Date Published: April 18, 2019
Publisher: Public Library of Science
Author(s): Ian M. Grettenberger, Shimat V. Joseph, Gadi V.P. Reddy.
Bagrada hilaris (Burmeister) (Hemiptera: Pentatomidae) is an invasive stink bug species that feeds on cruciferous plants and can cause substantial damage to crops. Little is known about the dispersal behavior of B. hilaris, but movement is important because of the way this pest moves from senescing weed hosts into crop fields. Perhaps, B. hilaris residing on declining weed hosts become starved, which alters their normal locomotor activity and initiates dispersal. We examined the influence of starvation on the locomotor behavior of multiple life stages of B. hilaris under laboratory and outdoor conditions. We starved nymph (2nd/3rd and 4th/5th instars) and adult (female and male) stages for 0, 24, and 48 h. We measured distance moved in the laboratory and then distance moved and turning ratio outdoors. In the laboratory, the younger nymphs moved shortest distances when starved for 24 h, whereas late-instar nymphs (4th-5th instars) and adult B. hilaris that were starved moved farther than non-starved individuals. In the outdoor setting, environmental conditions, specifically surface temperature were important in determining how starvation affected distance moved. Starved insects were more responsive (moved farther) for a given change in temperature than non-starved insects. At lower temperatures, B. hilaris tended to move farther when non-starved and at higher temperatures, moved longer distances when starved, at least for certain stages. Increased starvation also led to more directional movement. Our results indicate that starvation influences aspects of movement for B. hilaris and that these effects can be influenced by temperature.
Bagrada hilaris (Burmeister) (Hemiptera: Pentatomidae), commonly known as bagrada bug or painted bug, is an invasive stink bug species that damages crops in California and the desert southwest [1,2]. The bagrada bug preferentially feeds on cruciferous crops (family: Brassicaceae) such as broccoli (Brassica oleracea var. italica Plenck), cauliflower (B. oleracea L. var. botrytis), turnip (B. rapa L. var. rapa), kale (B. oleracea L. acephala), bok choy (B. rapa L. var. chinensis), arugula (Eruca sativa Mill.) and mizuna (B. rapa L. nipposinica), and several other minor cruciferous crops [1,3,4]. This pest was first reported in North America in Los Angeles Co., California in 2008  and then found in 2012 in the Central Coast region of California . In California, B. hilaris could damage many different cruciferous crops and likely caused crop losses in the millions of US dollars when it first appeared in the region.
Many polyphagous stink bug species feed primarily on plant fruiting structures and throughout the growing season search for fruits at a suitable developmental stage. To do so, they disperse among crop and non-crop hosts that differ in phenology [26,27]. The crucifer specialist B. hilaris differs in that it disperses within the landscape, at least in its introduced range in the U.S., based largely on the availability of any green tissue, leaves, stems, or fruiting structures, of its cruciferous hosts, which include weed and crop hosts [1,8]. Populations of B. hilaris appear to build and remain on weed hosts until the plants senesce, at which point they disperse in search of other hosts. A similar scenario can occur if B. hilaris populations develop on cruciferous crops that are then harvested or destroyed. Because hosts can rapidly disappear during the warmest part of the season when B. hilaris activity is the highest, starvation is likely one of the most important factors that influence movement and dispersal at this key period during the year. Our results demonstrate that starvation of B. hilaris can influence their dispersal behavior and orientation towards food. Our results indicate that life stages vary at times in their dispersal capacity and how they are affected by starvation. Environmental factors can also affect the influence of starvation on walking by B. hilaris. The change in locomotor behavior of B. hilaris with starvation provides insights into their dispersal behavior when feeding on senescing weed hosts.
Adults and all nymph stages of B. hilaris can cause serious damage to cruciferous crops at germinating and seedling stages. Improving our understanding of the dispersal behavior of B. hilaris, and what factors influence this behavior, is an important step towards developing an integrated pest management program for this pest. Our laboratory results show that starvation for short periods can increase distances moved through walking. Results from our outdoor experiment show that temperature is an important determinant of how starvation affects distance travelled. Starvation can affect how B. hilaris moves in search of food, with increased directionality of movement when insects are starved. Orientation of B. hilaris towards food also increased with starvation. Overall, our results improve our understanding of the movement of B. hilaris within the landscape. This is important for elucidating the role of non-crop hosts as sources of B. hilaris moving into fields and knowing what factors elicit dispersal behavior of B. hilaris between non-crop hosts or, more importantly, from non-crop hosts to nearby planted crops. A better understanding of dispersal dynamics of B. hilaris can also help develop refined management plans that account for movement of this pest using prediction of damage risk, targeted scouting, or management tactics that take advantage of movement such as trap cropping.