larvae. Nymphs of Chrysopa sp. (Neuroptera, Chrysopidae) were also placed in petridishes in different numbers (25, 50, and 100, respectively) of the second instar larvae. Numbers of dead larvae attacked by the predators were counted on the next day.

RESULTS AND DISCUSSIONS

There was a significant difference in survival of S. litura larvae on both sprayed and unsprayed plots (Fig. 1). The number of eggs hatched in both plots were almost the same, but the survive larvae in the sprayed plot were more abundant. The effect of significant on the survival of the second instar larvae. In the unsprayd plot, almost no survive larvae was found after the fourth instar. In the sprayed plot, however, many larvae were still survive after the fourth instar, and some of them reached the sixth instar. 

The life table for one generation of S. litura in the sprayed and unsprayed plots are shown in Table 1. Egg mortality in both plots was about 3%. It is assumed the mortality factor of the egg is due to infertility. The mortality of the first instar larvae in the sprayed plot was about 60%, lower than that of the unsprayed plot (93%). The mortality of the second instar larvae in the sprayed plot was 38%, while in the unsprayed plot was 61%. Survival rate of the first instar larvae in the sprayed plot dropped more quickly than that of the unsprayed plot (Fig. 1). It was indicated that the factors influencing mortality of the first and second instar larvae in the sprayed plot was rainfall, whereas in the unsprayed plot were parasites, predators, such as Chrysopa sp. Orius sp., spiders and ants, and rainfall.

Table 1.  A one generation life table of Spodoptera litura F. in a soybean field sprayed and unsprayed with insecticide. Tsukuba, Japan, September 1987

X		lx	dxf	dx	100 qx
Sprayed plot
Egg*		1000	Infertility	23	2.3
Larva	L1	977	Rainfall	588	60.2
	L2	389	Rainfall	146	37.5
	L3	243	Frog, rainfall	105	43.2
	L4	138	Unknown	98	71.0
	L5	40	Unknown	37	92.5
	L6	3	Unknown	3	100.0
Unsprayed plot
Egg*		1000	Infertility	31	3.1
Larva	L1	969	Parasite, Orius, Chrysopa, spiders, ants, rainfall	900	92.9
	L2	69		42	60.9
	L3	27	Parasite,  Chrysopa, spiders, ants, rainfall	15	55.6
	L4	12	Unknown	11	91.7
	L5	1	Unknown	1	100.0
	L6	0

X  

.

X  = age intervals at which the sample were taken;

l_x  = the number of survivors at the beginning of age class X;

d_xf      = the mortality factors responsible for d_x;

d_x = the number of deaths during the age interval X;

q_x = the mortality rate per age interval, q_x = d_x/l_x;

* = only one or two days exposured in the field.

The mortality of the third instar larvae in the sprayed plot was about 43%, lower than that of the unsprayed plot (56%). The factors influencing larval mortality in this larval stage in the sprayed plot were frog and rainfall, while in the unsprayed plot were parasites, predators, such as Chrysopa sp., spiders, ants and frog, and rainfall.

The final instar of the larvae in the sprayed plot was the sixth instar, whereas in the unsprayed plot was the fifth. Presumably, this difference is due to the difference in larval mortality between both plots during the first and second instar. The factors influencing mortality of the fifth and sixth instar larvae were not known, because the larvae moved down to the soil surface during the night and crawl away to surrounding area of the survey site.

In this experiment, the effect of insecticide application on natural enemies was not clear. It seem from the larval mortality of young instar (first to third stage) in the sprayed and unsprayed plots. In the sprayed plot, the mortality of S. litura from the egg laying to the third instar larvae was about 99%, whereas in the unsprayed plot was about 86% (Table 1). It seems the plot size used in this experiment is too small, so that some natural enemies from the surrounding area can easily migrate into the sprayed plot.

Results of the experiment showed that the larval mortality caused by parasite in the first, second, and third instar larvae were 19%, 13%, and 8%, respectively (Table 2).

