Kudzu bug, Megacopta cribraria, is a soybean pest and a recent invader to the United States. Since reaching Maryland in 2013, concern for expansion to the Northeast has grown. This project sought to determine the cold tolerance and overwintering behavior of the kudzu bug. Two measures of physiological cold tolerance were analyzed: the super cooling point (SCP), the point at which an insect’s body freezes, and the lower lethal temperature to kill 50% of the population (LLT50). Average SCP across three populations in Maryland and Virginia over September to December of 2015 was 9.3±6.3°F. LLT50 average value across the same populations in September and November of 2015 was 22.8°F with a 95% confidence interval of 21.2°F to 24.1°F. A caged and field experiment determined the timing of the overwintering movement and location. Both indicate that the adult kudzu bug move from the vines into the leaf litter, in which they overwinter, between November and December. Soybean producers can evaluate yearly pest pressure by using the SCP, LLT50, and timing of winter movement to predict spring population numbers. Physiological thresholds with climate modeling can predict range expansion. Under moderate climate change, the expectation is that the kudzu bug will to move into central Pennsylvania and coastally up to Massachusetts by the end of the century.
The goal of this project was to determine the cold tolerance of the kudzu bug, Megacopta cribraria, and predict its pest potential in the Northeast. Kudzu bug was first discovered in Georgia in 2009 and has since rapidly expanded to Maryland in 2013. Concern for the kudzu bug centers on soybean yield losses. In a study, untreated fields in Georgia and South Carolina showed yield losses of soybeans reaching up to 47% with an average of 18%.
Kudzu bug populations in the United States come from one female lineage originating from a subtropical island in Japan. Expansion of the kudzu bug range north of Maryland is unknown given the contrasting climate of the Northeast. Host plant availability, physiological tolerance, and biotic interactions often limit the distributions. The common host plants of the kudzu bug, both kudzu and soybean, extend well beyond its current distribution. As a recent invader, the kudzu bug has few natural enemies and competitors. Therefore, it is likely that physiological cold tolerance is limiting their northward expansion.
Observational evidence from 2014 to 2016 suggests that the kudzu bug has not expanded its range in Maryland. The halting of northward expansion bolsters the potential of cold tolerance limiting the kudzu bug distribution. However, winter temperatures are rising faster than summer temperatures under climate change. With changing winter extremes, the kudzu bug is likely to expand northward. No studies have explored kudzu bug cold tolerance within the United States. Thus this project aims at evaluating cold tolerance measures and well as characterize the overwintering behavior of the kudzu bug in Maryland.
The research goal was to determine and communicate the cold tolerance of the kudzu bug for integrated pest management use. Three objectives subdivide this overarching goal:
- Measure key responses, such as thermal limits on survivability, to cold tolerance in association with overwintering across a range of kudzu bug populations. I completed a study on the cold tolerance responses of super cooling point and lower lethal temperature to kill 50% of the population across three populations based in Maryland and Virginia.
- Observe kudzu bug phenology and movement to overwintering locations in the fall to determine temperature and realized overwintering population size. Kudzu bug fall movement and overwintering location was determined in conjunction with observations of the fall phenology.
- Create a model for integrated pest management and communication of findings with soybean producers and the agricultural community at large. Temperature limits were compared to the current kudzu bug distribution and expansion predicted using a climate change scenario.
Two physiological parameters, the super cooling point (SCP) and lower lethal temperature to kill 50% of the population (LLT50) were measured to assess kudzu bug cold tolerance. Three populations of kudzu bug were used from Virginia and Maryland to represent different USDA plant hardiness zones. Each population was sampled once a month from October to December during the fall of 2015. Individuals collected from populations were collected using a sweep net, placed in mesh insect cage, and brought back to the University of Maryland for testing.
Super Cooling Point:
To determine the kudzu bug SCP, ten males and ten females were assessed for each population over the fall months. Individual bugs were attached to a thermocouple wire and placed in a glass vial with a cotton ball on the bottom. Thermocouple wires were connected to a data logger set to record temperature via a computer program. Vials were submerged in a recirculating bath and cooled from room temperature to -13°F at a cooling rate of 0.41°F/min. Visual analysis of the individual’s temperature against time showed the temperature point before the exothermic reaction of ice nucleation, known as the SCP. SCP was analyzed using an ANOVA with factors for month, population, and sex. A post hoc tukey analysis was preformed on significant model factors.
