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1997 Interactive Effects of Weeds and Defoliating Insects in Soybean (Glycine Max). Charles Frost Grymes Louisiana State University and Agricultural & Mechanical College

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Recommended Citation Grymes, Charles Frost, "Interactive Effects of Weeds and Defoliating Insects in Soybean (Glycine Max)." (1997). LSU Historical Dissertations and Theses. 6421. https://digitalcommons.lsu.edu/gradschool_disstheses/6421

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTERACTIVE EFFECTS OF WEEDS AND DEFOLIATING INSECTS IN SOYBEAN (Glycine max)

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy

in

The Department of Plant Pathology and Crop Physiology

by Charles F. Grymes M.S., Texas A&M University, 1993 B.S., Texas A&M University, 1990 May 1997

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UMI Microform 9736016 Copyright 1997, by UMI Company. All rights reserved.

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UMI 300 North Zeeb Road Ann Arbor, MI 48103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

I would like to thank all of those who have made fulfillment of this degree

possible. I especially thank God for providing me good health, a caring family, and

the patience and drive to take on any task. To my wife Lorissa, you had more to do

with my completion of this degree than you realize. You have been much more than

just a wife. You've provided love, encouragement, and friendship, and I'm sure

you won’t let me forget who worked to put me through school. I love you very

much and look forward to our future together. To my daughter Celia Deanne, you

are only one year old at the completion of this dissertation. Your presence in your

mother's and my life has been nothing but pure enjoyment. The only thing I regret

is that this degree kept me from spending quality time with you and your mother.

To my parents, Gordon and Billie, thank you for your love and support over the

years. You always guided me in the right direction and encouraged me to do my

best. I would have never gone to college in the first place if you had not insisted on

it. To my brother and sister, Derrol and Melanie, your support and harassment will

never be forgotten. To my grandparents, Herman and Velma Liere, your love and

support through the years will always be appreciated. To Bernard and Kathleen

Deckert, I realize it was hard to let your daughter marry someone who would take

her so far away from home. I appreciate your understanding and support.

Dr. Jim Griffin, or "Doc", you have not been just a major professor, but a

colleague and good friend whom I truly look up to. I appreciate you giving me the

ii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. chance to be part of your program. It has been an enlightening experience in the

classroom, the office, and the field. I don't believe I would have enjoyed graduate

school anywhere else as much I did with you and all the "bull". I feel like I not

only gained knowledge in the classroom, but also how to work and communicate

well with others. I will be forever grateful for this experience.

To my committee members, Dr. B. Rogers Leonard, Dr. David Boethel, Dr.

David Jordan, and Dr. John Russin, thank you for guidance and support on this

project. Dr. Leonard and Dr. Boethel, thanks for taking someone with a "weed

science" mentality and molding me into a more rounded individual. I would also

like to thank Dr. Leonard and Dr. Jordan for all the extra assistance and patience

that was required for me to complete my work on the branch stations.

To my fellow graduate students, Pat Clay, Dr. Donnie Miller, Dr. David Black,

Ron Strahan, Doug Fairbanks, Chris Corkern, and Joe Pankey, I truly enjoyed

working with every one of you. I realize that my projects consumed much of your

time and plucking leaves was never any fun. I appreciate you hanging in there with

me until the job was done. I would also like to thank all the student workers who

helped me over the years. There are too many to name them all, but I will never

forget everything you did. I would also like to thank all the staff at the Winnsboro

and St. Joseph locations of the Northeast Research Station for your support and

many hours of hard work. I am also indebted to Rosanne Mascarenhas for her

assistance with rearing insects, Dr. T. Reagan for the use of his droplet analysis

iii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. equipment, Ed Ostheimer for his computer assistance, and D. Rester for his

expertise and assistance with the deposition study.

I would also like to thank the staff in the Department of Plant Pathology and

Crop Physiology. Mrs. Pat, Mrs. Charletta, and Ms. Jill, you made my life a lot

easier. I appreciate your patience and understanding. Dr. Johnnie Snow, it was

certainly a pleasure working with you and your department. While I will always be

an "Aggie" at heart, I realize that the degree and experience at LSU is what will get

me somewhere.

iv

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ACKNOWLEDGEMENTS ...... ii

ABSTRACT...... vi

CHAPTER I INTRODUCTION ...... 1 Literature Cited ...... 13

II SOYBEAN (iGlycine max) RESPONSE TO WEED INTERFERENCE AND DEFOLIATION...... 20 Introduction...... 20 Materials and Methods ...... 22 Results and Discussion ...... 25 Literature Cited ...... 33

III HEMP SESBANIA AND SICKLEPOD INFLUENCE ON INSECTICIDE DEPOSITION AND SOYBEAN LOOPER CONTROL...... 37 Introduction...... 37 Materials and Methods ...... 39 Results and Discussion ...... 45 Literature Cited ...... 56

IV SUMMARY ...... 58

VITA ...... 61

v

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

Field experiments were conducted to evaluate the influence of simulated insect

defoliation and full season weed competition on soybean [Glycine max (L.) Merr.J

growth and yield. Weeds were johnsongrass [Sorghum halepense (L.) Pers.],

common cocklebur {Xanthium strumarium L.), and hemp sesbania [Sesbania

exaltata (Raf.) Rybd. ex A.W. Hill] at 15, 3, and 12 plants/6 m of row. Simulated

defoliation at R2 and R5 soybean growth stages was accomplished by removal of 0,

1, or 2 leaflets per soybean trifoliate, which approximated 0, 33, and 66%

defoliation, respectively. Averaged across defoliation levels and stages,

johnsongrass, common cocklebur, and hemp sesbania reduced soybean yields 30,

15, and 14%, respectively, in 1994 compared with no weed interference. In 1995,

common cocklebur did not affect yield, whereas johnsongrass reduced yield 35%.

As defoliation level increased, a linear decrease in soybean yield was observed.

Averaged across weeds and defoliation stages, 33 and 66% defoliation reduced

soybean yield 6 and 20% in 1994 and 12 and 33% in 1995, respectively.

Defoliation at R5 resulted in 10% lower yield than defoliation at R2 in one of two

years. Yield reduction due combinations of weeds and defoliation was additive.

Field experiments evaluated the influence of hemp sesbania and sicklepod [Senna

obtusifolia (L.) Irwin and BarnebyJ on insecticide deposition within the soybean

canopy and resultant soybean looper [Pseudoplusia includens (Walker)] control.

Dye-sensitive cards placed in top, middle, and bottom portions of the soybean

vi

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. canopy measured spray droplet deposition for the insecticide thiodicarb applied at

504 g ai/ha in 94 L/ha spray volume with a ground sprayer. Spray droplet

deposition was highest on cards placed in the top of the soybean canopy, and weeds

reduced deposition 26 to 43 % compared with weed-free soybean. Thiodicarb

deposition within the middle and bottom levels of the canopy was not reduced by

weeds. Weeds, however, did not influence thiodicarb efficacy against soybean

looper in the field or in laboratory feeding bioassays. Control of both weeds and

defoliating pest is important; however, management strategies for soybean looper

may not need to be altered when weeds are present.

vii

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INTRODUCTION

Soybean [Glycine max (L.) Merr.] is the dominant oilseed crop in the world

accounting for 20 to 25 % of the total fat and oil production (Smith and Huyser

1987). Soybean oil is commonly used in shortening and margarine and in a variety

of other products. Meal is a major source of protein for livestock feed. Soybean

was first grown in the United States in 1765 on a farm in Thunderbolt, Georgia

(Hymowitz and Harlan 1983). In Louisiana, soybean has been an important cash

crop since the 1960's (Morrison and McCormick 19%). In 1994, soybean was

grown on approximately 465000 ha in Louisiana, with an estimated production value

of $189 million (Anonymous 1995).

Soybean grown in the Gulf Coast states is exposed to stresses imposed by more

species of pests, during longer periods of the year, and more frequently than in any

other area of the United States (Newsom and Boethel 1985). Pest management

strategies, however, have been primarily directed toward only a single class of pest

such as weeds, insects, or pathogens, with little concern for interactions that may

occur.

Adequate rainfall and soil moisture, warm temperatures and mild winters, along

with nutrient-rich soils in Louisiana provide an environment conducive to growth of

a broad spectrum of weed species (Jordan et al. 1987; Sanders 1996). Weeds

primarily reduce crop yields by directly competing with the crop for limited supplies

1

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of water, nutrients, and light (Ross and Lembi 1985). Some weeds can also have

allelopathic properties whereby chemicals released into the rhizosphere inhibit the

germination and/or growth of other plants. Competition and allelopathy are often

collectively referred to as weed interference (Anderson 1996). The degree of

interference and ultimately yield reduction associated with weeds are dependent on

weed species and density. A single giant foxtail Setaria( faberi Herrm.) plant/30 cm

of soybean row (30000/ha) resulted in a 13% yield reduction, whereas one smooth

pigweed (Amaranthus hybridus L.) at the same density reduced yield 25 % (Nave and

Wax 1971). McWhorter and Hartwig (1972) reported 23 to 43% yield losses from

johnsongrass [Sorghum halepense (L.) Pers.] competition, with differences

dependent upon the soybean variety. Common cocklebur ( Xanthium strumarium L.)

reduced yields of the same varieties 63 to 75%. Mosier and Oliver (1995) reported

yield losses of 57 and 60% with one common cocklebur plant/30 cm of row

(32000/ha) under irrigated and non-irrigated conditions, respectively. Full season

competition by common cocklebur populations of 3300, 6600, 13000, and 26000

plants/ha reduced soybean yields 10, 28, 43, and 52%, respectively (Barrentine

1974). Full season competition from hemp sesbania [Sesbania exaltata (Raf.) Rybd.

ex A.W. Hill] at densities up to 5500 plants/ha did not reduce soybean yields, but

populations of 8100 to 129000 plants/ha reduced yields 10 to 80% (McWhorter and

Anderson 1979). In Arkansas, sicklepod [Senna obtusifolia (L.) Irwin and Barneby]

spaced 10 and 30 cm apart (98000 and 32000/ha) reduced soybean yield 41 and

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31%, respectively (Bozsa et al. 1989). Thurlow and Buchanan (1972) observed 19

to 35% soybean yield losses from sicklepod at densities of 7.7/m2 (77000/ha). In

general, if the crop can be maintained free of weeds for 4 to 5 weeks after

emergence, later emerging weeds cause little or no yield loss (Ross and Lembi

1985), but can reduce harvest efficiency and crop quality. High weed populations

may render mechanical harvest difficult. Elevated moisture and foreign material in

harvested seed can lead to grade reductions that lower the value of the crop and

reduce economic returns. Application of postemergence herbicides to wild

poinsettia (Euphorbia heterophylla L.) did not increase weed control or soybean

yield over that following only a preemergence application, but percent moisture and

foreign material in the harvested soybean seed were reduced (Willard and Griffin

1993). Balloonvine (Cardiospermum halicacabum L.) produces a seed the same

shape and size as soybean seed; therefore, it is difficult to separate from the crop

seed (Jordan et al. 1987). Reduction in price received for soybean at the elevator

due to high moisture and presence of foreign material or weed seed in many cases

can justify the additional cost of herbicides.

Many insect pests feed on soybean during vegetative and reproductive growth

stages (Turnipseed and Kogan 1987), but several species cause enough damage to

justify control measures (Tynes and Boethel 1996). In Louisiana, soybean looper

[Pseudoplusia includens (Walker)], velvetbean caterpillar [Anticarsia gemmatalis

(Hubner)], and stink bugs [Nezara viridula (L.), Acrostemun hilare (Say), and

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Euchistus servus (Say)] frequently reach threshold levels that necessitate use of

insecticides (Baldwin et al. 1996). Other important insect pests of soybean include

corn earworm [Helicoverpa zea (Boddie)], bean leaf beetle [ Ceratoma trifurcata

(Forster)], and threecomered alfalfa hopper [Spissistilus festinus (Say)]. Soybean

looper, velvetbean caterpillar, green cloverworm [Plathypena scabra (Fabricius)],

Mexican bean beetle ( Epilachna varivestis Mulsant), and bean leaf beetle all feed on

soybean foliage. These pests reduce photosynthetically active leaf area of a plant,

thereby reducing yield in some cases. Soybean looper larvae habitually feed in the

lower half to two-thirds of the soybean canopy (Herzog 1980) and are most injurious

to soybean from August to October (Steffey et al. 1994). A single soybean looper

larva can consume up to 114 cm2 of leaf tissue before pupation (Boldt et al. 1975;

Reid and Greene 1973). Wier and Boethel (1996) reported soybean yield losses of

48 and 94% at defoliation levels of 74 and 94%, respectively, from full bloom (R2)

to pod development (R5) (Fehr et al. 1971).

