Category Archives: Entomology

UGA Entomologists Dr. Phillip Roberts and Dr. Mike Toews put this information together on corn earworms in cotton.

During the past week we have received a few reports of escaped corn earworm CEW) larvae in Bt cottons which exceed recommended thresholds. Bt cottons are not immune to CEW and never have been. All Bt cottons should be scouted for CEW and growers should be prepared to react in a timely manner if thresholds are exceeded. We have planted Bt cottons for over 20 years.  The technology has moved from a single Bt gene to two gene and now three Bt genes. The addition of Bt genes was for two reasons primarily; 1) to improve efficacy and increase the spectrum of activity and 2) for resistance management. The slide below is a general rating for Bt cottons for various caterpillar pests.

Activity on CEW varies by technology, however all technologies should be scouted. Entomologists in Georgia and other areas of the cotton belt believe we are seeing changes in the susceptibility of CEW to some Bt genes. We have been fortunate in Georgia in that only a small percentage of Bt cottons have required treatment for escaped CEW in recent years. However we have observed changes in performance of Bt corn in recent years, i.e. seeing more damage to corn ears. We are also seeing more feeding on squares in Bt cotton which was very rare 5 years ago. One aspect of Bt cotton that we must not forget is that all Bt cottons continue to provide excellent control of tobacco budworm.

When scouting Bt for CEW cotton scouts should examine the top 12 inches of the plant for eggs and larvae and also examine one bloom, one bloom tagged boll (be sure to look under bloom tag), and an additional boll lower in the canopy. If any damage is observed on the plant the entire plant should be searched. It is important to size larvae as small (< ¼ inch) or large (> ¼ inch). Once larvae reach ¼ inch in length it is likely those larvae will survive on the Bt plant and continue to feed. When we observe escaped CEW larvae they are often associated with fruiting structures near the uppermost white bloom.  So make sure you check blooms, bloom tagged bolls, and small bolls closely. The slide below shows various images of CEW in cotton.

The threshold for CEW larvae in Bt cotton is when 8 larvae ¼ inch or greater in length are found per 100 plants. When treating escaped CEW in Bt cotton coverage and penetration of the canopy with sprays will be important.  We must get the insecticide to the target as larvae will likely be down in the plant canopy. Control of larvae in bolls and under bloom tags will be difficult.

Pyrethroids have been the standard treatment for CEW for many years.  In parts of the cotton belt pyrethroid activity on CEW has deteriorated. For example some states in the Mid-South do not recommend pyrethroids for control of CEW due to pyrethroid resistance and field control failures. We annually monitor CEW susceptibility to pyrethroids in Georgia using adult vial tests. Basically we capture adult CEW moths in pheromone traps and place those in pyrethroid treated vials and monitor survival. During recent years and especially during 2016 we observed increased survival of CEW in these tests which suggests susceptibility is changing. However, we have not observed or been made aware of any field control issues when pyrethroids have been used for CEW control. With that said, we have made few field applications of pyrethroids for control of CEW in any crop during recent years. Bottom line is it will be important for us to check behind pyrethroid applications targeting CEW. There are non-pyrethroid alternatives that will provide very good control of CEW.  The slide below illustrates CEW survival in pyrethroid treated vials.

With this heat, it gets dry quick. I found a few plants with evidence of spider mites yesterday. This of course is not good since it is early. When we are scouting fields, look for spider mites. Look for the reddening on the “V” part of the leaf. UGA Extension Entomologist Dr. Phillip Roberts says early detection is critical.

**The best management practice is to NOT flare spidermites with unnecessary insect sprays. If our retention is good (plant bugs), stink bug threshold is low, and aphids have crashed, we need to hold back. It’s about to get hotter, and dry weather makes them worse too.

Some of our earliest planted cotton is now blooming. I thought it would be good to go through an insect update.

Aphids / Beneficial Insects

As reported last week, we found the aphid fungus in Thomas County. UGA Extension Entomologist Dr. Phillip Roberts located the fungus in Tift County last week. Once it’s found in a field, it takes about a week to ‘crash’ the aphids. There are still reports of aphids this week, and I’ve had a question or two. Remember, UGA has not found any yield difference in treating aphids. We’ve had more aphid populations this year, but I would hesitate on treating, especially when they are in ‘hot spots.’ We need to watch beneficials.

In one field this week, I saw many, many lady beetles. On one plant, I saw almost all of its life stages, confirming how many beneficials are in our fields now. The lady beetles can’t eat all these aphids, but the aphids are bringing them to the field. I’ve seen some parasitic wasps too.

Lady beetle puapae

Lady beetle larvae – “Baby alligator”

Plant Bugs

I haven’t talked about plant bugs this year. In fields I go in, retention has been good. 80% retention is our goal. I have seen a plant bug here and there. Dr. Roberts says that in the state, they have been spotty, and only a small acreage has been treated to date.

