Insect Strip

Insect Strip

Friday, August 8, 2014

Increase of Southwestern Corn Borers and Potential for Sorghum Midge

Southwestern Corn Borer

Southwestern corn borer (SWCB) moth activity and egg laying has started the second generation infestation period across the Texas High Plains. This is because SWCB moths captured in moth traps has been increasing dramatically the past two weeks. And, John Quillin, crop consultant, called Monday reporting he had found SWCB  eggs at the red-line developmental stage. These eggs will have hatched by the time this newsletter is published.  Eggs are usually laid in groups of two to three, but there may be more in a group. The female lay the creamy white eggs overlapping each other like fish scales. Each egg develops three red band after one day and by about the fourth day the black head of the developing larvae can be seen in the egg. By this time the larvae will hatch in about another day. 

Red-lined southwestern eggs
Photo: TAMU Entomology
Years ago, before Bt-corn hybrids and delayed planting, we would talk about second generation SWCB infestations occurring after tasseling. This is because early planting was a primary practice so fields could be harvested before plants started lodging from SWCB stalk girdling. 

Southwestern corn borer girdled and lodged plant
Photo: TAMU Entomology
Bt corn technology has been so successful in controlling SWCB infestations that it has changed the landscape of SWCB infestations. The moth trapping project (funded by Texas Corn Producers) the last three years has shown pockets of higher SWCB moth activity across moth trapping sites in a county and across the High Plains. This is different from extreme moth activity across the entire High Plains region back in the 70’s to the 90’s. Still non-bt corn fields and refuge corn is at risk of SWCB infestations even in areas of low moth activity. 

Another factor influencing the landscape of SWCB infestations is later planting dates for irrigation management. The later the planting date there is a higher probability of non-bt corn lodging from SWCB infestations. As I drive around the High Plains, I have seen corn fields in the late whorl stage, pre-tassel corn, fields just beginning to silk, and corn in the dough stage. Having so many different growth stages changes management decisions for the second generation SWCB infestations. It becomes even more important to scout the whorl and pre-tassel corn for egg and newly hatched larvae. The threshold for spraying insecticides when 20 to 25 percent of the plants are infested with eggs or newly hatched larvae should still apply to these later planted growth stages. 

Southwestern Corn Borer Moth Trap Captures 2014
County / Trap location #
3-Jun
10-Jun
17-Jun
24-Jun
1-Jul
8-Jul
15-Jul
22-Jul
29-Jul
5-Aug
Castro / 1


3
13
10

14



Castro / 2


32
21
5

3



Dallam / 1
0
6
15
30
10
10
6
17
95
152
Dallam / 2
3
10
13
105
101
45
5
2
63
222
Deaf Smith / 1
0
0
35
29
48
28
50
38
122
268
Deaf Smith / 2
0
0
25
14
0
113
101
82
242
492
Deaf Smith / 3
0
0
44
53
63
81
90
41
302
663
Deaf Smith / 4
0
0
227
178
349
36
300
488
960
416
Gray / 1
24
24
17
51
10
5
14



Gray / 2
0
0
0
4
0
2
2



Hale / 1
0
0
87
25
0
0
3
3
0
0
Hale / 2
0
2
0
0
0
0
0
0
2
1
Hansford / 1
0
15
32
111
51
40
6
0
37

Hartley / 1
1
4
93
167
58
22
16
84
273
436
Hutchinson / 1
0
0
0
18
2
14
0
4
4

Hutchinson / 2
0
0
7
12
25
6
3
8
36

Lipscomb / 1
2
0
0
1
2
1
0
1

25
Lipscomb / 2
0
0
0
0
0
0
1
3

24
Moore / 1
0
0
8
52
3
13
8
10
75
350
Moore / 2
0
5
40
105
62
145
60
5
160
320
Ochiltree / 1
57
27
7
31
5
18
0
13
8
7
Parmer / 1
36
2
200
203
13
52
1
13

18
Parmer / 2




8
97
5
4

7
Randall / 1
1
1
2
3
0
19
43
37
26

Randall / 2
1
27
4
2
3
6
36
9
3

Swisher / 1
0
0
5
0
0

0
0
0

Swisher / 2
0
0
0
0
3

0
0
0

Average
5
5
34
47
31
33
28
37
127
213


Sorghum Midge

Conditions may be just right to have the “perfect storm” for sorghum midge infestations this season. Sorghum acreage is higher this year and much of it was planted late after failed out cotton. And, johnsongrass is everywhere in the bar ditches and is a perfect alternate host for midge. 

Sorghum Midge
Photo: Pat Porter
Sorghum midge, Stenodiplosis sorghicola, is a small orange-red colored fly about the size of a gnat. A female midge will lay a single egg in each of 50 flowering spikelets in single day. The eggs hatch in 2 to 3 days and the larvae feed within the floret for 9 to 11 days. This destroys the grain kernels, resulting in blasted seed heads. The total life cycle is about 14 to 16 days, but there are overlapping generations of midge  in a single head. This is because females on a single day will emerge, laying eggs in the yellow flowers for that day. These females die, but the next day more midge emerge and lay eggs in the next set of yellow flowers on the sorghum head. Since it takes from 7 to 9 days for a individual sorghum head to flower and a field may have an extended period (2 to 3 weeks) of when sorghum heads are maturing, there are multiple generations of midge. And, midge are doing the same in johnsongrass. Heads that were infested may have all kernels blasted or varying degrees of normal kernels with non-kernel-bearing spikelets. Fortunately, once flowering is completed the heads are no longer at risk from midge.

Infestation periods on the Texas High Plains generally do not begin until after July. This is because midge do not over-winter on the High Plains and the midge migrate from south Texas as sorghum and johnsongrass develop across the state.

So, with all this being said, the short version is that our late planted sorghum may be at risk. Blayne Reed, Extension Agent - IPM for Hale and Swisher counties, reported finding midge in his August 1 issue of his Plains Pest Management Newsletter. He wrote “In our program we do have several fields’ currently in bloom and we are still finding some sub-threshold midge in spots. Our highest midge find this week was 0.42 per head, which is fairly high and needs to be watched daily.” 

Fields should be scouted daily during flowering when midge is a threat. Midge will be active when temperatures warm to 85o F, which is usually between noon and one o’clock. Midge can be seen and counted on spikelets or flying about the sorghum heads if you are careful not to disturb the head. Other methods for sampling are by placing a clear plastic bag over the sorghum head to trap adults or quickly beat the head in a white bucket and look for the midge before they fly away.

Sorghum midge blasted heads
Photo: ICRISSAT.org
The Texas A&M AgriLife Extension entomologists have developed an economic threshold calculator for determining when it is economical to treat midge infestations. This calculator is located at https://insects.tamu.edu/extension/apps/sorghummidgecalculator/index.php. The basic components of the calculator are Control Cost ($/acre), Grain value ($/cwt), and number of flowering heads/acre.

Once an insecticide has been applied the residue will only be effective for 1 to 2 days after treatment. Then if midge are still present 3 to 5 days after the first application another application should be applied. Some of the standard insecticides listed in our Extension guide, “Managing Insect and Mite Pests of Texas Sorghum,” are chlorpyrifos (numerous products), Baythroid© XL, Karate© and Warrior©, Asana© XL, Lannate© (methomyl), and Mustang Max©. But, other labeled products are Cobalt© and Cobalt Advanced©, Diamond©, Delta Gold©, Proaxis©, and Stallion©.