In approximately 90% of studies investigating the effect of defoliation on

soybean yield, artificial injury was used to simulate actual insect defoliation (Ostlie

and Pedigo 1984). Hinson et al. (1978) reported soybean seed yield losses of 8, 21,

31, and 30% with 67% leaf defoliation imposed 3, 17, 31, and 42 days after

flowering, respectively, whereas yields were reduced only 4% with 33% defoliation.

Defoliation of soybean 33, 66, and 100% at first bloom reduced yield 15, 20, and

36%, respectively, and 19, 37, and 67%, respectively, when defoliated 4 weeks

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. later (Todd and Morgan 1972). Most studies have shown soybean to be most

tolerant to defoliation up to R3 (pod initiation) (Fehr et al. 1971), and prior to R3,

soybean can tolerate up to 30% defoliation (Turnipseed and Kogan 1987). Research

has been conducted using various simulated defoliation techniques including removal

with a hole puncher or cork-borer, clipping portions of a leaflet (Poston et al. 1976),

and excision of entire leaflets (Kalton et al. 1949; Todd and Morgan 1972). Even

though the methodology has been refined, the hole puncher or clipping techniques

are generally no more effective in representing a percentage defoliation than simply

removing the entire leaflet (Turnipseed and Kogan 1987).

Soybean is also susceptible to many pathogens (Athow 1987; Ross 1987). Bean

pod mottle virus, vectored by the bean leaf beetle, is common in Louisiana (Ross

1987). Bacterial blight [Pseudomonas syringae pv. glycinea (Coerper) Young, Dye,

Wilke] is the major bacterial disease of soybean. A number of fungal pathogens can

cause leaf defoliation of soybean. Common diseases include foliar blight

(Rhizoctonia solani Kuhn), ffog-eye leaf spot (Cercospora sojina Hara), brown spot

(Septoria glycines Hemmi), downy mildew [Peronospora manshurica (Naum.) Syd.

ex Gaum], and red crown rot [Calonectria crotalariae (Loos) Bell and Sobers]

(Berggren 1989; Phillips 1989; Whitam 1996). In Louisiana, ffog-eye leaf spot and

foliar blight have caused soybean yield losses as high as 20 and 80%, respectively

(Whitam 1996).

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In field situations, pests only rarely occur singly, but more commonly as

complexes. The concept of integrated pest management (IPM) has been around

since the 1950's with the original intent to promote judicious use of synthetic

organic insecticides (Holtzer et al. 1996). The IPM concept emphasized the

integration of multiple tactics for the management of pests in an ecologically and

economically sound manner (Berry 1995). The most important aspects of an IPM

program include use of pesticides, host plant resistance, biological control, and

cultural practices (Holtzer et al. 1996). Researchers in the plant protection

disciplines often fail to address interactions associated with multiple pest complexes

that occur under field conditions. Consequently, such interactive effects are not

considered in economic evaluations of pest management programs. While the IPM

concept is familiar to scientists in many disciplines, investigations of pest complexes

have been hindered by unfamiliarity of scientists with experimental methodology

outside their respective disciplines (Higgins 1985). Furthermore, the complexity of

dealing with a diversity of species can be overwhelming (Newsom and Boethel

1985). To conduct meaningful interactive research, Higgins (1985) suggested that

pests with relatively simple life cycles and substantial data bases should be selected.

This should enhance the possibility that integrated management practices could be

developed.

Considerable research has addressed insect and disease interactions. Girdling of

soybean stems by threecornered alfalfa hopper increased the severity of stem

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anthracnose disease [Colletotrichum truncatum (Schw.)] (Russin et al. 1987).

Severity of stem canker disease [Diaporthe phaseolorum (Cke. & Ell.) Sacc. var.

caulivora Athow & Caldwell] was greater when soybean was defoliated by soybean

looper larvae (Russin et al. 1989a). In other studies, defoliation of soybean by

soybean looper increased numbers of juvenile and cyst stages of soybean cyst

nematode (Heterodera glycines Ichinohe) in roots and soil (Russin et al. 1989b).

Additionally, population densities of root-knot nematode [Meloidogyne incognita

(Kofoid & White) Chitwood] were greatest for plants defoliated by soybean looper

(Russin et al. 1993). Padgett et al. (1994) reported that red crown rot incidence and

stem canker severity were less on soybean defoliated by soybean looper, but stem

canker severity was increased when soybean stems were girdled by the threecornered

alfalfa hopper.

Research has also addressed interactions between weeds and pathogens. Some

weeds can serve as alternate hosts for the soybean stem canker (Black et al. 1996b)

and aerial blight (Black et al. 1996a) pathogens common in Louisiana soybean. The

stem canker pathogen was isolated from hemp sesbania and hairy indigo 0 (ndigofera

hirsuta Harvey), but not from johnsongrass or barnyardgrass [Echinochloa crus-galli

(L.) Beauv.] (Black et al. 1996b). Berner et al. (1991) reported less mycelial

growth by Calonectria crotalariae, the causal agent of red crown rot, exposed to the

herbicide glyphosate [iV-(phosphonomethyl)glycine]. Furthermore, incidence of red

crown rot in the soybean field was reduced when glyphosate was applied preplant.

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Paraquat (l,l'-

solani sclerotia production in laboratory studies and reduced Rhizoctonia foliar

blight severity in field-grown soybean with high disease pressure (Black et al.

1996c).

The relationship between weeds and insects has been investigated to a lesser

extent. Insect populations can be influenced by weeds. Collins and Johnson (1985)

reported that oviposition by velvetbean caterpillar adult moths was almost three

times greater in hemp sesbania, common cocklebur, and morningglory [Ipomoea

lacunosa L. and Ipomoea hederacea (L.) Jacq.] infested soybean than in weed-free

soybean. The nectar produced by morningglories ( Ipomoea sp.) and hemp sesbania

was a carbohydrate source required by soybean looper and velvetbean caterpillar for

normal egg production. Altieri et al. (1981) observed lower populations of the

predator Geocoris sp. in weed-free soybean in comparison with soybean infested

with sicklepod. Conversely, velvetbean caterpillar and southern green stink bug

[Nezara viridula (L.)] were more abundant in weed-free plots. The populations of

velvetbean caterpillar and stinkbug may have been lower due to reduced soybean

biomass under weedy conditions. Predator insects, Coleomegilla maculata

(DeGeer), Onus insidiosus (Say), and Nabis spp., also were more abundant in

weedy soybean, whereas Mexican bean beetle populations were higher in weed-free

soybean (Shelton and Edwards 1983).

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Herbicides also can affect insect populations. Soybean looper larvae survived

for 13.8 days on soybean leaves treated with fluazifop-butyl [(±)-2-[4-[[5-

(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid] compared with 15 days

for non-treated leaves (Angello et. al 1986b). Mexican bean beede larvae reared on

soybean plants treated with sethoxydim [2-[l-(ethoxyimino)butyl]-5-[2-(ethyIthio)

propyl]-3-hydroxy-2-cyclohexen-l-one] herbicide took longer to pupate than larvae

on non-treated plants, while larvae reared on soybean plants treated with fluazifop-

butyl had lower pupal weights (Angello et al. 1986a). Mexican bean beede adults

also preferred feeding on untreated soybean plants rather than plants treated with

sethoxydim or fluazifop-butyl. Huckaba and Coble (1990) observed lower larval

and adult soybean thrips [Sericothrips variabilis (Beach)] numbers on soybean

treated postemergence with napthalam [2-[(l-napthaIenylamino)carbonyl] benzoic

acid] plus dinoseb [2-(l-methylpropyl)-4,6-dinitrophenol]. Populations of flower

bug [Onus insidiosus (Say)], damsel bugs (Nabis spp.), leafhoppers (Cicadellidae),

and tarnished plant bug ( Lygus lineolaris Palisot de Beauvois) were reduced with

two applications of MSMA (monosodium salt of methylarsenic acid) herbicide

applied to cotton (Baker et al. 1985). Higher levels of sugarcane borer [Diatraea

saccharalis (F.)] in sugarcane have been reported following an application of 2,4-D

[(2,4-dichlorophenoxy)acetic acid] (Ingram et al. 1947). Less parasitism (18%) of

sugarcane borer eggs by Trickogramma minutum Riley occurred on plants treated

with 2.4-D.

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Little research has investigated the impact of both weeds and insects on soybean.

Helm et al. (1992) observed soybean yield reductions resulting from defoliation by

soybean looper or competition from velvetleaf (Abutilon theophrasti Medik.). In

some plots, soybean yield reduction by combination of the two pests was additive.

Higgins et al. (1984) investigated the influence of velvetleaf competition and

simulated green cloverworm defoliation on soybean. They concluded that the

economic injury level for green cloverworm did not change when velvetleaf was

present. Robbins et al. (1990) investigated the relationships among soybean cyst

nematode, threecornered alfalfa hopper, and three weeds (common cocklebur, pitted

momingglory, or sicklepod). Yield loss attributed to each pest was additive.

Herbicides, insecticides, and fungicides are commonly applied to prevent pests

from reaching levels of economic consequence. The effectiveness of a pesticide is

related to many environmental, chemical, and physical factors (Johnstone 1985).

Temperature and humidity affect the stability of a spray droplet containing the

pesticide. Wind coupled with small spray droplet size is conducive to pesticide

drift, resulting in poor control of the pest and possible injury to non-target crops.

The herbicide 2,4-D can injure susceptible plants several kilometers downwind of

the target area (Matthews 1992). To achieve optimum pest control, good coverage

of the target weed (Field and Bishop 1988), insect (Hutchins and Pitre 1985), or

pathogen (Royal et al. 1990; Walker and Huitink 1989) is often critical. Greater

soybean looper mortality with permethrin [(3-phenoxyphenyl)methyl(±)-m. trans-

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3-(2,2-dichloroethenyl)-2,2-dimethylcyclopropanecarboxylate] insecticide in the

upper one-third of the soybean canopy was associated with greater pesticide

coverage in this region (Hutchins and Pitre 1985). Walker and Huitink (1989)

reported greater propiconazole [l-[[2-(2,4-dichlorophenyl)-4-propyl-l,3-dioxolan-2-

yl]methyl]-l//-l,2,4-triazole] fungicide coverage provided greater control of sheath

blight (Rhizoctonia solani Kuhn) on rice (Oryza sativa L.). Surfactants and crop oil

concentrates have improved herbicide effectiveness by increasing droplet spread

across the leaf, thereby enhancing leaf surface penetration (Ashton and Monaco

1991). Variations in sprayer speed, spray volume and pressure, nozzle type and

orientation, and droplet size are among the many factors investigated as a means to

improve pesticide distribution within the crop canopy (Carlton et al. 1982; Kirk et

al. 1994; Salyani and Whitney 1989; Walker and Huitink 1989). Soybean looper

mortality with permethrin insecticide was greater when insecticide was applied with

ground equipment than by airplane (Hutchins and Pitre 1985). This difference was

associated with increased spray droplet deposition in the target area with the ground

equipment.

These studies were conducted under weed-free conditions and the impact of weeds

on insecticide deposition and subsequent pest control were not considered. Leaves and

stems of tall growing weeds potentially could intercept insecticide spray droplets

resulting in less insecticide coverage within the crop canopy. Royal et al. (1990)

reported that chlorothalonil (tetrachloroisophthalonitrile) fungicide deposition into

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peanut decreased and disease incidence increased with increasing densities of

sicklepod, Florida beggarweed [Desmodium tortuosum (Sw.) DC.], or common

cocklebur. Hutchins and Pitre (1984) observed less coverage with methomyl [S-

methy[-,/V-[(methylcarbamoyI)oxy]thioacetimidate] and methyl parathion [0,0-

dimethyl O-p-nitrophenyl phosphorothioate] insecticides within the median one-third of

the canopy in narrow-row (17.8 cm spacing) soybean compared with wide-row (96.5

cm spacing). Soybean looper larval mortality also was reduced in the median one-

third of the canopy in the narrow-row soybean in relation to the wide row soybean.

