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2.3 Importance of predators
2.3.1 Hopper consumption by birds
Hopper bands are attacked by numerous predators. Several species of birds and one hymenopteran species were particularly effective. In Section 2.1.2.2 the loss under natural conditions of bands treated with botanicals was already mentioned. Even at the station itself the sparrows proved themselves to be effective locust catchers. Not only did they pick up all the larvae which escaped from the holding cages, they also made an active attempt to get into the cages. In various experiments, the number of larvae devoured or killed by the sparrows was measured over a period of 3 h (Tab. 8).
Some hundred hoppers (3rd or 4th instar) were reared in a field cage (2x2x2 m) which was set up near to nesting places of birds (Passer simplex saharae). Some holes were made in the cage's side wall big enough for the hoppers to escape, but too small for the birds to invade.
Tab. 8 Hopper consumption by Passer simplex saharae
|
Trial |
Time |
No. of birds |
No. of hoppers killed |
Hoppers/bird |
|
1 |
30 |
4 |
63 |
15.8 |
|
2 |
30 |
4 |
49 |
12.3 |
|
3 |
30 |
5 |
61 |
12.2 |
|
4 |
30 |
4 |
55 |
13.8 |
|
5 |
30 |
4 |
48 |
12.0 |
|
6 |
30 |
5 |
51 |
10.2 |
The trials were conducted on two days between 9.00 and 12-00 a.m. The temperatures increased from 23° C to 27° C. The majority of the hoppers (nearly 2/3) were eaten immediately. The killed hoppers left on the ground were also consumed within 3 h of the trials being completed.
Tab. 9 Field observation of hopper band consumption by birds
|
Year |
Initial hopper band |
Instar |
Bird species |
Time of attack |
Hoppers left |
|
1993 |
130 000* |
4th |
Cursoris cursor |
6 |
<1000 |
|
1994 |
180 000* |
4th |
various |
4 |
<5 000 |
|
1995 |
190 000* |
2nd |
various |
10 |
<500 |
|
1995 |
250 000 |
2nd |
various |
11 |
<5 000 |
|
1995 |
180 000 |
3rd |
Passer luteus |
10 |
<300 |
|
1995 |
1 100 000* |
2nd to 3rd |
various |
9 |
<50 000 |
|
1995 |
500 000* |
4th |
various |
10 |
<5 000 |
various: Passer luteus, Lanius spec. and others
*These hopper bands were treated with sublethal dosages of neem oil (content of active ingredient less than 300 ppm/l) 3 to 5 days before the end of observations. The mortality rate caused by neem oil was below 40 % (cage control), although hopper mobility was reduced, i.e. the daily distances migrated decreased from nearly 1000 m to less than 250 m.
Similar observations without precise quantification were carried out in 1993 in Brumduid. In the field trials conducted at that location, not only untreated bands but also bands treated with neem oil and Metarhizium spores (C. Kooyman, personal communication) were consumed almost completely by various bird species.
2.3.2 Influence of predatory wasps
Among the predatory insects a sphecid wasp was observed, since it is a very effective hopper predator. Populations of some thousands of wasps followed hopper bands, caught the hoppers and carried them into their holes dug in the ground and arranged in so-called wasp fields of up to more than 100 m2 in size (Tab. 10). These activities were repeated each day after the hopper bands had reached the 3rd instar, and were reduced after the hopper bands had undergone moult to the 5th instar.
Table 10 Impact of sphecid wasps on hopper bands
|
Field no. |
No. of larvae/hopper band |
Size of wasp field (m) |
Mean no. of holes (n=3) |
Total no. of holes |
Mean no. of larvae/hole (n=6) |
|
1 |
80 000 - 150 000 |
36 |
40 |
1 440 |
|
|
2 |
80 000 - 150 000 |
33.75 |
52.7 |
1 778 |
11.3 |
|
3 |
> 300 000 |
108 |
52.3 |
5 652 |
13.1 |
|
4 |
10 000 - 15 000 |
10.5 |
28 |
294 |
|
|
5 |
80 000 - 150 000 |
72 |
50 |
3 600 |
|
|
6 |
> 500 000 |
107.25 |
55.3 |
5 931 |
10.6 |
|
7 |
600 000 - 700 000 |
112.5 |
49 |
5 513 |
The investigation of around 50 holes revealed an average of 11 paralysed hoppers per hole. Assuming that the wasps' activity were extended to 10 to 12 days, all hopper bands in Table 10 would be reduced to less than 20 % of their original size.
The observation of and experiments with predators demonstrate that hopper bands of up to around 100 000 individuals can be heavily decimated or even totally exterminated by predators, which is highly consistent with earlier findings. In the case of plagues, however, the importance of predators remains doubtful, but should nevertheless be investigated, especially when the predator populations also increase during the course of such a plague.