Pest management - Ecological Applications: Toward a Sustainable World - page 152

Fig. 6.5 Pesticide
problems amongst
cotton pests in the San
Joaquin Valley,
California. (a) Target
pest resurgence: cotton
bollworms ( Heliothis
zea ) resurged because
the abundance of their
natural predators was
reduced - the number
of damaged bolls was
higher. An increase (b)
in cabbage loopers
( Tr ich o plu s i a ni ) and (c)
in beet army worms
( Spodoptera exigua )
were seen when plots
were sprayed against
the target lygus bugs
( Lygus hesper us ) - both
are examples of
secondary pest
outbreaks. (After van
den Bosch et al., 1971.)

Control Azodrin

(a)

Bollworm population

40
30
20
10
0

40
30
20
10
0

Treatment

23 30 6 13 20 27 5 23 30 6 13 0 7 5

Aug Sep Oct Aug Sep Oct

Predator population

500
400
300
200
100
0

500
400
300
200
100
0

23 6 21 28 6 23 6 21 28 6

Aug Sep Oct Aug Sep Oct

Damaged bolls

60
50
40
30
20
10
0

60
50
40
30
20
10
0

23 30 6 13 20 27 5 23 30 6 13 20 27 5

Aug Sep Oct Aug Sep Oct

(b)

Bidrin used against Lygus .
Spray dates: Jun 8, Jun 17,
Jun 28, Jul 14
Untreated

100
80
60
40
20
0

Jul 18 Jul 25 Aug 2 Aug 9 Aug 16 Aug 23 Aug 30

(c)

180
160
140
120
100
80
60
40
20
0

Control
Treatments with toxaphane-DDT
Two treatments

Jul 6 Jul 15 Jul 22 Jul 29 Aug 5 Aug 12

Thus, in their model of the role of dispersal rates of pests and their predators on
target pest resurgence, Trumper and Holt (1998) found that resurgent effects were
greatest when predator dispersal rate was low and pest dispersal rate was high
(Figure 6.6). In a related context, recolonization by natural enemies can be speeded
up by providing adjacent refuges for predators from the pesticide. Lee et al. (2001)
planted grassland strips as natural enemy refuges adjacent to a corn crop ( Zea mays )