Parrot et al. (1973) reported greater deposits of azinphos-methyl [O, O-dimethyl 5-[(4-

oxo-1,2,3-benzotriazin-3(4//)-yl) methyl] phosphorodithioate] insecticide along with

higher boll weevil (Anthonomus grandis grandis Boheman) mortality on ffego bract

cotton in relation to normal bract cotton.

Weed and insect pests frequently occur together in the same field. Development of

integrated pest management strategies, however, has been most often directed toward a

single class of pest such as weeds or insects. When a multiple pest complex is present,

both individual and interactive effects on crop yield may occur. To economically

produce crops in the Southern United States, weeds, insects, and diseases must be

managed. A decision to apply a pesticide is based on the economic return expected

from controlling a specific pest without consideration of the subsequent impact on

other pests that may be present. Interactions among pests should be considered before

economic thresholds for pest complexes can be developed.

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This dissertation addresses the combined stresses of weed competition and

defoliation on weed and soybean growth and soybean yield. Previous research

investigating interactive effects of weeds and insects was conducted in Iowa (Higgins

et al. 1984) and Illinois (Helm et al. 1992) with indeterminate soybean and

velvetleaf. However, in Louisiana determinate soybean is grown and velvetleaf is of

minor importance. The agreement of these studies in respect to the additive

response to defoliation and weed interference on soybean yield would be of

significance considering the differences in environment and weed spectrum.

Additionally, the effect of weeds on thiodicarb [dimethyl-W, A’-fthiobis

[(methylimino)carbonyloxy]] bis[ethanimidothioate]] insecticide deposition into the

soybean canopy and subsequent soybean looper control is investigated. Results of

this research will help delineate the possible interactive effects of multiple pests in

soybean and help justify use of multiple pest control measures to maximize

economic returns in a soybean production system.

LITERATURE CITED

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Angello, A. M., J. R. Bradley, Jr., and J. W. VanDuyn. 1986a. Plant mediated effects of postemergence herbicides on Epilachna varivestis (Coleoptera: Coccinellidae). Environ. Entomol. 15:216-220.

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Angello, A. M., J. R. Bradley, Jr., and J. W. VanDuyn. 1986b. Plant mediated effects of postemergence herbicides on Pseudoplusia includens. J. Agric. Entomol. 3:61-65.

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Barrentine, W. L. 1974. Common cocklebur competition in soybean. Weed Sci. 22:600-603.

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Berner, D. K., G. T. Berggren, and J. P. Snow. 1991. Effects of glyphosate on Calonectria crotalariae and red crown rot of soybean. Plant Dis. 75:809-813.

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Black, B. D., J. L. Griffin, J. S. Russin, and J. P. Snow. 1996a. Weeds hosts for Rhizoctonia solani, causal agent for Rhizoctonia foliar blight of soybean (Glycine max). Weed Technol. 10:865-869.

Black, B. D., G. B. Padgett, J. S. Russin, J. L. Griffin, J. P. Snow, and G. T. Berggren, Jr. 1996b. Potential weed hosts for Diaporthe phaseolorum var. caulivora. causal agent for soybean stem canker. Plant Dis. 80:763-765.

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Black, B. D., J. S. Russin, J. L. Griffin, and J. P. Snow. 1996c. Herbicide effects on Rhizoctonia solani in vitro and Rhizoctonia foliar blight of soybean (Glycine max). Weed Sci. 44:711-716.

Boldt, P. E., K. D. Biever, and C. M. Ignoffo. 1975. Lepidopteran pest of soybeans: Consumption of soybean foliage and pods and development time. J. Econ. Entomol. 68:480-482.

Bozsa, R. C., L. R. Oliver, andT. L. Driver. 1989. Intraspecific and interspecific sicklepod (Cassia obtusifolia) interference. Weed Sci. 37:670-673.

Carlton, J. B., L. F. Bouse, W. J. Walla, and H. P. O'Neal. 1982. Mechanical factors affecting aerial spray coverage of soybeans. World Agric. Aviation. 9:35-38.

Collins, F. L. and S. J. Johnson. 1985. Reproductive response of caged adult velvetbean caterpillar and soybean looper to the presence of weeds. Agric. Ecosystems Environ. 14:139-149.

Fehr, W. R., C. E. Caviness, D. T. Burmood, and J. S. Pennington. 1971. Stage of development descriptions for soybeans. Glycine max (L.) Merrill. Crop Sci. 11:929- 931.

Field, R. J. and N. G. Bishop. 1988. Promotion of stomatal infiltration of glyphosate by an organosilicone adjuvant reduces the critical rainfall period. Pestic. Sci. 24:55-62.

Helm, C. G., M. Kogan, D. W. Onstad, L. M. Max, and M. R. Jeffords. 1992. Effects of velvetleaf competition and defoliation by soybean looper (Lepidoptera: Noctuidae) on yield of indeterminate soybean. J. Econ. Entomol. 85:2433-2439.

Herzog, D. C. 1980. Sampling soybean looper on soybean. Pages 141-168 in M. Kogan and D. C. Herzog, eds. Sampling Methods in Soybean Entomology. Springer- Verlag, New York.

Higgins, R. A. 1985. Approaches to studying interactive stresses caused by insects and weeds. Pages 641-649 in R. L. Shibles, ed. Proc. World Soybean Res. Conf. Westview Press, Boulder, CO.

Higgins, R. A., L. P. Pedigo, and D. W. Staniforth. 1984. Effect of velvetleaf competition and defoliation simulating a green cloverworm (Lepidoptera: Noctuidae) outbreak in Iowa on indeterminate soybean yield, yield components, and economic decision levels. Environ. Entomol. 13:917-925.

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Hinson, K., R. H. Nino, and K. J. Booto. 1978. Characteristics of removed leaflets and yield response of artificially defoliated soybeans. Proc. Soil Crop Sci. Soc. Fla. 37:104-109.

Holtzer, T. O., R. L. Anderson, M. P. McMullen, and F. B. Peairs. 1996. Integrated pest management of insects, plant pathogens, and weeds in dryland cropping systems of the great plains. J. Prod. Agric. 9:200-208.

Huckaba, R. M. and H. D. Coble. 1990. Effect of herbicides on soybean thrips {Sericothrips variabilis) in soybeans (Glycine max). Weed Technol. 4:475-477.

Hutchins, S. H. and H. N. Pitre. 1984. Effects of soybean row spacing on spray penetration and efficacy of insecticides applied with ground and aerial equipment. Environ Entomol. 13:948-953.

Hutchins, S. H. and H. N. Pitre. 1985. Differences in penetration and efficacy of insecticide sprays applied by aerial and ground equipment to soybean. J. Entomol. Sci. 20:34-41.

Hymowitz, T. and J. R. Harlan. 1983. Introduction of soybeans to North America by Samuel Bowen in 1765. Econ. Bot. 37:371-379.

Ingram, J. W., E. K. Bynum, and L. J. Charpentier. 1947. Effect of 2,4-D on sugarcane borer. J. Econ. Entomol. 40:745-746.

Johnstone, D. R. 1985. Physics and meteorology. Pages 35-67 in P. T. Haskell, ed. Pesticide Application: Principles and Practice. Clarendon Press, Oxford.

Jordan, T. N., H. D. Coble, and L. M. Wax. 1987. Weed control. Pages 429-460 in J. R. Wilcox, ed. Soybeans: Improvement, Production, and Uses. Am. Soc. Agron., Crop Sci. Soc. Am., and Soil Sci. Soc. Am., Madison, WI.

Kalton, R. R., C. R. Weber, and J. C. Eldredge. 1949. The effect of injury simulating hail damage to soybeans. Iowa Agric. Exp. Stn. Res. Bull. 359:736-796.

Kirk, I. W., L. F. Bouse, J. B. Carlton, E. Franz, M. A. Latheef, J. E. Wright, and D. A. Wolfenbarger. 1994. Within-canopy spray distribution from fixed-wing aircraft. Transactions of ASAE. 37:745-752.

Matthews, G. A. 1992. Pesticide Application Methods, 2nd Edition. Longman Singapore Publishers. Singapore. 405 pp.

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McWhorter, C. G. and J. M. Anderson. 1979. Hemp sesbania ( Sesbania exaltata) competition in soybeans (Glycine max). Weed Sci. 27:58-64.

McWhorter, C. G. and E. E. Hartwig. 1972. Competition of johnsongrass and cocklebur with six soybean varieties. Weed Sci. 20:56-59.

Morrison, W. C. and L. L. McCormick. 1996. The soybean crop, history in Louisiana. Pages 1-8 in J. Honeycutt, ed. Louisiana Soybean Handbook. Louisiana Coop. Ext. Serv., Baton Rouge. Publ. 2624.

Mosier, D. G. and L. R. Oliver. 1995. Common cocklebur ( Xanthium strumarium) and entireleaf momingglory (Ipomoea hederacea var. integriuscula) interference on soybeans {Glycine max). Weed Sci. 43:239-246.

Nave, W. R. and L. M. Wax. 1971. Effect of weeds on soybean yield and harvesting efficiency. Weed Sci. 19:533-535.

Newsom, L. D and D. J. Boethel. 1985. Interpreting multiple pest interactions in soybean. Pages 232-255 in R.E. Frisbie and P.L. Adkisson, eds. Integrated Pest Management in Major Agricultural Systems. Texas Agric. Exp. Stn., College Station, TX. MP-1616.

Ostlie, K. R. and L. P. Pedigo. 1984. Water loss from soybeans after simulated and actual insect defoliation. Environ. Entomol. 13:1675-1680.

Padgett, G. B., J. S. Russin, J. P. Snow, D. J. Boethel, and G. T. Berggren. 1994. Interactions among the soybean looper (Lepidoptera: Noctuidae), threecornered alfalfa hopper (Homoptera: Membracidae), stem canker, and red crown rot in soybean. J. Entomol. Sci. 29:110-119.

Parrott, W. L., J. N. Jenkins, and D. B. Smith. 1973. Frego bract cotton and normal bract cotton: How morphology affects control of boll weevils by insecticides. J. Econ. Entomol. 66:222-225.

Phillips, D. V. 1989. Fungal leaf spots. Pages 14-21 in P. D. Colyer, ed. Soybean Disease Atlas, 2nd Edition. Louisiana State Univ. Agric. Center Exp. Stn., Baton Rouge.

Poston, F. L., L. P. Pedigo, R. B. Pearce, and R. B. Hammond. 1976. Effects of artificial and insect defoliation on soybean net photosynthesis. J. Econ. Entomol. 69:109-112.

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Reid, J. C. and G. L. Greene. 1973. The soybean looper: pupal weight, development time, and consumption of soybean foliage. Fla. Entomol. 56:203-206.

Robbins, R. T., L. R. Oliver, and A. J. Mueller. 1990. Interaction among a nematode (Heterodera glycines), an insect, and three weeds in soybean. J. Nematol. Suppl. 22:729-734.

Ross, J. P. 1987. Viral and bacterial diseases. Pages 729-755 in J. R. Wilcox, ed. Soybeans: Improvement, Production, and Uses, 2nd Edition. Am. Soc. Agron., Crop Sci. Soc. Am., and Soil Sci. Soc. Am., Madison, WI.

Ross, M. A. and C. A. Lembi. 1985. Applied Weed Science. Burgess Publ. Co., Minneapolis, MN. 340 pp.

Royal, S. S., B. J. Brecke, D. L. Colvin, and F. M. Shokes. 1990. The effects of three weed species on fungicide deposition in peanuts. Proc. South. Weed Sci. Soc. 43:103.

Russin, J. S., M. B. Layton, and D. J. Boethel. 1989a. Severity of soybean stem canker disease affected by insect-induced defoliation. Plant Dis. 73:144-147.

Russin, J. S., M. B. Layton, D. J. Boethel, E. C. McGawley, J. P. Snow, and G. T. Berggren. 1989b. Development of Heterodera glycines on soybean damaged by soybean looper and stem canker. J. Nematol. 21:108-114.

Russin, J. S., E. C. McGawley, and D. J. Boethel. 1993. Population development of Meloidogyne incognita on soybean defoliated by Pseudoplusia includens. J. Nematol. 25:50-54.

Russin, J. S., L. D. Newsom, D. J. Boethel, and A. N. Sparks, Jr. 1987. Multiple pest complexes on soybean: Influences of threecornered alfalfa hopper injury on pod and stem blight and anthracnose diseases and seed vigour. Crop. Prot. 6:320-325.

