and Integrated Pest Management - part - usaid

DEVELOPMENT OF ACTION CONTROL
THRESHOLDS FOR MAJOR RICE PESTS
J.P. Bandong and J. A. Litsinger
Senior Research Assistant and Entomologist, respectively
Departinent of Entomology, International Rice Research Institute
P.O. Box 933, Manila, Philippines
Economic thresholds (ET) are determined by (i) damage functions relatinlg
insect pest abundance to yield loss and (ii) control functions relating the degree
of control from insecticides (Smith & Litsinger 1985). These are evaluated
economically based on prevoiling costs. However, the use of ET values derived
firom controlled single insect pest studies have proven unreliable as lhere can be a
large site interaction as well as the influence of other pests. In addition, since
scouting is commonly (lope weekly there needs to be an allowance for time
delays in applying ETs. Therefore, in practice, action control thresholds (ACTs)
are derived empirically from trial and error verification on farmers' fields and used
in place of ETs.
DEVELOPING TIE KNOWLEDGE BASE
Sites
Verification trials were conducted in farmers' fields in four irrigated rice
locations representing distinct sites. Guimba in Nueva Ecija was supplied by
deep well pumps while Zaragoza in Nueva Ecija, Laguna, and South Cotabato
sites were irrigated by river systems (Baladong & Litsinger 1986, Litsinger et al.
1980, IRRI 19(4, 1985a, b). We aimed to develop ACTs for sites o"
rccommend'ition dcnains which are usually irrigation systems.
Yield Losses by Crop Growth Stage
Developmert of ACTs involved determiring yield loss, preferably
partitioned by growth stage, and relating various ACT values to those economic
losses. Variability in yild loss was evident across sites from 5% (0.19 t/ha) in
Laguna to 37% (0.80 t/ha) in Guimba (Table 1). ACTs therefore will be higher
in Lagt.ia and lower in Guimba. Higher yield losses occurred in the vegetative
(38%) and reproductive -tages (38%) than ripening (24%) (Table 1). If an ACT
triggered an iasecticide deci'ion during a growth stage without economically
si,;n'icant yield loss, then the ACT was too low and was raised.
Major Pests
Insect pests attacking the vegetative stage comprised mainly the whorl
maggot lydrellia philippina Fcrino, the rice green semi-looper Nararga