Insect Strip

Insect Strip

Friday, July 11, 2014

Spider Mites Populations Building in Corn

General Observations

Corn is in all different growth stages from whorl stages to tasseling or ear development. Western bean cutworm moth activity is increasing and fall armyworm moth activity has dropped - off some but they continue to be present.  The first flight of southwestern corn borer is ending and will begin again in about 3 weeks.

Spider Mites

Temperatures in the 90’s and the dryer conditions the past couple of weeks have provided good conditions for spider mite populations to be increasing rapidly. Some fields have already been sprayed for early season infestations before corn reaches the tassel grow stages. Other fields are now tasseling with mite populations developing on the lower 1/3 part of the plant.

Some of the spider mite predators are present, but they mostly are in large numbers. While collecting spider mite infested leaves to infest one of my spider mite trials, I noticed more small spider than any of the other spider mite predators. Kaj Overstreet, crop consultant, is finding the six-spotted thrips and even some spider mite destroyer beetles. I have seen some six-spotted thrips, but not any of the spider mite destroyer beetles in fields I have been in. These predators may be able to contain the spider mites in some fields, but in other fields spider mite can develop into damaging infestations.

Once corn reaches the tassel growth stage and during grain development, research has shown that the spider mite reproductive capacity increases 4 times that of when spider mites are on the whorl stages. And, during the grain development stages canopy temperatures are hotter which shortens spider mite developmental rate. During these conditions spider mites can develop from an egg to an adult in as little as 5 days. This causes spider mite populations to increase exponentially, unless predator populations or the spider mite disease, neozygites sp., can contain the infestation.

Since spider mites are such a small animal, it is mind boggling how they (collectively thousands) can cause so much damage and yield loss. The following table shows the extent of damage that spider mites can cause to corn after tassel and during grain development.

Table 1. Damage of Banks grass mite on corn and yield losses. Archer and Bynum. 1990. J. Econ. Entomol. 83(3): 1069-1073. 
Treatment No.
Whole Plant Damage Ratinga
% Leaves Infested
Yield (lbs./A)
% Yield loss
1
0.0 e
25 d
10,138 a
---
2
2.0 d
67 c
9,668 ab
4.6
3
3.0 c
77 b
9,437 ab
6.9
4
5.2 b
92 a
9,092 bc
10.3
5
6.9 a
99 a
8,739 c
13.8
Means in each column followed by the same letter are not significantly different (P=0.05; Duncan’s multiple range test [SAS Institute 1985]).
a Damage rating on a 1-10 scale when 1 = 1-10% of the leaf area damaged by mite feeding to 10 = 91-100% damage.

The table illustrates that even at relatively low levels of damage (2 to 3) reduction in yield was from between 5% to 7%. And, once damage reached levels of 5 to 7 the yield losses could be from 10% to 14%. Although yield losses did not exceed 14% in this study, other researchers have estimated losses as high as 20% to 40%. Other trials showed yield losses from two-spotted spider mites were equal to that by the Banks grass mites.  These losses demonstrate the importance of sampling fields for progression of spider mite damage in order to make good mite management decisions. But, it can be difficult making visual assessments of spider mite damage. The next table provides a scale and description of damage that an individual can use for determining feeding damage from spider mites (Table 2). By using the damage rating scale individuals can estimate feeding damage for individual plants or sample at several locations within the field and come up with an average damage rating for a field. When using the damage rating scale, a miticide should probably be sprayed at damage rating 3. This corresponds to previous recommendations that 1) fields should be scouted at least once a week to know the dynamics of the mite/predator populations and damage and 2) consider treating if mite colonies are beginning to establish on the ear leaf and mite densities (eggs, immatures, adults) and damage continue to be increasing.