Salyani, M. and J. D. Whitney. 1989. Spraying speed effect on deposition inside citrus trees. ASAE Paper No. 89-1048. ASAE, St. Joseph, MI.

Sanders, D. E. 1996. Most troublesome weeds in Louisiana. Pages 105-108 in J. Honeycutt, ed. Louisiana Soybean Handbook. Louisiana Coop. Ext. Serv., Baton Rouge. Publ. 2624.

Shelton, M. D. and C. R. Edwards. 1983. Effects of weeds on the diversity and abundance of insects in soybeans. Environ. Entomol. 12:296-298.

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Smith, K. J. and W. Huyser. 1987. World distribution and significance of soybean. Pages 1-23 in J. R. Wilcox, ed. Soybeans: Improvement, Production, and Uses. Am. Soc. Agron., Crop Sci. Soc. Am., and Soil Sci. Soc. Am., Madison, WI.

Steffey, K. L., M. E. Gray, and L. G. Higley. 1994. Introduction to identification and diagnosis of injury. Pages 17-19 in L. G. Higley and D. J. Boethel, eds. Handbook of Soybean Insect Pests. Entomol. Soc. Am., Lanham, MD.

Thurlow, D. L. and G. A. Buchanan. 1972. Competition of sicklepod with soybeans. Weed Sci. 20:379-384.

Todd, J. W. and L. W. Morgan. 1972. Effects of hand defoliation on yield and seed weight o f soybeans. J. Econ. Entomol. 65:567-570.

Turnipseed, S. G. and M. Kogan. 1987. Integrated control of insect pests. Pages 779-817 in J. R. Wilcox, ed. Soybeans: Improvement, Production, and Uses, 2nd Edition. Am. Soc. Agron., Crop Sci. Soc. Am., and Soil Sci. Soc. Am., Madison, WI.

Tynes, J. S. and D. J. Boethel. 1996. Soybean insect pests. Pages 131-138 in J. Honeycutt, ed. Louisiana Soybean Handbook. Louisiana Coop. Ext. Serv., Baton Rouge. Publ. 2624.

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Whitam, K. 1996. Soybean diseases. Pages 116-124 in J. Honeycutt, ed. Louisiana Soybean Handbook. Louisiana Coop. Ext. Serv., Baton Rouge. Publ. 2624.

Wier, A. T. and D. J. Boethel. 1996. Symbiotic nitrogen fixation and yield of soybean following defoliation by soybean looper (Lepidoptera: Noctuidae) during pod or seed development. J. Econ. Entomol. 89:525-535.

Willard, T. S. and J. L. Griffin. 1993. Soybean (

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SOYBEAN {Glycine max) RESPONSE TO WEED INTERFERENCE AND DEFOLIATION

INTRODUCTION

Soybean [Glycine max (L.) Merr.] in the Gulf Coast states is exposed to stress

imposed by more species of pests, during longer periods of the year, and more

frequently than any other area of the United States (Newsom and Boethel 1985).

Development of integrated pest management strategies has been most often directed

toward a single class of pest such as weeds, insects, or pathogens. Weeds reduce

soybean yield through competition for water, nutrients, and light. Williams and

Hayes (1984) reported soybean yield reductions of 59 to 88% from johnsongrass

[Sorghum halepense (L.) Pers.] competition. McWhorter and Hartwig (1972)

reported yield losses of 23 to 43% from johnsongrass competition with differences

attributed to soybean variety. Common cocklebur ( Xanthium strumarium L.) reduced

yields of the same varieties 63 to 75%. Full season competition by common

cocklebur populations of 3300, 6600, 13000, and 26000 plants/ha reduced yields 10,

28, 43, and 52%, respectively (Barrentine 1974). Full season competition from hemp

sesbania [Sesbania exaltata (Raf.) Rybd. ex A. W. Hill] at densities up to 5500

plants/ha did not reduce soybean yields; however, populations of 8100 to 129000

plants/ha reduced yield 10 to 80% (McWhorter and Anderson 1979).

In Louisiana, insects requiring control measures most frequently include the

soybean looper [Pseudoplusia includens (Walker)] and velvetbean caterpillar

20

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[Anticarsia gemmatalis (Hubner)] (Baldwin et al. 1994). These two pests in their

larval stage are most injurious to soybean from August to October (Steffey et al.

1994). A single soybean looper larva can consume up to 114 cm2 of leaf tissue

before pupation (Boldt et al. 1975; Reid and Greene 1973). In approximately 90%

of studies investigating the effect of defoliation on soybean yield, artificial injury

was used to simulate actual insect defoliation (Ostlie and Pedigo 1984). Hinson et

al. (1978) observed soybean seed yield losses of 8, 21, 31, and 30% with 67%

artificial defoliation at 3, 17, 31, and 42 days after flowering, respectively, whereas

yields were reduced only 4% with 33% defoliation. Tumipseed and Kogan (1987)

reported that most studies have shown soybean to be most tolerant to defoliation up

to R3 (pod initiation) (Fehr et al. 1971), and that prior to R3, soybean can tolerate

up to 30% defoliation.

Little research has addressed weed and insect relationships. Collins and Johnson

(1985) reported that oviposition by velvetbean caterpillar adult moths was increased

nearly three-fold when exposed to hemp sesbania, common cocklebur, and

momingglory [Ipomoea lacunosa L. and Ipomoea hederacea (L.) Jacq.] infested

soybean compared with weed-free soybean. Helm et al. (1992) observed soybean

yield reductions from defoliation by both soybean looper and velvetleaf {Abutilon

theophrasti Medicus) competition, and combinations of the two had an additive

effect in some plots. Higgins et al. (1984a) investigated the influence of velvetleaf

competition and simulated green cloverworm [Plathypena scabra (Fabricius)]

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defoliation on soybean. Simulated defoliation was achieved by removing leaf area

with cork borers during the same time period green cloverworms were defoliating

surrounding fields. They concluded that economic injury levels for green

cloverworm did not change when velvetleaf was present. Robbins et al. (1990)

investigated the relationships among soybean cyst nematode, threecornered alfalfa

hopper, and one of three weeds [common cocklebur, pitted morningglory (Ipomoea

lacunosa L.), or sicklepod [Senna obtusifolia (L.) Irwin and Barneby]]. The yield

loss attributed to each pest was additive.

To manage the diverse pest problems of soybean in Louisiana, both herbicides

and insecticides are used. Research to delineate the effects of weeds and defoliating

insects alone and in combination will help justify pest control practices. Previous

research investigating interactive effects of weeds and insects were conducted in

Iowa (Higgins et al. 1984a) and Illinois (Helm et al. 1992) with indeterminate

soybean and velvetleaf. However, in Louisiana determinate soybean is grown and

velvetleaf is of minor importance. The objective of this research was to determine

the effect of full-season johnsongrass, common cocklebur, and hemp sesbania

interference and simulated insect defoliation on weed and soybean growth

parameters and soybean yield.

MATERIALS AND METHODS

Field experiments were conducted at the Ben Hur Research Farm near Baton

Rouge, Louisiana in 1994 and 1995 on a Mhoon silty clay loam soil (fine-silty.

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mixed nonacidic, thermic Fluventic Haplaquepts). 'Asgrow 6785', a determinate

group VI soybean, was planted 16 June 1994 and 14 June 1995 in rows spaced 76

cm apart. Metalochlor [2-chlonWV-(2-ethyl-6-methylphenyl)-/V-(2-methoxy-l-

methylethyl) acetamide] at 1.1 kg ai/ha and glyphosate [/V-(phosphonomethyl)

glycine] at 1.8 kg ai/ha were broadcast as a tank mix over the entire experimental

area immediately after planting both years to control emerged weeds and to provide

residual control of annual grasses. Seeds of johnsongrass, common cocklebur, and

hemp sesbania were planted in peat pellets in a greenhouse the same day that

soybean was planted in the field. Two weeks after planting, seedlings were

transplanted five cm from soybean in each row of the four-row plot. The plastic

wrap was removed from each peat pellet prior to transplanting to facilitate weed root

growth. Plots were maintained free of other weeds by hand removal and mechanical

cultivation both years. Methyl-parathion ( 0 ,0-dimethyl-(9-p-nitrophenyl

phosphorothioate) and thiodicarb [dimethyl-#,/V,-[thiobis[(methylimino)

carbonyloxy]] bis[ethanimidothioate]] insecticides were applied as needed to control

insects.

The experimental design was a randomized complete block with a factorial

arrangement of treatments replicated four times. Plots were four rows wide and 6 m

in length. Weed treatments (Factor A) consisted of johnsongrass, common

cocklebur, and hemp sesbania at densities of 15, 3, and 12 plants/6 m of row,

respectively, and a weed-free control. These densities were selected because they

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were sufficient to reduce soybean yield approximately 20% in previous studies

(Barrentine 1974; McWhorter and Anderson 1979; McWhorter and Hartwig 1972;

Oliver 1988; Williams and Hayes 1984). The intent was to evaluate low to

moderate weed densities that economically may not justify control measures. Hemp

sesbania and some common cocklebur did not survive transplanting the second year.

Therefore, hemp sesbania was not evaluated and cocklebur was limited to one row

per plot in 1995. Defoliation levels (Factor B) were imposed by removing 0, 1, or

2 leaflets per soybean trifoliate to approximate 0, 33, and 66% defoliation,

respectively. No preference was given to removing lateral or center leaflets from

each trifoliate. Simulated defoliation (Factor C) was initiated at R2 (full bloom) or

R5 (beginning seed development) soybean growth stages (Fehr et al. 1971), which is

when soybean loopers commonly defoliate soybean (Steffey et al. 1994). Soybean

plants were manually defoliated at R2 on 16 to 18 August 1994 and 14 to 16 August

1995. Defoliation at R5 was performed on 7 to 9 September 1994 and 11 to 13

September 1995. All treatments were defoliated by replication because several days

were required to complete the procedure. Soybean from only the two center rows of

each plot was defoliated. At each defoliation stage, soybean plants from a one-m

section of row was removed from selected plots to determine actual leaf area

removal for each defoliation level. Leaf area was measured on a Li-Cor LI-3100

Area Meter1. Measurements revealed that removal of one leaflet per trifoliate

‘LICOR, Inc. Lincoln, NE 68504.

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resulted in a reduction in leaf area of 31 to 36% and two leaflet removal a 64 to

69% reduction.

Soybean and weed heights were measured three weeks after defoliation at R5.

Soybean, common cocklebur, and hemp sesbania heights were measured from the

ground to the apex of the plant, whereas johnsongrass was measured from the

ground to the tip of the longest leaf. Above-ground portions of weeds were

harvested on 21 October 1994 and 17 October 1995, dried, and weighed. Soybean

was harvested on 2 November 1994 and 26 October 1995 using a plot combine.

Soybean seed moisture and 100-seed weight were measured after harvest. Seed

yield was adjusted to 13% moisture. Data were subjected to the General Linear

Models procedure (SAS Institute 1988) to test for main treatment effects and

interactions among treatments. For soybean seed yield, weed by year and

defoliation stage by year interactions were observed, so all data are presented

separately for 1994 and 1995. Means were separated using Fisher’s Protected LSD

at the 0.05 probability level. Defoliation level responses were further evaluated

using single degree-of-ffeedom contrasts.