Feeding abilities of Orius sp. as a predator of S. litura vary, depend on the age, larval stage, and population of the prey (Table 3). The feeding ability of the predator nymph was 27 larvae per day at high population level of the first instar stage of S. litura, where as of the predator adult was 23 larvae per day. However, their feeding abilities were different from the former when were given to the second instar larvae. The feeding ability was not dependent on the prey population, which was about 2 larvae per day. It is assumed Orius sp. is aisn effective predator against the first instar larvae of S. litura.

Table 2.    Effect of parasites to Spodoptera litura F. in a soybean field. Tsukuba, Japan, October 1987

Insect stadia infested *		Insect population infested	Insect population picked up	Parasitism (%)
Egg		538	538	0
Larva	L1	500	100	19.0 ± 2.65
	L2	500	74	13.4 ± 3.72
	L3	500	54	7.7 ± 5.42
	L4	500	25	0

*  Larval stage: L1= instar I; L2= instar II; L3= instar III; L4= instar IV, respectively

Table 3.    Effect of predator Orius sp. on the survival of Spodoptera litura F. larvae in the laboratory. Tsukuba, Japan, October 1987

Number of larvae	Number of dead larvae caused by
	Nymph *	Adult
First instar
10	3.0 ± 1.00	2.3 ± 0.57
25	19.7 ± 3.21	5.3 ± 3.21
50	27.3 ± 11.15	22.7 ± 2.89
Second instar
10	1.3 ± 0.58	2.0 ± 1.00
25	2.3 ± 1.53	2.0 ± 1.00
50	3.7 ± 0.58	3.3 ± 0.58

* Final instar nymph

Feeding abilities of effective predator Chrysopa sp. against S. litura larvae vary with the prey population. The feeding percentage reached 51% at high population level of the prey (Table 4). Djuwarso et al. (1) reported that feeding speed of Chrysopa sp. was 58 second instar larva or 25 larvae were fed up within 24 minutes.

Table 4.    Effect of predator Chrysopa sp. on the second instar larvae of Spodoptera litura F. in the laboratory. Tsukuba, Japan, October 1987

Number of larvae	Number of dead larvae	Predatism (%)
25	25	100
50	43	86
100	51	51

CONCLUSIONS

Population of natural enemies decreases by spraying with insecticides; consequently, the survival rate of S. litura larvae is increased after insecticide application. Among the natural enemies, Orius sp. and Chrysopa sp. are the most effective predators. Orius sp. attacts 23 larvae of the first instar, and Chrysopa sp. attacks about 50 larvae of the second instar.

ACKNOWLEDGMENTS

We wish to express our gratitude to Dr. Kinya Kushibuchi, Director General and Dr. Hajime Kato, Director of Department of Plant Protection of National Agriculture Research Center for their encouragements.

REFERENCES

1. Djuwarso, T., A. Naito, H. Matsuura, and A. Kikuchi. 1984. Life lable of tobacco cutworm, Spodoptera litura F. in the soybean field. Technical Report at National Agriculture Research Center, Tsukuba, Ibaraki. 8 p.

2. Harnoto, R. Hamada, and K. Kiritani. 1984. Effect of sublethal dosages of carbaryl on the fecundity of Spodoptera litura F. (Lepidoptera, Noctuidae). App. Ent. Zool. 19(3): 404-405.

3. Naito, A., Harnoto, and A. Iqbal. 1983. Current problems of majors soybean production area in Indonesia. CRIFC Seminar, Oct. 14. 1983.

4. Sumarno and Harnoto. 1983. Kedelai dan cara bercocok tanamnya. Puslitbangtan, Bogor. 54 p.

5. Tengkano, W. dan M. Soehardjan. 1985. Jenis hama utama pada berbagai fase pertumbuhan tanaman kedelai, pp. 295-330. Dalam S. Somaatmadja et al. (Eds). Kedelai. Puslitbangtan, Bogor.