Table 2. Mite damage rating scale used to estimate spider mite feeding damage on corn. 
Rating
% leaf area damage per plant
Description of Damage
1
1-10
A few small mite colonies and associate damage (chlorotic spots) along the midrib of the lowest leaves
2
11-20
Mite colonies and damage spread along the midribs on the lowest leaves on a plant
3
21-30
Mite colonies and damage spreading out from the midrib on the lowest leaves and small colonies may occur on leaves up to the ear.
4
31-40
Mites and damage cover most of the leaf area on the 1-2 lowest leaves and mite colonies and damage extend along the midrib to the ear leaf.
5
41-50
Mites have killed one leaf, bottom 2-3 green leaves heavily infested and damaged, and mite colonies on 1-2 leaves above the ear.
6
51-60
Mites have killed or nearly killed the bottom two leaves and colonies and damage extend beyond the midribs on two leaves above the ear.
7
61-70
Mites have killed or nearly killed the bottom three leaves, all leaves up to the ear significantly damaged, and mite colonies and damage found on most to all leaves on the plant.
8
71-80
Mites have killed or nearly killed all leaves up to the ear and mites and damage occur on most to all leaves on the plant.
9
81-90
Most leaves on the plant killed by mite feeding and only leaves in upper third of plant alive.
10
91-100
Very little green area left on plant or plant dead.
Archer et al., 1989. Techniques for screening maize for resistance to mites. In: Toward Insect Resistant Maize for the Third World: Proceedings of the International Symposium on Methodologies for Developing Host Plant Resistance to Maize Insects. (Ed: Mihm, J. A.) CIMMYT, Mexico D.F., pp. 178-183.

We now have Comite II, Oberon 4SC, Onager 1E, Portal, and Zeal labeled for use in corn.  All of these miticdes do not adversely affect natural predators of spider mites which is key to sustained spider mite suppression. The efficacy of these miticides are slower than the old organophosphate and bifenthrin (Brigade) chemicals. So, we often do not see immediate control. It often takes 10 to 14 days before seeing any noticeable reduction in spider mite infestations. The important thing to remember about these products is that good spray coverage is critical for optimum control and applications should be made before infestations reach a damage rating 5. If spider mite populations are damaging corn at ratings above 5 when applications are applied, the miticides will reduce the mite population. But the level of control will not be acceptable. The other products (Brigade 2EC, Dimate 4E, and Hero) are harsher on beneficials. And, data have shown that spider mites infestations can flare following usage of these three products in the Texas High Plains.

Wednesday, July 2, 2014

Moth Trap Catches and Spider Mites

Moth Activity

Fall armyworm (FAW) moths continue to be active (July 1 catches), but they seem to be tapering off compared to trap catches for June 17 and June 24 (See FAW moth trap catches table). Whorl feeding damage is easier to find in none Bt hybrids and even in refuge plants. the damage we are finding now is from the egg laying during June 10 to June 24. With  the continued FAW moth activity, damaging larval infestation is still a threat to corn, grain grain and forage sorghums in the whorl stages, now to corn that is beginning to tassel and other crops that are a host to the FAW. For the grain crops there are not any definitive thresholds for deciding when to treat. In the June 20 issue, I compared the thresholds used by different states (Indiana, Kentucky, and Texas).  So, I can not provide you with a specific recommendation for spraying. Whorl infestations are much harder to control because of the difficulty of getting the spray in the whorl where larvae are feeding.  The use of older insecticide chemistries have not provided good effective control when applied to whorl infestations. Newer insecticide such as Belt, Beseige, and Prevathon have provided better results than the older insecticides. A recent trial, by Dr. Pat Porter, indicated the pyrethroid in Beseige could have improved mortality of the larvae. However, if the field were also infested with spider mites, the pyrethroid would cause spider mite infestation to flare. 

Fall Armyworm Moth Trap Catches
County / Trap location # Jun 03 Jun 10 Jun 17 Jun 24 Jul 01
Dallam / 1 0 0 0 39 0
Dallam / 2 0 0 0 19 5
Deaf Smith / 1 0 0 0 17 22
Deaf Smith / 2 0 0 5 3 3
Deaf Smith / 3 0 0 11 19 40
Deaf Smith / 4 0 0 0 6 20
Hale / 1 - - - - 125
Hale / 2 0 0 156 275 133
Hansford / 1 0 0 23 16 -
Hartley / 1 0 0 129 52 130
Hutchinson / 1 15 0 72 36 -
Hutchinson / 2 0 12 67 19 -
Lipscomb / 1 10 39 210 737 395
Lipscomb / 2 2 24 140 460 112
Moore / 1 0 0 88 155 70
Moore / 2 0 0 60 135 72
Ochiltree / 1 16 0 174 77 -
Parmer / 1 34 182 765 298 19
Parmer / 2 -
- - 22
Randall / 1 0 61 248 560 354
Randall / 2 29 58 264 104 137
Swisher / 1 1 0 3 0 2
Swisher / 2 0 0 3 7 3
There has been a noticeable increase in Western bean cutworm (WBC) moths this last week in Dallam, Hartley, and Moore Counties (See WBC moth trap catches table).  John Quillin, crop consultant, sent me a photograph this week of a WBC egg mass on a corn plant. We can expect the peak WBC activity to continue for the next 4 to 6 weeks, but moths could be caught in traps even longer. The moth activity this year is beginning almost a month latter that they did in 2012, but similar to the beginning activity in 2013.