RESULTS AND DISCUSSION

Johnsongrass, common cocklebur, and hemp sesbania height and dry weight

were not affected by defoliation level or defoliation stage (Table 2.1). It was

anticipated that additional light interception by weeds in defoliated soybean would

increase weed growth. Higgins et al. (1984b) also reported no significant

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ON - 97 99 ______g/plant - 110 111 112 123 117 215 a 113 0.5276 0.9936 ------55 b - 141 145 100 99 174 174 a 65 b 0.8819 0.3064 0.3877 ------c m Height Dry weight -- 159 143 155 143 114 98 158 154 127 127 b 114 b 168 168 a 0.6432 0.7631 0.2613 PiF) ( (PiF) R5 Hemp sesbaniab 0 160 144 33 Quadratic Common cocklebur 66 Linear Johnsongrass 177 a* Factor Level 1994 1995 1994 1995 maturity maturity as influenced by soybean defoliation level and in stage and 1994 1995, Defoliation level (%) Contrast Defoliation stage(table con’d.) R2‘ Weed None Table 2.1. Weed height three weeks after defoliation at R5 and dry weight at soybean

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -j 0.1328 0.5016 0.3913 0.97960.9732 0.1827 0.7922 0.0001 P*F 0.3572 0.6490 0.1141 0.7628 0.0671 0.0001 0.3433 0.0001 "Means "Means followed by the same letter within a column are not significantly different using 'Felir et al. 1971 bData availablebData for only. 1994 Fisher’s Protected LSD at PsO.O5. Level by stage 0.1293 Weed by level by stage 0.2799 0.2638 0.9093 0.9843 Defoliation levelDefoliation stageWeed by stage 0.4797 0.3714 0.9443 0.4840 0.5344 0.6847 0.6075 Weed Weed by level

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differences in velvetleaf dry weight in response to simulated green cloverworm

defoliation. They did, however, observe linear increases in velvetleaf leaf area and

leaf number as soybean defoliation level increased. In another study, shading of

three week old velvetleaf plants reduced height, leaf number, and dry weight (Bello

et al. 1995). Since weeds in the present study were taller than soybean at

defoliation, light interception was not a limiting factor to weed growth. Mosier and

Oliver (1995) observed that most of the leaf area of common cocklebur competing

with irrigated soybean was in the upper portion of the plant 12 weeks after planting.

Differences in height and dry weight were observed among weeds. Johnsongrass

was taller than common cocklebur both years, but dry weight of common cocklebur

was 3.3 times that of johnsongrass in 1994 (Table 2.1). For weed height and dry

weight, none of the possible interactions were significant indicating that weeds

responded similarly to defoliation.

Soybean height three weeks after defoliation at R5 was not influenced by weed

interference, defoliation level, or defoliation stage either year (Table 2.2). Soybean

yield response to weed interference and defoliation was additive both years. No

interaction was observed between weeds, defoliation levels, or defoliation stages

(Table 2.2). In 1994, averaged across defoliation levels and stages, johnsongrass,

common cocklebur, and hemp sesbania reduced soybean yield 30, 15, and 14%,

respectively, compared with weed-free soybean. In 1995, johnsongrass reduced

yield 35 %, but common cocklebur interference resulted in yield equivalent to weed-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. lo vo i - 10.85 10.55 10.60 ------0.7938 0.0001 10.49 10.49 1 g/100 12.77 10.85 11.87 9.78 12.80 12.78 10.62 12.72 ------______

- 2600 13.392010 11.57 0.7668 0.3802 2180 b 12.59 2410 a 2600 a ------2340 2350 b kg/ha 92 2310 b 2590 a 12.53 90 2720 a* 0.8449 0.1182 0.2380 0.0001 0.0001 0.0001 ------Height Seed yield Seed weight c m 8787 91 91 2550 2400 2280 87 91 1910 c 1700 b 12.67 88 1.000 0.6431 ------(P±F) (P±F) R5 87 91 R2‘ 87 90 2310 Quadratic 0 66 87 90 2030 Hemp sesbamah 87 90 33 Common cocklebur Linear Johnsongrass Defoliation stage (table (table con’d.) Defoliation level (%) Contrast influenced by influenced weed interference and soybean defoliation level and in stage and 1994 1995. Factor Level 1994 1995 1994 1995 1994 1995 Weed None 86 Table 2.2. Soybean height three weeks after defoliation at R5 and soybean yield and weight as 100-seed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission u> o 0.1073 0.2041 0.0001 0.1270 0.6946 0.7421 0.2390 0.0518 0.0632 0.0001 0.4142 0.0170 0.2644 0.0081 0.86150.8122 0.8851 0.3449 0.00010.0001 0.7032 PzF 0.6254 0.7421 0.0001 0.7920 0.4644 0.1342 0.0001 0.8227 0.6609 0.4461 0.0877 0.6640 0.0856 0.4883 0.4899 0.2333 0.8975 0.4866 Pi0.05. 'Fehretal. 1971 bData available bData for only. 1994 ‘Means followed by the same letter within a column are not significantly different using Fisher’s Protected Level Level by stage 0.3428 0.2415 LSD at Weed by level by stage 0.7289 Defoliation stage Defoliation level Weed Weed by stage Weed by level

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31

free soybean. The lack of response for common cocklebur the second year is not

apparent, but may be related to the lower yield for the weed-free control that year.

The weed densities selected for this study were expected to reduce soybean yield

approximately 20% (Barrentine 1974; McWhorter and Anderson 1979; McWhorter

and Hartwig 1972; Oliver 1988; Williams and Hayes 1984). It is possible that

greater differences and interactions may have been observed if higher weed densities

had been included. Contrast analysis revealed a linear relationship between soybean

seed yield and defoliation level both years (Table 2.2). Defoliation levels of 33 and

66% reduced yield 6 and 20%, respectively, in 1994 and 12 and 23%, respectively,

in 1995. These data agree with those of Tumipseed (1972) and Todd and Morgan

(1972) in which soybean yield decreased as defoliation level increased. In the

present study, soybean yield in 1994 was equivalent for defoliation at R2 and R5,

but in 1995 defoliation at R5 resulted in 10% less yield than defoliation at R2. Fehr

et al. (1977) also reported greater yield losses with defoliation at R5 than R2. In

another study, soybean yield was reduced 40% with 100% defoliation at temporal

mid-point of seed filling (R6.3), but only 20% at three-quarter point of seed filling

(R6.6), (Board et al. 1994). Soybean defoliated at R2 in 1995 in the current study

may have compensated for the reduction in photosynthetic leaf area by delaying

senescence and by delaying the decline of photosynthetic rates associated with aging

(Higley 1992). These responses would result in sustained photosynthate production

later into the growing season.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32

Regrowth in the early-defoliated plants may also help explain why yield was

equivalent for the defoliation stages the first year and greatest for defoliation at R2

the second year. By the time soybean was in the R5 growth stage, during seed fill,

plants had compensated for defoliation at R2 by producing new leaf area. Regrowth

was not measured in this study, however, additional growth was visually apparent in

the defoliated plots. In a study where soybean was defoliated 30% by soybean

looper at V I1, leaf area of defoliated soybean was equivalent to leaf area of non­

defoliated soybean 22 days after defoliation (Russin et al. 1989), indicating

regrowth.

Soybean seed weight was not influenced by weed interference either year (Table

2.2). Yield loss may be partially explained by reductions in seed weight caused by

defoliation. As with yield, a linear decrease in seed weight was observed as

defoliation level increased. Seed weight was reduced at least 4.6 and 11.4% by 33

and 66% defoliation, respectively. In 1995, seed weight was also 3.3% lower in

soybean defoliated at R5 than at R2. As with soybean yield, 100-seed weight was

equivalent for defoliation stages in 1994. Tumipseed (1972) also reported seed

weight reductions for soybean defoliated 33% at pod-fill, but not when defoliated at

bloom stage. Board et al. (1994) reported seed weight reductions of 34% with

complete defoliation at R6.3, however, only a 14% reduction was observed at R6.6.

In other studies, yield loss by defoliation during the early reproductive stages up to

R4 was primarily related to reduced pod numbers (Board and Harville 1993: Goli and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 33

Weaver 1986), whereas defoliation during R5 and R6 was related to reduced seed

weight (Goli and Weaver 1986).

Results of this study indicate that weeds and defoliation each can negatively

influence soybean yield when both stresses were imposed. Johnsongrass, common

cocklebur, and hemp sesbania reduced soybean yield 14 to 30% when compared with

no weed interference. The degree of yield reduction was dependent on the weed and

defoliation level, and to a lesser extent on defoliation stage. Soybean yield response

to weed interference and simulated insect defoliation was additive in this study.

Velvetleaf competition and insect defoliation have also resulted primarily in additive

responses on soybean yield in Iowa (Higgins et al. 1984a) and Illinois (Helm et al.

1992) studies. Furthermore, in Arkansas soybean yield losses with combinations of a

nematode, threecomered alfalfa hopper, and weeds were additive (Robbins et al.

1990). The agreement of these studies, especially with the diversity in soybean type

and environmental conditions, clearly show the importance o f controlling both weeds

and defoliating insects in soybean pest management programs.

LITERATURE CITED

Baldwin, J. L., D. J. Boethel, and B. R. Leonard. 1994. Control Soybean Insects 1994. Louisiana Coop. Ext. Serv., Baton Rouge. Publ. 2211. 15 p.

Barrentine, W. L. 1974. Common cocklebur competition in soybean. Weed Sci. 22:600-603.

Bello, I. A., M. D. K. Owen, H. M. Hatterman-Valenti. 1995. Effect of shade on velvetleaf (Abutilon theophrasti) growth, seed production, and dormancy. Weed Technol. 9:452-455.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34

Board, J. E. and B. G. Harville. 1993. Soybean yield component responses to a light interception gradient during reproductive period. Crop Sci. 33:772-777.

Board, J. E., A. T. Wier, and D. J. Boethel. 1994. Soybean yield reductions caused by defoliation during mid to late seed filling. Agron. J. 86:1074-1079.

Boldt, P. E., K. D. Biever, and C. M. Ignoffo. 1975. Lepidopteran pest of soybeans: Consumption of soybean foliage and pods and development time. J. Econ. Entomol. 68:480-482.

Collins, F. L. and S. J. Johnson. 1985. Reproductive response of caged adult velvetbean caterpillar and soybean looper to the presence of weeds. Agric. Ecosystems Environ. 14:139-149.

Fehr, W. R., C. E. Caviness, D. T. Burmood, and J. S. Pennington. 1971. Stage of development descriptions for soybeans Gtycine max (L.) Merrill. Crop Sci. 11:929-931.

Fehr, W. R., C. E. Caviness, and J. J. Vorst. 1977. Response of indeterminate and determinate soybean cultivars to defoliation and half-plant cut-off. Crop Sci. 17:913-917.

Goli, A. and D. B. Weaver. 1986. Defoliation responses of determinate and indeterminate late-planted soybeans. Crop Sci. 26:156-159.

Helm, C. G., M. Kogan, D. W. Onstad, L. M. Max, and M. R. Jeffords. 1992. Effects of velvetleaf competition and defoliation by soybean looper (Lepidoptera: Noctuidae) on yield of indeterminate soybean. J. Econ. Entomol. 85:2433-2439.

Higgins, R. A., L. P. Pedigo, and D. W. Staniforth. 1984a. Effect of velvetleaf competition and defoliation simulating a green cloverworm (Lepidoptera: Noctuidae) outbreak in Iowa on indeterminate soybean yield, yield components, and economic decision levels. Environ. Entomol. 13:917-925.

Higgins, R. A., D. W. Staniforth, and L. P. Pedigo. 1984b. Effects of weed density and defoliated or undefoliated soybeans (Glycine max) on velvetleaf (Abutilon theophrasti) development. Weed Sci. 32:511-519.

Higley, L. G. 1992. New understandings of soybean defoliation and their implication for pest management. Pages 56-65 in L. G. Copping, M. B. Green, and R. T. Rees, eds. Pest Management in Soybean. Elsevier Applied Science, London, England.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35

Hinson, K., R. H. Nino, and K. J. Booto. 1978. Characteristics of removed leaflets and yield response of artificially defoliated soybeans. Proc. Soil Crop Sci. Soc. Fla. 37:104-109.

McWhorter, C. G. and J. M. Anderson. 1979. Hemp sesbania (Sesbania exaltata) competition in soybeans (Glycine max). Weed Sci. 27:58-64.

McWhorter, C. G. and E. E. Hartwig. 1972. Competition of johnsongrass and cocklebur with six soybean varieties. Weed Sci. 20:56-59.

Mosier, D. G. and L. R. Oliver. 1995. Soybean (Glycine max) interference on common cocklebur (Xanthium strumarium) and entireleaf morningglory (Ipomoea hederacea var. integriuscula). Weed Sci. 43:402-409.

Newsom, L. D and D. J. Boethel. 1985. Interpreting multiple pest interactions in soybean. Pages 232-255 in R. E. Frisbie and P. L. Adkisson, eds. Integrated Pest Management in Major Agricultural Systems. Texas Agric. Exp. Stn., College Station, TX. MP-1616.

Oliver, L. R. 1988. Principles of weed threshold research. Weed Technol. 2:398- 403.

Ostlie, K. R. and L. P. Pedigo. 1984. Water loss from soybeans after simulated and actual insect defoliation. Environ. Entomol. 13:1675-1680.

Reid, J. C. and G. L. Greene. 1973. The soybean looper: pupal weight, development time, and consumption of soybean foliage. Fla. Entomol. 56:203-206.

