South-West Plains IPM Update Volume 16, Issue 10

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Bollworm Egg Lay in Gaines County July 19, 2016

On Tuesday (7/19/2016) my field scout and I started to pick up some bollworm eggs in Western Gaines county. These eggs were being found on both the upper leaf surface (Figure 1&2) and on the bract of squares. Eggs were not laid in the terminal region, but were about 3 nodes below the terminal, this can be because of the low humidity and high temperatures we have had lately.  The egg populations in the two fields were averaging between 10 eggs per 100 plants to as high as 30 eggs per 100 plants. Eggs were both white and brown. The color of the egg can be an indication of how soon you will be seeing neonate larvae in the field. When eggs are first laid they are white, and within 2 to 3 days the eggs will turn brown. The bollworm egg is about the size of a pin head and has ridges running from the top center of the egg to where it attaches to the plant (Figure 3). These eggs are typically laid singly on the upper third of the plant, however they can deposit eggs on squares, bolls, stems, and flowers in the lower portion of the plants. This typically only occurs when the cotton is stressed or when we are experiencing periods of high temperatures with low humidity. 

There have been reports of high bollworm populations in the southern portions of Texas. This means that farmers and crop consultants in the rolling plains and southern high plains should be looking out for bollworms in cotton whether it is still squaring or past full bloom. Scouting for bollworms should occur about every five days. In non-Bt cotton examination of the upper third of the plant is usually sound enough to make management decisions, but in Bt cotton the entire plant should be inspected for bollworm eggs and larvae. Currently there are two recommended scouting methods the Terminal/Square inspection method and the whole plant inspection method. In the terminal inspection method, the field should be divided into four sections, and examine 25 plants in each of the four sections for a total of 100 terminals. Also in each of the four sections of the field 25 quarter grown squares or larger should be randomly pulled and checked for damage. When pulling squares the scout should avoid flared or yellow squares. Scouting for bollworms using the whole plant inspection method the field should also be divided into four sections, and a minimum of 20 plants per a section should be checked for a minimum of 80 sites per field. When looking at the plant the scout should examine every square, boll, bloom, or leaf on the plant. Scouts should also pay attention to bloom tags, or the dried bloom still stuck to the boll.

There are two types of economic threshold for bollworms in cotton, and are based on either the number of larvae per acre (common for the High Plains and the Panhandle of Texas), or the number of larvae per 100 plants (common in eastern portion of Texas).  To calculate the number of larvae per acre you need to calculate the plant population per acre, this can be done by counting the number of plants in roughly 13 row feet and multiply by 1,000 to obtain the number of plants per acre. Next you need to calculate the average number of larvae per plant from your checked plants and then multiply by the number of plants per acre. For example, you averaged 40 plants in 13 row feet across the field, multiply by 1,000 and you get a plant population of 40,000 plants per acre. When scouting you averaged 2 bollworms per plant, now multiply 2 bollworms per plant by 40,000 plant per acre and you get 80,000 bollworms per acre.  The economic threshold for bollworms is also dependent on whether the cotton is Bt or non-Bt, and the growth stage at which the field is in. The economic threshold for Bt cotton in or past the bloom stage using the worms per acre calculation is 5,000 worms/acre with >5% damaged fruit (Table1), where using the number of larvae per 100 plants the threshold is 8-12 worms >0.25” per 100 plants with >5% damaged fruit (Table 2). Table 3 is the threshold for non-Bt cotton when scouting using the terminal inspection method. 

Figure 1. White cotton bollworm egg found on the upper leaf surface of cotton in Western Gaines County, Texas. (photo credit: Tyler Mays)

Figure 2. Brown cotton bollworm egg found on the upper leaf surface of cotton in Western Gaines County, Texas. (photo credit: Tyler Mays)

Figure 3. Magnified photo of a cotton bollworm egg (top) to show the ridges running from the top of the egg down the side to where the egg contracts the leaf. Looper eggs (bottom) which can sometimes be mistaken for bollworm eggs. (photo credit: cottonbugs.tamu.edu)

Table 1. Whole Plant Inspection Method Economic Threshold for Cotton Bollworms based on the Number of Larvae per acre
		Cotton Type
Cotton Stage	Worm Size	Non-Bt	Bt
Pre-Bloom	All	≥30% damaged squares with worms present
Bloom to Harvest	< 0.25”	10,000 worms/acre	Do not Treat
	>0.25”	5,000 worms/acre	5,000 worms/acre with >5% damaged fruit.
Table was derived from the economic threshold section for bollworms/budworms and cottonbugs.tamu.edu/fruit-feeding-pests/bollworm-and-tobacco-budworm/

Table 2. Whole Plant Inspection Method: Economic Threshold for Cotton Bollworms Based on Number of Larvae per 100 Plants.
	Cotton Type
Cotton Stage	Worm Size	Non-Bt	Bt
Pre Bloom	All	≥30% damaged squares with worms present
Bloom to Harvest	>0.25”	10-15 worms /100 plants	Do no treat
	<0.25	8-12 worms/100 plants	8-12 worms/100 plants with >5% damaged fruit
Table was derived from the economic threshold section for bollworms/budworms and cottonbugs.tamu.edu/fruit-feeding-pests/bollworm-and-tobacco-budworm/

Table 3. Terminal Inspection Method: Bollworm Economic Threshold for Cotton Bollworm based on Number of Larvae per 100 Terminals.
Cotton Stage	Natural Enemies	Cotton Type
		Non-Bt	Bt
Pre-Bloom	Present or Absent	15-30% damaged squares and worms present	Use Whole Plant Inspection Method
Bloom to Harvest	Present	8-12 small worms per 100 terminals and >5% damaged fruit
	Absent	4-5 small worms per 100 terminals with eggs present and >5% damaged fruit.
Table was derived from the economic threshold section for bollworms/budworms and cottonbugs.tamu.edu/fruit-feeding-pests/bollworm-and-tobacco-budworm/

South-West Plains IPM Update Volume 16, Issue 9.

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South-West Plains IPM Update Volume 16, Issue 8.

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South-West Plains IPM Update Volume 16, Issue 7

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