Kelly Tindall, Michael Stout, Bill Williams, Boris Castro and Eric Webster

Weed and insect pests perpetually cause problems for Louisiana farmers. In addition to their individual effects, insects, weeds and their management practices can interact. Uncontrolled weeds can serve as alternate hosts for insect pests. Many insect pests of rice also feed on a broad range of other grasses, several of which are common weeds in rice fields. Additionally, cultural practices used for weed control can affect management of insect pests and vice versa. For example, flooding rice fields at the two-to-three-leaf stage to control red rice can increase the severity of damage by the rice water weevil, the most significant insect pest in south Louisiana. Conversely, delaying floods until the four-to-five-leaf stage allows the rice root system to become more tolerant to rice water weevil infestation, but compromises weed control. 

Interactions between herbicides and insecticides may be toxic to plants and may require special consideration before pesticide applications are made. In rice, for example, injury can occur to plants when applications of certain insecticides (methyl parathion, malathion or carbaryl) are made within 15 days of application of the herbicide propanil.
    
An interdisciplinary team of LSU AgCenter scientists has been investigating these types of interactions among weeds, insects and pest management strategies in rice during the past three years at the Northeast, Macon Ridge and Rice research stations. Recent research has focused on interactions between rice stink bugs and barnyard-grass. Rice stink bugs feed on develop-ing grains, causing partially filled seeds, reduced milling quality and “pecky” rice – rice with discolored kernels. Also, rice stink bugs can reduce yield by feeding on flowers, causing them to become sterile.

Barnyardgrass competes with rice plants, reducing the number of tillers, panicles and seeds per panicle. Because barnyardgrass is also a preferred host for the rice stink bug (Figure 1), experiments were conducted to evaluate the influence of barnyardgrass on rice stink bug population dynamics in a rice field. In addition, the potential impact of rice stink bugs on rice yield and quality in weedy fields was studied. Rice was grown in the presence and absence of barnyardgrass and examined for yield losses associated with weeds and rice stink bugs.

Two important findings were made with respect to rice stink bug behavior. Rice stink bugs were found in barnyardgrass-infested rice fields without damaging rice because rice panicles had not yet emerged. Barnyardgrass produced seed heads before rice panicles emerged, and rice stink bugs were feeding on barnyardgrass, not rice. But because rice stink bug densities were higher on rice when barnyardgrass was present than when it was absent (Figure 2), more rice plants suffered damage in barnyardgrass-infested plots as rice panicles began to emerge. Rice plants are most vulnerable to rice stink bug injury during the early stages of grain-filling.

Grain yields were reduced by more than 30 percent in the presence of barnyardgrass and rice stink bugs (Figure 3). Incidence of pecky rice was 30 percent higher in weedy plots than weed-free plots (Figure 4). These results show that barnyardgrass not only causes direct yield losses through competition with rice but also causes additional indirect losses (pecky rice) because of the higher numbers of rice stink bugs in barnyardgrass-infested plots.

The timing of barnyardgrass seed head emergence relative to rice panicle emergence is equally important. Rice stink bug infestations appear to be more severe in barnyardgrass-infested rice plots than in weed-free rice plots when barnyardgrass seed heads emerge and begin to reach late maturity before rice panicles emerge. If barnyardgrass seed heads are present at the same time rice panicles are present, however, rice stink bugs prefer to feed on barnyardgrass and remain on it until it becomes unsuitable. This allows some rice plants to escape early damage during panicle development.

Preliminary results from a large plot demonstration conducted at Woodsland Plantation in Richland Parish demonstrated that rice stink bug populations were up to 10 times greater in barnyardgrass-infested rice compared to barnyardgrass-free rice. Because barnyardgrass can influence rice stink bug populations, producers need to be proactive in scouting weedy fields for rice stink bugs to minimize stink bug injury to rice. If barnyardgrass seeds are present before rice panicles emerge, rice stink bug populations may build up and attack vulnerable rice as panicles emerge. On the other hand, rice may gain protection from rice stink bugs if rice panicles and barnyardgrass seed heads are present at the same time.

Future research plans include further investigating the effects of the timing of barnyardgrass emergence and determining the influence weedy field borders can have on rice stink bug populations on rice.

Acknowledgment
This research was partially funded by the Louisiana Rice Research and Promotions Board.

Kelly Tindall, Graduate Assistant; Michael Stout, Associate Professor, Department of Entomology; Bill Williams, Associate Professor, Northeast Research Station, St. Joseph, La.; Boris Castro, Assistant Professor, Department of Entomology; and Eric Webster, Associate Professor, Department of Agronomy & Environmental Management, LSU AgCenter, Baton Rouge, La.