Robbins, R. T., L. R. Oliver, and A. J. Mueller. 1990. Interaction among a nematode (Heterodera glycines), an insect, and three weeds in soybean. J. Nematol. Suppl. 22:729-734.

Russin, J. S., M. B. Layton, and D. J. Boethel. 1989. Severity of soybean stem canker disease affected by insect-induced defoliation. Plant Dis. 73:144-147.

SAS Institute. 1988. SAS/STAT User's Guide, Release 6.03 Edition. SAS Institute, Cary, NC. 1028 p.

Steffey, K. L., M. E. Gray, and L. G. Higley. 1994. Introduction to identification and diagnosis of injury. Pages 17-19 in L. G. Higley and D. J. Boethel, eds. Handbook of Soybean Insect Pests. Entomol. Soc. Am.. Lanham. MD.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36

Todd, J. W. and L. W. Morgan. 1972. Effects of hand defoliation on yield and seed weight of soybeans. J. Econ. Entomol. 65:567-570.

Tumipseed, S. G. 1972. Response of soybeans to foliage losses in South Carolina. J. Econ. Entomol. 65:224-229.

Tumipseed, S. G. and M. Kogan. 1987. Integrated control of insect pests. Pages 779-817 in J. R. Wilcox, ed. Soybeans: Improvement, Production, and Uses, 2nd Edition. Am. Soc. Agron., Crop Sci. Soc. Am., and Soil Sci. Soc. Am., Madison, WI.

Williams, C. S. and R. M. Hayes. 1984. Johnsongrass (Sorghum halepense) competition in soybeans (Glycine max). Weed Sci. 32:498-501.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER III

HEMP SESBANIA AND SICKLEPOD INFLUENCE ON INSECTICIDE DEPOSITION AND SOYBEAN LOOPER CONTROL

INTRODUCTION

Weed and insect control can be major expenses in soybean [Glycine max (L.)

Merr.]. Depending on location and agronomic practices, Louisiana soybean

producers annually spend from 62 to 86 dollars/ha for herbicides and from none to

32 dollars/ha for insecticides (Wegenhoft 19%). Weeds reduce soybean yield

through competition for water, nutrients, and light. Hemp sesbania [Sesbania

exaltata (Raf.) Rybd. ex A.W. Hill] and sicklepod [Senna obtusifolia (L.) Irwin and

Barneby] are among the most troublesome weeds in several southern states (Bridges

and Baumann 1992). Hemp sesbania is an erect, annual, herbaceous plant that will

reach a height of four m (Anonymous). Full season competition from hemp

sesbania at densities up to 5500 plants/ha did not reduce soybean yields, but

populations of 8100 to 129000 plants/ha reduced yield 10 to 80%, respectively

(McWhorter and Anderson 1979). Sicklepod is an annual, herbaceous plant that

grows up to two m tall (Anonymous). Densities of 32000 and 98000 sicklepod/ha

reduced soybean yield 31 and 41%, respectively (Bozsa et al. 1989).

In Louisiana, soybean loopers [Pseudoplusia includens (Walker)] occasionally

reach population levels requiring the use of insecticides (Baldwin et al. 1996).

Soybean loopers are defoliating insects that habitually feed in the lower one-half to

two-thirds of the soybean canopy (Herzog 1980). This pest reduces the

37

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photosynthetically active leaf area of a plant, thereby reducing yield in some cases.

Wier and Boethel (1996) observed 48 and 95% yield losses when soybean looper

defoliated 'Clark' soybean 74 and 94%, respectively, during full bloom to pod

development (R2 to R5) (Fehr et al. 1971).

Even though weeds and insects frequently coexist in the same field, development

of integrated pest management strategies has been most often directed toward a

single class of pest. Furthermore, a decision to apply a pesticide is based on the

economic return expected from controlling a specific pest without consideration of

the subsequent impact on other pests that may be present.

Weed presence in a soybean field may influence insecticide deposition into

canopy resulting in reduced insecticide efficiency. This may be especially important

for weeds such as hemp sesbania and sicklepod, which are tall-growing and capable

of producing a canopy over soybean late in the growing season. Weed leaves and

stems potentially could intercept the insecticide spray droplets before reaching the

crop, resulting in less insecticide deposition. Decreased insecticide coverage of

soybean planted in narrow rows compared with wide rows (18 vs. 97 cm) has

resulted in a reduction in soybean looper control (Hutchins and Pitre 1984).

Methomyl [S-methyl-iV-[(methylcarbamoyl)oxy]thioacetimidate] and methyl

parathion (O, O-dimethyl-O-p-nitrophenyl phosphorothioate) insecticide coverage of

soybean was lower within the median one-third of the canopy in narrow-row

soybean spacing than in wide-row spacing. No differences in coverage between row

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39

spacings were noted in the terminal or lower-thirds of the canopy. Furthermore, in

their study soybean looper larval mortality also was reduced in the median one-third

of the canopy in the narrow-row soybean. Royal et al. (1990) reported that

chlorothalonil (tetrachloroisophthalonitrile) fungicide deposition into peanut

decreased with increasing density of sicklepod, Florida beggarweed (Desmodium

tortuosum (Sw.) DC.), or common cocklebur (Xanthium strumarium L.). Four

sicklepod or Florida beggarweed/7.6 m of row reduced chlorothalonil deposition by

10%, while four common cocklebur plants/7.6 m resulted in a 20% reduction.

Furthermore, late season disease incidence also increased with increasing weed

density.

In order to manage the diverse pest problems in soybean, herbicides and

insecticides are often needed. Herbicide use may be even more important if weeds

alter insecticide efficiency. Furthermore, insecticide programs may need revision

when weed control programs fail. The objective of this research is to evaluate

insecticide deposition into the soybean canopy and soybean looper control when

hemp sesbania and sicklepod are present.

MATERIALS AND METHODS

Field experiments were conducted at the Macon Ridge location of the Northeast

Research Station near Winnsboro, Louisiana and at the Northeast Research Station

near St. Joseph, Louisiana in 1995 and 1996. Soil type was a Gigger silt loam (fine-

silty, mixed nonacidic, thermic Typic Fragiudalfs) at Winnsboro and a Commerce

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silty clay loam (fine-silty, mixed, nonacid, thermic Aerie Fluvaquents) at St.

Joseph. Soybean fields with natural infestations of hemp sesbania and sicklepod

were used. To ensure adequate densities, fields were overseeded with each weed

species at soybean planting with a hand spreader. Soybean varieties were 'DPL

3627’ and 'Asgrow 6785' in the 1995 and 1996 experiments, respectively. Soybean

was planted at Winnsboro on 5 June 1995 and 28 May 1996 and at St. Joseph on 12

June 1995 and 23 May 1996. The experimental design was a randomized complete

block replicated four times at Winnsboro and five times at St. Joseph. Plot size was

four 102-cm rows wide and 12 m long at Winnsboro and 9 and 11m long at St.

Joseph in 1995 and 1996, respectively. Treatments consisted of either weeds, hemp

sesbania and sicklepod, or no weeds in combination with no insecticide or thiodicarb

[dimethyl-A,N' -[thiobis[(methylimino)carbonyloxy]]bis [ethanimidothioate]] [Larvin

3.21 Flowable (F), 504 g (ai)/ha] applied to control soybean looper. Within five

weeks after planting, all plots were cultivated once so that any desired weeds were

present only in the non-cultivated band. All weeds were hand-removed from the

weed-free plots and weeds other than hemp sesbania and sicklepod from the weedy

plots. Weed-free plots were maintained the remainder of the season by hand

removal. Methyl-parathion was applied in early August to reduce the native

predator and parasitoid populations across the test sites. This treatment has been

shown to increase population densities of soybean loopers to economic damage

‘Rhone-Poulenc Ag Company, Research Triangle Park. NC 27709.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41

levels (Shepard et al. 1977). Soybean looper populations were then monitored

weekly using a 38-cm diameter sweep net until larval numbers reached economic

threshold levels (Baldwin et al. 1996). Soybean was at R4 (Fehr et. al 1971) growth

stage when populations reached this level in all experiments, except St. Joseph in

1996 where this level was reached at R5.

Number of larvae in each plot was determined before insecticide application by

placing a 91-cm ground cloth between the two center rows and shaking plants on

both sides over the cloth (Kogan and Pitre 1980). This same procedure also was

used to estimate larval mortality three days after treatment (DAT)2. Bean leaf beetle

[Ceratoma trifurcata (Forster)] populations were also determined at St. Joseph in

1996 using this same technique. Light penetration within the soybean canopy was

measured using a Decagon Sunfleck Ceptometer3 positioned between the two center

rows, parallel to the row, and 30 cm above ground level. Light readings also were

measured above the soybean canopy to give an estimate of full sunlight. These

measurements were made on the day of or the day before thiodicarb application

between 1230 and 1330 hours when no clouds were present to reduce variation in

light intensity due to cloud cover and sun position. Measurements from 30 cm

above ground level were compared to the above-canopy measurements and expressed

Abbreviations: DAT, days after treatment; PAR, photosynthetically active radiation.

3Decagon Devices Inc., Pullman. WA 99163.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42

as reduction in photosynthetically active radiation (PAR)2 penetrating the canopy.

Cloudy weather occurred at the time soybean was sprayed at Winnsboro in 1996 and

no PAR data were collected.

Thiodicarb was applied with nozzles maintained at a height to allow spray

deposition on both the weed and soybean foliage. This height was 180 cm above the

soil surface and approximately 101 to 111 cm above the top of the soybean canopy

in three of the four experiments. Because the predominate weed species was

sicklepod at Winnsboro in 1996, the nozzle height was 155 cm above the soil and

approximately 53 cm above the soybean canopy. An additional control treatment

included insecticide application with the nozzle height 48 cm above a weed-free

soybean canopy, which is more typical for on-farm applications. Thiodicarb

applications were made using a high clearance sprayer equipped with a four-row

boom with 51-cm nozzle spacing and calibrated to deliver a spray volume of 94

L/ha. Nozzles used were TX-84 hollow cone in all experiments except St. Joseph in

1995 where TX-124 nozzles were used. Data collected at each application date are

presented in Table 3.1.

Before insecticide application, 10.5 x 14-cm Kromekote5 dye-sensitive cards were

placed in the top, middle, and bottom-thirds of the soybean canopy to measure spray

deposition. Top level cards were placed at the average soybean terminal height for

4Spraying Systems Company, Wheaton, IL 60189-7900.

5Champion International Corp., Stanford, CT 06921.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43

Table 3.1. Application data for studies conducted at the Winnsboro and St. Joseph locations of the Northeast Research Station in 1995 and 1996. 1995 1996

Factor Winnsboro St. Joseph Winnsboro St. Joseph

Application date 25 August 31 August 26 august 6 September

Air temperature (°C) 36 34 30 29

Relative humidity (%) 62 86 95 80

Wind speed (km/h) 8-13 0-3 3-10 0-3

Wind direction North Northeast West North

Sprayer speed (km/h) 5 10 5 10

Spray pressure (kPa) 6.4 5.2 5.8 6.6

Soybean height (cm) 79 102 69 74

Hemp sesbania density 3.4 4.5 0.5 7.4 (plants/row m)

Hemp sesbania height (cm) 221 204 117 163

Sicklepod density (plants/row m) 1.0 2.5 3.4 2.1

Sicklepod height (cm) 141 109 107 100

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44

each experiment. The middle level cards were positioned half way between the

terminal and the soil surface. All bottom cards were placed 10 cm above the soil

surface. Soybean and weed height, measured from the soil surface to the terminal at

application time, and weed densities are shown in Table 3.1. Cards were attached to

a steel rod placed vertically in the soybean row. Ten steel rods were placed

randomly throughout each plot in the center two rows at Winnsboro. Five rods

were placed in each plot at St. Joseph. After application of thiodicarb amended with

Rhodamine WT6 dye (7 and 10 ml/ha in 1995 and 1996, respectively) to the

appropriate plots, cards were allowed to dry, and placed in waterproof bags for

deposition analysis. Cards were analyzed using a Hewlett Packard ScanJet Hex7

scanner using the Droplet Analyzer Program8 computer software. This program

analyzed four 6.45-cm2 sample areas from each card at a resolution of 400 dots/6.45

cm2 to estimate percent deposition.