Sting Bugs

It is now time to scout for stink bugs in blooming fields. Scouts need to crack bolls and look for warts and calloused walls. Treatment decision is based on % of bolls damaged.

Silverleaf Whiteflies

Dr. Roberts says whiteflies are a month early. This is usually an indication of a rough year. We need to look for adults. The precense of adult whiteflies influence deicisions of other pests. Treatment for whiteflies must be timely as well. For more on thresholds and insecticides, visit this previous post on Silverleaf Whitefly Management.

Probably the biggest agriculture pest topic last year, whiteflies became a problem very fast. Because of our warm winter, white are already present, likely meaning another bad year. The goal in silverleaf whitefly (SLWF) management is to initiate control measures just prior to the period of most rapid pest population development. UGA Extension Entomologists Dr. Phillip Roberts and Dr. Mike Toews put together some info on SLWF:

Risk for SLWF

• Hairy leaf > smooth leaf cotton.
• Late planted > early planted cotton.
• Hot and dry > rainy conditions.

Scouting

SLWF adults (solid white wings) and immatures will be found on the underside of leaves. SLWF populations are best estimated from the 5th main stem leaf below the terminal. Main stem leaves are attached directly to the main stem by their petioles. The top or first main stem leaf is defined as the uppermost leaf which is 1 inch or larger in diameter. Adults and nymphs should be counted on the 5th main stem leaf below the terminal.

Adult whitefly in our UGA On-farm Variety Trial on June 29th. Leaves are considered infested when 3 adults are observed. (This counts adults that fly off when leaf is turned over.

SLWF Threshold

Treat when 50 percent of sampled leaves (sample 5th expanded leaf below the terminal) are infested with multiple immatures (≥5 per leaf).

Leaves are considered infested if 5 immatures are observes.

Insecticides

Insect Growth Regulators (Knack and Courier): use of IGRs are the backbone of SLWF management programs in cotton. Effects on SLWF populations are generally slow due to the life stages targeted by IGRs, however these products have long residual activity and perform very well when applied on a timely basis.

Use of other insecticide options which are active on all life stages have quicker effects on SLWF infestations but lack the residual of IGRs.

SLWF is an areawide cross commodity problem. When all parties use sound SLWF management programs all will benefit.

Steps for Efficient Sampling of SLWF

1. Familiarize yourself with the general location of the 5th main stem leaf in each field.
2. Select plants at random at least 25 paces into the field and at least 10 paces apart, being careful to keep your shadow from passing over the plant you plan to sample.
3. Turn the 5th leaf over slowly by its tip or petiole and count the leaf as infested with adults if it has 3 or more adults on it. Include in your counts any adults that fly up from the leaf as you turn it over.
4. Detach the leaf by the petiole from the main stem. If it fails to snap off easily, you have likely sampled a leaf that is too high on the plant. Recheck your leaf position to make sure you are sampling the 5th leaf.
5. Examine the bottom of the leaf for the presence of immature SLWFs. Count the leaf as infested if it has 5 or more immatures on the underside of the leaf. Sample at least 30 plants (leaves) per field.
6. Calculate the percentage of leaves infested with adults and the percentage of leaves infested with immatures.
7. Treatment is recommended when 50 percent of sampled leaves are infested with immature SLWFs.

For the first time, I am getting to see what ‘consistent aphids’ looks like in the field. Normally, aphid populations are sporadic, seen only in ‘hot spots.’ This season, they are prevalent in our oldest and youngest cotton. We looked at a field this week where every single plant was affected by aphids in the top 3 – 4 leaves (below). The only reason we don’t talk much about it is UGA research on treating aphids has never found a consistent yield response. The best thing is to limit stress on the plant, and our rainfall is helping us here.

Leaves of 8-leaf cotton curl due to stress of aphids

Classic symptoms of aphids in our cotton variety trial.

Aphid Fungus

But the real new is that yesterday, we confirmed the ‘aphid fungus’ in Thomas County. UGA Extension Entomologist Dr. Phillip Roberts already reported the ‘aphid fungus’ being spotted in Tifton this week. This means, aphids will soon crash.

We observed the aphid fungus, (Neozygites fresenii), which causes aphid populations to crash on the Tifton Campus on June 26, 2017.  We have also received some early observations from both north and south of Tifton.  The presence of grey, fuzzy aphid cadavers is indicative of the naturally occuring fungus (below).  Once observed in the field we would expect aphids to crash within a week.  We typically first observe the fungus in fields with high aphid infestations; and more specifically areas of those fields which were initially infested.  All scouts should be on the lookout for the fungus and be sure to report to growers.

Aphid fungus – Photo by Dr. Phillip Roberts