Field Survey on the Overwintering of Syrphids in Changbai Mountains and Experiments on artificial protection of the Overwintering Syrphid Flies

Gao, JF, Zhang,GX, Qin,Y-C, Yu,K, Li,M-H & Qin, Y

Chinese Journal of Biological Control (1993) 9,3: 142-143

There are 130 days a year in Changbai Mountains, in which the temperature is below zero. The temperature in winter can reach minus 35°C. The ground is covered with snow 100 days per year.

Under natural conditions, the pupae of Syrphids overwinter in a 4cm soil Iayer. They emerge from middle of April to early May next year. We investigated the emergence of Syrphids from 1988 to 1990. We selected blacksoil, sand, South slope und North slope as experimental sites. 500 individuals were transfered into every site. Next year the fields were covered with gauze. Every 3 days the number of adults was calculated. The living pupae, witch didn‘t emerge until 10th of May, were thought to be emergence pupae. The results were as follows:

Table 1: Observation of the emergence rate under natural conditions

ExperimentalSite	Year	number of observed pupae	number of emergence	emergence rate (%)	date of earliest emergence 	
blacksoil
	1988
1989
1990	487
500
500	114
105
213	23,4
21,0
42,6	27.4.
29.4.
3.5.	
sand	1988
1989
1990	469
491
500	247
262
247	52,7
53,4
49,4	26.4.
24.4.
1.5.	
south slope	1988
1989
1990	500
500
482	231
243
264	46,2
48,6
54,8	21.4.
22.4.
24.4.	
north slope	1988
1989
1990	483
500
486	113
147
149	23,4
29,4
30,4	1.5.
9.5.
4.5.	

From table 1 it can be seen that the overwintering mortality of Syrphids was related to the overwintering place. 50% of the pupae in sandsoil and South slope emerged into adults. But only 20-30% of pupae in blacksoil und North slope.

We made experiments in 1990 and 1991 by protecting artificaliy the overwintering sites of Syrphids. We planted Chinese Cabbage in pots with sandsoil. Early in August aphids were transferred onto the cabbage. In early September the pots were covered with gauze and Syrphid adults were released into the pot. Early in October the pupae were obtained. When the temperature was minus 5°C, the pots were tranferred to the four following surroundings. 1. Basement 2. Indoor 3. A 30cm deep ditch outdoors, the pot was at first put into the ditch, then covered with a board, at last covered with rice straw. 4. Outdoors, no protective measures.
The temperature in basement in winter was - 3°C, indoor minus 10°C. In the middle of April next year pots were removed outdoors. The results are Iisted in table 2.

Table 2. Emergence rates of Syrphids in different overwintering places

Overwintering site	Year	Number of pupae	Number of emerged flies	emergence rate (%)	minimum temperature (°C)	
basement	1990
1991	472
500	397
407	84,1
81,4	-3	
indoor	1990
1991	500
463	386
399	77,2
86,2	-10	
in covered ditch	1990
1991	500
500	312
347	62,4
69,4	-17	
ck	1990
1991	500
500	247
232	49,4
46,4	-35	

From table 2 it can be seen that the emergence rate in basement und indoor reach above 80%, but it was below 49% under natural conditions.
We compared the ratio of sex, lifespan and the number of eggs under different overwintering conditions. The results were Iisted in table 3.

Table 3. Survey of the emerged Syrphid adults in different overwintering conditions

Overwintering site	sex ratio	mean lifespan (d)	Mean number of eggs/adult	highest egg number/adult	maximum lifspan (d)	
basement	1:1,1	31,0	114	187	37	
indoor	1:1,1	30,5	117	157	34	
in covered ditch	1:1,7	27,0	84	112	30	
ck	1:2,3	24,0	72	103	27	

From table 3 it can be seen that the overwintering place influences not only the emergence rate but also significantly the lifespan of adults, the ratio of sex and the number of eggs
The experiments proved that by protecting the overwintering sites of Syrphids their number increased in the following year.