A feeding bioassay also was conducted at the Winnsboro site. Thirteen (1995)

and ten (1996) randomly selected center soybean leaflets were collected immediately

after thiodicarb application from the top, middle, and bottom third of the crop

canopy from the same position at which cards were placed. Leaflets were placed in

6WRK Incorporated, Manhattan, KS 66502.

7Hewlett-Packard Company, Boise, ID 83707.

8D. Lambregts and M. Mailander (Unpublished), Department of Environmental and Agricultural Engineering, Louisiana State University, Baton Rouge, LA 70803.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45

9-cm petri dishes containing a moistened piece of 9-cm filter paper. Fourth and

fifth instar soybean Iooper larvae from a laboratory strain (USDA-ARS Southern

Insect Management Laboratory, Stoneville) reared on pinto bean wheat germ diet

(Thomas et al. 1993) were placed in the petri dishes within 1 h after insecticide

application and mortality was evaluated 2 DAT. Plots were mechanically harvested

using a plot combine to determine yield. Seed yield was adjusted to 13% moisture.

Each experiment was analyzed separately due to variation in weed densities.

Data were analyzed using the General Linear Models procedure (SAS Institute

1988). Larval mortality percentage data were transformed by square root or arcsine

square root where appropriate for analysis and then retransformed for presentation.

If a significant treatment effect was measured at the 0.05 probability level, single

degree of freedom contrast analysis (0.05 probability level) was used to make

comparisons between selected treatments. Card deposition data were analyzed as a

split-plot with weed treatment as the whole-plot and card placement as the sub-plot

for individual treatments. The CORR procedure (SAS Institute 1985) was used to

make correlations between deposition and thiodicarb effectiveness on soybean looper

in the field and in the feeding bioassay.

RESULTS AND DISCUSSION

For the control treatment where nozzles were maintained 48 cm above the weed-

free soybean canopy, thiodicarb deposition on cards ranged from 3.9 to 11.1%.

Deposition for the control treatment was not significantly different from the weed-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46

free treatment in which nozzles were maintained high enough to cover both weeds

and soybean at Winnsboro in 1995 (P=0.0267) and St. Joseph in 1996 (P=0.0095).

The control treatment with nozzles 48 cm above the soybean canopy was not

included at St. Joseph in 1995 and insecticide deposition was not detectable on cards

at Winnsboro in 1996 due to use of an ineffective dye solution. For thiodicarb

efficacy against native field soybean Iooper, the control treatment resulted in two to

18% larval survival and was not significantly different from the weed-free treatment

with the high boom level at Winnsboro in 1995 (/>=0.0267) and 1996 (/>=0.0038)

and St. Joseph in 1996 (P=0.0004). For all parameters measured in the four

experiments, results obtained for this control treatment were not significantly

different from the weed-free treatment in which nozzles were maintained high

enough to provide spray deposition of the weed and soybean canopy, so data for the

control treatment was excluded from analysis, and treatment comparisons were made

only for nozzles maintained at a single height.

Hemp sesbania and sicklepod influenced insecticide deposition at Winnsboro in

1995 (P=0.0164) and at St. Joseph in 1995 (P=0.0010) and 1996 (P=0.0149).

The treatment by card location interaction was significant at St. Joseph in 1995

(P=0.0214), but not Winnsboro in 1995 (P=0.0594) and St. Joseph in 1996

(P=0.2117). In all experiments, differences in droplet deposition between weedy

and weed-free plots were observed on cards placed in the top of the canopy, but not

in the middle or bottom portions of the canopy (Figure 3.1). There was a strong

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47 2 0 Winnsboro 1995

15 --

1 0

LSD (0.05) =2.6

12 St. Joseph 1995

1 0 -

8 -

LSD (0.05)=2.3

lO St. Joseph 1996

8 - -

LSD (0.05) = 2.0

bottom m iddle top Card location within soybean canopy

Figure. 3.1. Influence of hemp sesbania and sicklepod on thiodicarb deposition on dye sensitive cards placed in the bottom, middle, and top of the soybean canopy. The presence of a represents significant difference between the two data points.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48

tendency for a decrease in deposition between the top and the middle of the canopy

for both weedy and weed-free soybean. At Winnsboro in 1995, 17.0 and 12.5 %

deposition on cards in the top of the soybean canopy was observed for weed-free and

weedy soybean, respectively, with weeds resulting in a 26% reduction in deposition.

Deposition in the middle of the canopy was at least 28% less than the bottom o f the

canopy. The senescence of soybean leaves in the lower portion of the canopy where

bottom cards were placed could help explain this difference. At St. Joseph in 1995,

11.5 and 6.5% deposition on cards at the top of the canopy was observed in weed-

free and weedy soybean, respectively, a 43 % reduction due to hemp sesbania and

sicklepod. Spray deposition was reduced 30% in the top of the soybean canopy at

St. Joseph in 1996 when weeds were present.

Thiodicarb efficacy against native field soybean looper larvae at 3 DAT is

expressed as percent larval survival based on the pre-count density in each individual

plot. Treatment effects were significant at Winnsboro in 1995 (P=0.0202) and

1996 (/>=0.0001) and at St. Joseph in 1995 (/>=0.0191) and 1996 (P=0.0002). In

1995 at Winnsboro and St. Joseph when no insecticide was applied, soybean looper

larval survival was less than 41 % as compared with 54 to 144% at the same

locations in 1996 (Table 3.2). At the time of insecticide application in 1995, larval

populations had began to decline through pupation, and there may have been some

natural mortality from disease occurring. Contrast analysis revealed no differences

in survival from thiodicarb between the weedy and weed-free soybean at the 0.05

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. VO 101.8 survival* Bean leafbeetle _____ 1.6 0.8 6.2 3.4 19.48 19.48 0.0008 12.45 0.0042 iu mh 18.3 144.0 92.2 F P F P F P 0.01 0.9230 0.07 0.7919 0.01 0.9291 9.90 0.0188 5.35 0.0393 22.85 0.0004 13.3 137.0 54.2 140.2 F P Soybean looper larval survival* 0.26 0.6216 1995 1996 1996 21.1 1.3 25.7 F P Winnsboro St. Joseph Winnsboro St. Joseph St. Joseph 5.58 0.0501 0.17 0.6889 13.85 0.0029 17.40 0.0042 3.22 0.0980 10.45 0.0103 Weedy Weed-free 4.5 2.3 22.0 Treatment/Coutrasts bMean density bMean ofhemp sesbania and sicklepod was 3.4 and 1.0 (Winnsboro 1995), 4.5 and 2.5 (St. Joseph 1995), 0.5 and 3.4 ‘Survival is based on pre-count measurements ineach plot. •Single •Single degree offreedom contrasts. (Winnsboro 1996), 7.4 and 2.1/row m (St. Joseph 1996), respectively. No No insecticide/weedy vs thiodicaib/weedy Thiodicarb/weed-free vs thiodicarb/weedy No insecticide Contrasts0 No insecticide/weed-free vs thiodicarb/weed-free Thiodicarb Weedy No insecticide Weed-freeThiodicarb 40.8 9.0 Insecticide Weedh weeds and weeds thiodicarb application, and meaningful contrasts between selected treatment effects. and and bean leaf beetle survival at St. Joseph, LA in 1996, 3 days after treatment in response to Table 3.2. Soybean looper larval survival at Winnsboro and St. Joseph, Louisiana in and 1995 1996

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50

probability level. In three of the four experiments, larval survival was less when

thiodicarb was applied than not applied.

Correlations between mean deposition of the three card locations together for

each insecticide treatment and soybean looper larval survival were not significant

(Winnsboro 1995, r=0.4592, prob> |R| =0.1332; St. Joseph 1995, r=0.4661,

prob> |R | =0.0836; St. Joseph 1996, r=0.1421, prob> |R | =0.6133). Soybean

looper larvae habitually feed in the lower half to two-thirds of the soybean plant

(Herzog 1980), whereas the differences in insecticide deposition in the present study

were at the top of the soybean canopy. Furthermore, thiodicarb requires ingestion

by soybean looper to be effective (Thomson 1982). Significant treatment effects

(P<0.0006) were also observed for bean leaf beetle survival at St. Joseph in 1996

(Table 3.2). Thiodicarb reduced bean leaf beetle populations and control was

equivalent regardless of weed presence.

In the feeding bioassays at Winnsboro, the treatment effect was not significant at

the canopy bottom in 1995 (P=0.1991), but was significant at the bottom in 1996

(0.0182), middle in 1995 (0.0164) and 1996 (0.0008), and top of the soybean

canopy in 1995 (0.0484) and 1996 (0.0014). Mortality of soybean looper larvae on

leaflets from weedy and weed-free soybean treated with thiodicarb was similar when

collected from the middle or top of the soybean canopy both years, and at the

bottom in 1995 (Table 3.3). At the bottom of the canopy in 1996, greater mortality

was observed in weedy soybean than weed-free soybean. There was no significant

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. P 19% 95.0 F Top P 42.5 F 9.39 0.0221 192.02 0.0001 P 4.2 5.8 15.8 1996 1995 77.5 F 2.25 0.1939 0.27 0.6209 0.96 0.3728 iv or P Mortality 6.0 1995 46.0 F 18.36 0.0052 96.8 0.0002 10.8 11.0 11.0 0.0211 5.18 0.0631 4.09 0.0990 Bottom* Middle P F P 32.5 67.5 24.8 67.5 36.3 90.0 - - F Weed-free 9.5 Weed-free 28.8 47.5 Weedy T reatment/ContrastsT “Bottom, middle,“Bottom, and top represent the bottom, middle, and top-thirds ofdensity the bMean of soybeanhemp canopysesbania whereand sicklepod leaflets was were3.4 removed.and 1.0 (1995) and 0.5'Single and 3.4/rowdegree of freedom m contrasts.(1996), respectively. Insecticide No insecticide Weed* No insecticide/weed-free vs 1995 1996 Thiodicarb/weed-free vs thiodicarb/weedy thiodicarb/weed-free Thiodicarb Contrasts' mortality 2 mortality 2 days after treatment in the feeding bioassays, and meaningful contrasts between selected treatment Thiodicarb Table Table 3.3. Influence of weeds and thiodicarb at Winnsboro, Louisiana in and 1995 on 1996 soybean looper larval effects.

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correlation (r=0.2544, prob> |R | =0.1343) between thiodicarb deposition and

mortality in the feeding bioassay at Winnsboro in 1995. Because the only difference

in insecticide spray deposition between weedy and weed-free soybean was at the top

of the canopy (Figure 3.1), lower insecticide efficacy from the presence of weeds

wouid not be expected in the bottom and middle portions of the canopy. Even

though thiodicarb deposition in weedy soybean at the top of the canopy was lower

than in weed-free soybean, the amount deposited was probably sufficient to control

soybean Ioopers.

No differences in light penetration into the soybean canopy were observed

between the weedy and weed-free soybean at Winnsboro in 1995 (/*=0.1994) (Table

3.4). At St. Joseph in 1995 (P=0.0146) and 1996 (/>=0.0485) where thiodicarb

was to be applied, PAR was reduced more in weedy soybean than in weed-free

soybean, indicating that less light penetrated the weed infested canopy. These

findings support the differences observed within the top of the soybean canopy in

respect to insecticide deposition (Figure 3.1).

Weed interference and insecticide treatment significantly affected soybean yield at

St. Joseph in 1995 (/*=0.0002) and Winnsboro in 1996 (P=0.0276), but not St.

Joseph in 1996 (P=0.1154). Soybean yields at Winnsboro in 1995 were extremely

low (<134 kg/ha) due to environmental conditions, and since few conclusions can

be drawn, data are not presented. At St. Joseph in 1995 where thiodicarb was

applied, hemp sesbania and sicklepod reduced yields 69% in relation to weed-free

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Table 3.4. Photosynthetically active radiation (PAR) reduction into the soybean canopy as influenced by weeds at Winnsboro and St. Joseph, Louisiana in 1995 and 1996, and meaningful contrast between a selected treatment effect. ______PAR reduction*

Treatment/Contrast 1995 19%

Insecticide Weedb Winnsboro St. Joseph St. Joseph

No insecticide Weed-free 233 179 1431

No insecticide Weedy 806 781 1395

Thiodicarb Weed-free 467 504 1169

Thiodicarb Weedy 684 1157 933

Contrast' F P F P F P

Thiodicarb/weed-free vs thiodicarb/weedy - 6.58 0.0248 7.09 0.0207 ■Value expressed as a difference between PAR above the canopy and PAR within the canopy. bMean density of hemp sesbania and sicklepod was 3.4 and 1.0 (Winnsboro 1995), 4.5 and 2.5 (St. Joseph 1995), 7.4 and 2 .1/row m (St. Joseph 19%), respectively. 'Single degree of freedom contrast.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54

soybean (Table 3.5). Insecticide application, however, did not result in a yield

increase for weedy or weed-free plots, possibly because soybean looper populations

had already began to decline at time of application. At Winnsboro in 1996 where

thiodicarb was applied, soybean yield was not reduced by weeds in relation to weed -

free soybean. Thiodicarb application also did not result in a yield increase.