Western Bean Cutworm Moth Trap Catches
County / Trap location # Jun 03 Jun 10 Jun 17 Jun 24 Jul 01
Dallam / 1 0 0 0 7 264
Dallam / 2 0 0 0 16 117
Deaf Smith / 1 0 0 0 3 7
Deaf Smith / 2 0 0 0 0 3
Deaf Smith / 3 0 0 2 0 5
Deaf Smith / 4 0 0 3 10 8
Hale / 1 0 0 0 0 0
Hale / 2 0 0 0 0 0
Hansford / 1 0 5 0 4 -
Hartley / 1 0 0 2 8 85
Hutchinson / 1 0 0 2 0 -
Hutchinson / 2 0 5 1 5 -
Lipscomb / 1 0 0 0 0 1
Lipscomb / 2 0 0 0 0 0
Moore / 1 0 0 0 9 39
Moore / 2 0 0 2 3 35
Ochiltree / 1 0 0 0 0 -
Parmer / 1 0 1 0 3 6
Parmer / 2 - - - - 3
Randall / 1 0 0 3 2 4
Randall / 2 1 1 2 0 0
Swisher / 1 0 0 0 0 8
Swisher / 2 0 0 0 0 2

Southwester corn borer (SWCB) moths have been active for the past three weeks in counties with more corn acreage (Dallam, Deaf Smith, Hansford, Hartley, Moore and Parmer Counties).  This moth activity should begin to taper-off as we reach the end of moths emerging from the overwintering SWCB pupae. 
Also, I have heard reports of European corn borer moths being active. I would suspect our cooler  and wet weather may be contributing to this activity. 

Southwestern Corn Borer Trap Catches
County / Trap location # Jun 03 Jun 10 Jun 17 Jun 24 Jul 01
Dallam / 1 0 6 15 30 10
Dallam / 2 3 10 13 105 101
Deaf Smith / 1 0 0 35 29 48
Deaf Smith / 2 0 0 25 14 0
Deaf Smith / 3 0 0 44 53 63
Deaf Smith / 4 0 0 227 178 349
Hale / 1 0 0 87 25 0
Hale / 2 0 2 0 0 0
Hansford / 1 0 15 32 111 -
Hartley / 1 1 4 93 167 58
Hutchinson / 1 0 0 0 18 -
Hutchinson / 2 0 0 7 12 -
Lipscomb / 1 2 0 0 1 2
Lipscomb / 2 0 0 0 0 0
Moore / 1 0 0 8 52 3
Moore / 2 0 5 40 105 62
Ochiltree / 1 57 27 7 31 -
Parmer / 1 36 2 200 203 13
Parmer / 2 - - - - 8
Randall / 1 1 - 2 3 0
Randall / 2 1 27 4 2 3
Swisher / 1 0 0 5 0 0
Swisher / 2 0 0 0 0 3

The White-line sphinx caterpillar has caught the attention of everyone. I have received more calls about this caterpillar this week than another pest.  Jim Elzner, crop consultant, found in grain sorghum, but they did not appear to be causing any damage. Another caller found them in corn. But in both fields the caterpillars were mostly large sized worms and on the ground. They may be consumed the weeds they were eating and are now moving around looking for more weed or they are preparing to burrow in the soil and pupate. 


White-lined Sphinx caterpillars showing different coloration patterns, Photo credit: Ed Bynum
Spider Mites

Our rains and cooler temperatures from late May through June have kept spider mite populations from getting established in heavy numbers this spring. Thrips coming out of wheat two or three weeks ago may have helped keep spider mite numbers low. This past week in my corn seed treatment trial, spider mites were still present in low numbers. This week thrips were not that abundant and other spider mite predators were not very common.  Also, Greg Cronholm, retired IPM Extension agent - Hale and Swisher Counties, is not reporting many spider mite predators. This could be important if or when weather conditions become hotter and dryer, because we still have time for spider mites to build to damaging infestations. However, damaging infestations should not be as widespread as we had the past three years. Some fields may never develop damaging infestations and will not need miticide applications.