Numbers of rice water weevil larvae (Coleoptera : Curculionidae) and rice plant growth in relation to adult infestation levels and broadleaf herbicide applications

We conducted experiments to determine the numbers of rice water weevil (RWW , Lissorhoptrus oryzophilus Kuschel) larvae and the growth of rice (Oryza. sativa L.) plants resulting from combinations of adult-RWW-infestation leve ls and broadleaf herbicide (bensulfuron or MCPA [2-methyl-4-chlorophenoxyac etic acid]) applications. In the greenhouse, bensulfuron (applied either be fore or after adult RWW were added to rice plants) did not affect the numbe r of immature RWW or rice plant growth. In field plots, herbicides did not affect the numbers of immature RWW produced from Various adult-RWW-infestat ion levels in either 1989 or 1990, but affected various measures of plant g rowth. In both 1989 and 1990, adult-RWW-infestation levels affected the num ber of immature RWW in plots and affected all (1989) or most (1990) rice gr owth characteristics. In 1990, rice plant growth measurements and yield gen erally declined sharply between 0 and 12 adult RWW added, but they declined gradually at higher infestation levels. Interaction between herbicides and adult-RWW-infestation level was not significant in either year, suggesting largely independent action of herbicides and RWW on plant growth and no mo derating effect of herbicides on numbers of immature RWW. Our results under score that RWW and broadleaf weeds are each important pests of rice in Cali fornia, but the potential interaction from their management on immature RWW and corresponding rice plant growth appears to be of little relevance.