This research suggests that weeds capable of producing biomass above the soybean

canopy such as hemp sesbania and sicklepod can influence thiodicarb deposition into

the soybean canopy. A difference in thiodicarb deposition occurred only in the

terminal area of the soybean canopy, and deposition was much lower in the middle

and lower levels of the crop canopy, regardless of weed presence. Differences in

insecticide deposition within the crop canopy, however, were not reflected in

differences in soybean looper larval mortality. Thiodicarb has excellent activity on

soybean looper (Leonard et al. 1990). It is possible that different results may have

been obtained with use of a less efficacious insecticide or with an insecticide tolerant

soybean looper strain.

Weed management strategies should consider yield losses associated with weed

competition. Maximum yield observed in this study was 1490 kg/ha with yield loss

due to weeds in only one of three experiments. From an economical perspective, it

would probably not be advisable to apply any pesticide in this situation. Although

further research should consider other insecticide use strategies and weed species,

soybean looper management practices may not need to be altered when weeds are

present, assuming that an economic return can be expected.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U i t-fl 990 1490 St. Joseph - - F P 1996 P 0.0561 0.1404 1430 1010 1450 Yield 2.61 4.80 4.80 0.0561 420 1010 1150 1995 St. Joseph Winnsboro F P F 3.17 0.1055 2.31 0.1595 37.44 0.0001 Treatment/Contrasts bSingle degreebSingle offreedom contrasts. •Mean density ofhemp sesbania and sicklepod was 4.5 and 2.5 (St. Joseph 1995), 0.5 and 3.4 (Winnsboro 19%), 7.4 and 2.1/row m (St. Joseph 1996), respectively. No No insecticide/weedy vs thiodicarb/weedy No insecticide/weed-ffee vs Contrasts*1 No insecticide Weedy 690 700 1190 No No insecticide Weed-free Thiodicarb/weed-free vs thiodicarb/weedy thiodicarb/weed-free Thiodicarb Weedy Thiodicarb Weed-free 1370 Insecticide Weed* Louisiana Louisiana in and 1995 and Winnsboro, 1996 Louisiana in 1996, and meaningful contrasts between selected treatment effects. Table 3.5. Soybean yield response to weeds and thiodicarb application at St. Joseph,

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LITERATURE CITED

Anonymous. Weed Identification Guide. South. Weed Sci. Soc. Champaign, IL.

Baldwin, J. L., D. J. Boethel, and B. R. Leonard. 1996. Control Soybean Insects 1996. Louisiana Coop. Ext. Serv., Baton Rouge. Publ. 2211. 16 pp.

Bozsa, R. C., L. R. Oliver, and T. L. Driver. 1989. Intraspecific and interspecific sicklepod (Cassia obtusifolia) interference. Weed Sci. 37:670-673.

Bridges, D. C. and P. A. Baumann. 1992. Weeds causing losses in the United States. Pages 75-147 in D. C. Bridges, ed. Crop Losses Due To Weeds 1992. Weed Sci. Soc. Am., Champaign, IL.

Fehr, W. R., C. E. Caviness, D. T. Burmood, and J. S. Pennington. 1971. Stage of development descriptions for soybeans Glycine max (L.) Merrill. Crop Sci. 11:929- 931.

Herzog, D. C. 1980. Sampling soybean looper on soybean. Pages 141-168 in M. Kogan and D. C. Herzog, eds. Sampling Methods in Soybean Entomology. Springer-Verlag, New York.

Hutchins, S. H. and H. N. Pitre. 1984. Effects of soybean row spacing on spray penetration and efficacy of insecticides applied with ground and aerial equipment. Environ. Entomol. 13:948-953.

Kogan, M. and H. N. Pitre, Jr. 1980. General sampling methods for above-ground populations of soybean arthropods. Pages 30-60 in M. Kogan and D. C. Herzog, eds. Sampling Methods in Soybean Entomology. Springer-Verlag, New York.

Leonard, B. R., D. J. Boethel, A. N. Sparks, Jr., M. B. Layton, J. S. Mink, A. M. Pavloff, E. Burris, and J. B. Graves. 1990. Variations in response of soybean looper (Lepidoptera: Noctuidae) to selected insecticides in Louisiana. J. Econ. Entomol. 83:27-34.

McWhorter, C. G. and J. M. Anderson. 1979. Hemp sesbania (Sesbania exaltata) competition in soybeans {Glycine max). Weed Sci. 27:58-64.

Royal, S. S., B. J. Brecke, D. L. Colvin, and F. M. Shokes. 1990. The effects of three weed species on fungicide deposition in peanuts. Proc. South. Weed Sci. Soc. 43:103.

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SAS Institute. 1985. SAS User's Guide: Basics, Version 5 Edition. SAS Institute, Cary, NC. 1292 p.

SAS Institute. 1988. SAS/STAT User’s Guide, Release 6.03 Edition. SAS Institute, Cary, NC. 1028 p.

Shepard, M., G. R. Carner, and S. G. Tumipseed. 1977. Colonization and resurgence of insect pests of soybean in response to insecticides and field isolation. Environ. Entomol. 6:501-506.

Thomas, J. D., J. S. Mink, and D. J. Boethel. 1993. Diet influences in permethrin susceptibility and growth of soybean looper larvae (Lepidoptera: Noctuidae). J. Econ. Entomol. 86:1236-1240.

Thomson, W. T. 1982. Agricultural Chemicals: Book I Insecticides. Thomson Publ., Fresno, CA. 294 pp.

Wegenhoft, K. N. 1996. Costs of producing soybeans in Louisiana. Pages 30-38 in J. Honeycutt, ed. Louisiana Soybean Handbook. Louisiana Coop. Ext. Serv., Baton Rouge. Publ. 2624.

Wier, A. T. and D. J. Boethel. 1996. Symbiotic nitrogen fixation and yield of soybean following defoliation by soybean looper (Lepidoptera: Noctuidae) during pod or seed development. J. Econ. Entomol. 89:525-535.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER IV

SUMMARY

Field studies were conducted to evaluate the influence of simulated insect

defoliation in combination with season-long competition from weeds on weed and

soybean growth and soybean yield. Weeds consisted of johnsongrass [Sorghum

halepense (L.) Pers.] at 15 plants/6 m, common cocklebur (Xanthium strumarium

L.) at 3 plants/6 m, and hemp sesbania [Sesbania exaltata (Raf.) Rydb. ex A.W.

Hill] at 12 plants/6 m of row. Defoliation levels were imposed by removing 0, 1,

and 2 leaflets per soybean trifoliate to approximate 0, 33, and 66% defoliation,

respectively. Simulated defoliations were performed when soybean was at R2 (full

bloom) and R5 (beginning seed development).

Johnsongrass, common cocklebur, and hemp sesbania height and dry weight

were not affected by defoliation level or defoliation stage. Soybean yield response

to weed interference and defoliation was additive both years. In 1994, averaged

across defoliation levels and stages, johnsongrass, common cocklebur, and hemp

sesbania reduced soybean yield 30, 15, and 14%, respectively, in comparison to

weed-free soybean. Johnsongrass reduced yield 35% in 1995, whereas for common

cocklebur yield was equivalent for weedy and weed-free soybean. A linear

relationship between soybean yield and defoliation level was observed both years.

Defoliation levels of 33 and 66% reduced yield 6 and 20%, respectively, in 1994

and 12 and 23%, respectively, in 1995. Defoliation at R5 resulted in 10% lower

58

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yield than defoliation at R2 in only one year. Soybean 100-seed weight was not

influenced by weed interference either year. As with yield, a linear decrease in seed

weight was observed as defoliation level increased. Seed weight in 1995 was 3.3%

lower in soybean defoliated at R5 compared with defoliation at R2.

Soybean yield response to weed interference and simulated insect defoliation was

additive in this study, which is in agreement with other research. This is especially

noteworthy because of the variation in soybean varieties, growth habit, and

environmental conditions. Findings clearly show the importance of avoiding stress

to soybean from weed interference and defoliation. Controlling both weeds and

insects would be necessary for maximizing productivity.

Field experiments also evaluated the influence of hemp sesbania and sicklepod

[Senna obtusifolia (L.) Irwin and Barneby] on thiodicarb insecticide deposition

within the soybean canopy and on soybean looper [Pseudoplusia includens (Walker)]

control. Insecticide deposition was highest on dye-sensitive cards placed in the top

of the soybean canopy and hemp sesbania and sicklepod reduced insecticide spray

deposition 26, 43, and 30% in comparison with weed-free soybean. Weeds did not

reduce deposition in the middle or bottom levels of the canopy. Additionally, weeds

did not negatively affect thiodicarb efficacy against soybean looper in the field or in

a feeding bioassay. Even though thiodicarb deposition was lower in weedy soybean

at the top of the canopy than in weed-free soybean, the amount deposited was

sufficient for soybean looper control. It is possible that different results may have

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been obtained with use of insecticides less efficacious than thiodicarb or with a

soybean looper strain more tolerant to thiodicarb.

Overall results of these studies show the importance of controlling both weeds

and insects in a soybean production system. Pest management strategies should

consider yield losses associated with both weed competition and insects, as well as

the indirect effect weeds may have on insecticide deposition and insect mortality.

Soybean looper management may need to be altered when weeds are present. Lack

of adequate weed control early season, however, may reduce the economic incentive

to control insects later in the season. These studies clearly show the importance of

implementing integrated pest management strategies for control of weeds and insects

in a soybean production system.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. VITA

Charles Frost Grymes, known to his friends as Charlie, was born in Houston,

Texas on July 22, 1968 to Gordon and Billie Grymes. He grew up on rice farms

near Cypress, Texas and LaWard, Texas. His father also raised soybean, com, and

grain sorghum. Charlie grew to appreciate agricultural life as he worked many long

days with his family on the farm. He attended Industrial High School where he

played football and golf and graduated as salutatorian. He was also actively

involved with the band, 4-H, and FFA. Charlie obtained his Bachelor of Science

degree from Texas A&M University in Agronomy in 1990. While completing his

degree, he worked for Dr. Paul Nester with American Cyanamid Company in the

area of herbicide research. During this endeavor, Charlie became interested in the

weed science field. He completed his Master of Science degree in Agronomy from

Texas A&M University under the direction of Dr. J. M. Chandler with emphasis on

weed science. After graduation, he interned with Mr. James Whitehead, an

entomologist with American Cyanamid at Cheneyville, Louisiana. While working

for James, Charlie learned a great deal about cotton insect pests and their

management. His colleagues at this time suggested he pursue a Doctor of

Philosophy in weed science working on an interdisciplinary weed science and

entomology project. He moved to Baton Rouge, Louisiana in January 1994, three

days after marrying his wife Lorissa, to pursue a Ph. D. at Louisiana State

University under the direction of Dr. James L. Griffin. During his first year at

61

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L. S. U., Charlie decided to minor in entomology working on an interdisciplinary

project. During his graduate program at Louisiana State University, Charlie held

membership in the Weed Science Society of America, the Southern Weed Science

Society, and the Louisiana Plant Protection Association. He was a member of the

third place team at the Southern Weed Science Society Weed Contest in 1994.

Charlie also was awarded first place in the oral paper competition at the 1996

meeting of the Southern Weed Science Society and second place at the 1996

Louisiana Plant Protection Association Meeting. He was also awarded the Louisiana

Consultants Association Scholarship in 1995.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DOCTORAL EXAMINATION AND DISSERTATION REPORT

Candidate: Charles F. Grymes

Major Field: Plant Health

Title of Dissertation: Interactive Effects of Weeds and Defoliating Insects in Soybean (Glycine Max)

Approved:

jor Profed Chairman

Dei of the Graduate School

EXAMINING COMMITTEE:

Pate of Examination:

January 24, 1997

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