Insect Infestations in Relation to Rice Crop Phenology

Michael StoutDepartment of Entomology

LSU AgCenter/LSU A&M

When do Rice Pests Attack? AND

To What Extent can We Predict Attack?

Insect Infestations in Relation to Rice Crop Phenology

OR

Attack by a pest depends on several factors…

• Is the pest present, and at what levels?

--population dynamics and life history of pest

• Are environmental conditions conducive to attack?

-- pests only attack under certain conditions

• Is the plant susceptible to attack?

--pests specialize on certain plant parts

--suitability of plant parts change as plant develops

Attack by a pest depends on several factors…

• Is the pest present, and at what levels?

--population dynamics /life history of pest?

• Are environmental conditions conducive to attack?

-- pests only attack under certain conditions

• Is the plant susceptible to attack?

--pests specialize on certain plant parts

--suitability of plant parts change as plant develops

Attack by a pest depends on several factors…

• Is the pest present, and at what levels?

--population dynamics /life history of pest?

• Are environmental conditions conducive to attack?

-- pests only attack under certain conditions

• Is the plant susceptible to attack?

--pests specialize on certain plant parts

--suitability of plant parts change as plant develops

Attack by a pest depends on several factors…

• Is the pest present, and at what levels?

--population dynamics /life history of pest?

• Are environmental conditions conducive to attack?

-- pests only attack under certain conditions

• Is the plant susceptible to attack?

--pests specialize on certain plant parts

--suitability of plant parts changes as plant develops

If we had reliable answers to each of these questions, we could predict attack with reasonable certainty

Unfortunately, we don’t have reliable answers for many pests

Whether or not the pest is causing economic losses is a related, but separate

issue

Let’s apply these principles to the best-studied rice pest in Louisiana, the rice water weevil…

Adult overwintering

Larval feeding

Mating and oviposition

Adult feeding

Attack by a pest depends on several factors…

• Is the pest present, and at what levels?

--population dynamics /life history of pest?

• Are environmental conditions conducive to attack?

-- pests only attack under certain conditions

• Is the plant susceptible to attack?

--pests specialize on certain plant parts

--suitability of plant parts changes as plant develops

Population dynamics of the rice water weevil over the growing season

April May June% e

mer

genc

e fr

om o

verw

inte

ring Emergence of the

rice water weevil from overwintering

does not begin until early April in most years, and

may not be complete until late

May…

Population dynamics of the rice water weevil over the growing season

April May% e

mer

genc

e fr

om o

verw

inte

ring

June July August

Wee

vil p

opul

ation

den

sitie

s

…but less is known about population

fluctuations during the season and

return to overwintering

June

Attack by a pest depends on several factors…

• Is the pest present, and at what levels?

--population dynamics /life history of pest?

• Are environmental conditions conducive to attack?

-- pests only attack under certain conditions

• Is the plant susceptible to attack?

--pests specialize on certain plant parts

--suitability of plant parts changes as plant develops

Oviposition (egg-laying) by rice water weevil is triggered by flooding…

Treatment/Flood depth

Eg

gs

per

pla

nt

0

10

20

30

40

50

drysoil

wetsoil

1.3cm

5.1cm

10.2cm

cbc

bcb

a

Attack by a pest depends on several factors…

• Is the pest present, and at what levels?

--population dynamics /life history of pest?

• Are environmental conditions conducive to attack?

-- pests only attack under certain conditions

• Is the plant susceptible to attack?

--pests specialize on certain plant parts

--suitability of plant parts changes as plant develops

Suitability of rice for oviposition changes as rice grows and develops

Plant Age

2 leaf early 3 leaf late 3 leaf 4 leaf

Mea

n n

um

ber

of

L.

ory

zop

hil

us

egg

s/p

lan

t

0

2

4

6

8

10

12

14

16

Plant Age

3/4 Leaf Mid/ Late Tillering Panicle InitiationPanicle Differentiation

Me

an n

um

ber

of

L.

ory

zop

hil

us

larv

ae/p

lan

t

0

10

20

30

40

50

What are the implications for rice water weevil management?

• Rice planted early can escape, to some degree, rice water weevil infestation because weevil populations have not fully emerged before rice passes through susceptible stages

• Rice can be attacked by rice water weevils for most of its development, but is most susceptible during tillering

• Early flooding (e.g., pinpoint flooding) of rice extends the window of time in which rice is vulnerable to infestation

• Delayed flooding of rice can reduce the window of vulnerability

We lack basic information (on population dynamics, environmental effects, plant susceptibilty) for most

other rice pests…

So, we are less capable of predicting attack by these pests

Pests of young / early season rice

Intolerant of flooding• Fall armyworm• Thrips• Aphids• Colaspis

Tolerant /dependent on flooding

• Rice water weevil• Seed midges (feed on

seeds, likes floods)• Rice leafminer (likes

floods)• South American rice

miner

Pests of mid to late season rice

• Stem borers--population dynamics unknown, appears to require large stems, prefers reproductive stages

• Rice stink bug--feeds on developing grains, susceptibility changes as

grains mature

• Skippers, grasshoppers, other defoliators

Author

• Michael Stout• mstout@agcenter.lsu.edu