Lower Lethal Temperature:
To determine the kudzu bug LLT50, two hundred kudzu bugs were collected from each population over October and November. December was excluded due to insufficient population numbers. Furthermore, the Virginia population was excluded from November due to a fungal infection of Beauveria bassiana. Eight glass vials of five kudzu bugs were created per minimum temperature tested. Minimum temperatures used were 14.0, 21.2, 24.8, 28.4, 35.6°F to achieve a range of 100% to 0% death. Groups of vials were submerged in a recirculating bath and cooled from room temperature to the minimum temperature of the run at a rate of 0.27°F per min. The minimum temperature was maintained for twelve hours before warming back to room temperature at 0.27°F per min. Upon reaching room temperature the vials were transferred to petri dishes containing an edamame leaf. Groups of kudzu bug were allowed to recover at room temperature for twelve hours before checking mortality as defined by a lack of movement when simulated with a paintbrush. Counts of dead were run through a logit model with factors for month, population, minimum temperature, and all interactions. Significance was determined with the effect likelihood ratio test. LLT50 values were calculated for each month and population combination at the temperature where there was a 0.5 probability of survival.
Recognizing the potential challenge of field based behavioral observations, the objective is broken into two studies: field movement and cage movement.
Three locations in Maryland were sampled once a month from October to December during the fall of 2015. At each location, five sampling points were randomly chosen. A 0.5 by 0.5 meter square of vine, leaf litter, and top 3cm of soil were bagged separately per sampling point. Each layer was search for counts of nymphs and adults.
At the greenhouse complex of the University of Maryland, a 6x12x6ft mesh cage was constructed. The cage was provisioned with propagated kudzu plants, bins with soil, and bins with leaf litter for the fall of 2015. The caged was provisioned with propagated kudzu plants and bins of leaf litter for the fall of 2016. Each year the cage was stocked with +2,000 kudzu bugs and monitored from late October through December. The cage was destructively sampled each January to determine overwintering location and number of dead versus alive.
USDA plant hardiness zones determined minimum winter temperature isolines for the SCP and LLT50. These isolines were compared to the kudzu bug distribution from the Early Detection and Distribution Mapping System. To predict future range shifts, the intergovernmental panel on climate change (IPCC) scenario RCP 4.5 was applied to the isolines.
Started by Dr. Alan Leslie in the Lamp Lab, my work on the kudzu bug in Maryland has been chronicled on the website: http://mdkudzubug.org. I have also presented my findings at three presentations and in one publication to date.
Super Cooling Point:
Across all populations, months, and gender the overall SCP was 9.3±6.3°F. The SCP was not significantly affected by gender but was affected by population and month (F1,154=2.77, p=0.0984; F2,154=6.54, p=0.0019; F2,154=19.19, p=<0.0001 respectively). Though population was significant, there was no consistent latitudinal trend between the population locations. Furthermore, though the month was significant, there was no consistent lowering SCP trend over the fall.
Lower Lethal Temperature:
LLT50 across populations and months averaged 22.8°F with a 95% confidence interval of 21.2°F to 24.1°F. Month, temperature, population, and temperature*population were all significant factors (X=5.52, p=0.02; X=127.06, p=0.00, X=21.06, p=0.00, X=13.67, p=0.00 respectively). Month showed a lowering of LLT50 between September and October. Given the interaction between temperature and population, no consistent latitudinal trend was seen across populations.
Nymphs are found late into the fall but do not overwinter. Fall movement suggests that the adult kudzu bug overwinters in the leaf litter. Bugs move from the vines into the leaf litter between November and early December. During the sampling in November, the majority of samples with bugs were in the vines while in December the majority was found in the leaf litter. Other studies have additionally suggested adult bugs overwinter in leaf litter and under tree bark.
Caged movement of adults to overwintering locations confirms the field movement results. Adults moved from visible areas of the plants and cage into the leaf litter between mid November and mid December. The majority of adults were found overwintering in the leaf litter of the plants or the bins with leaf litter. Additionally, adults were observed to often overwinter in groups in the curled recesses of dead leaves.
The SCP isoline matches well with the current northern limit of the kudzu bug distribution. Cold tolerance is likely the predominate factor restricting northern expansion of the kudzu bug. The LLT50 isoline runs roughly through the middle of the current distribution. With warming winter temperatures the kudzu bug could conservatively move into south and central Pennsylvania as well as coastal areas as far north as Massachusetts by the end of the century.
Updates on research and monitoring have been recorded on mdkudzubug.org. Presentations were given at both professional and outreach meetings. Continuing work includes writing a publication for a scientific journal with detailed results.
The kudzu bug showed rapid expansion in range from initial discovery in 2009. Concerns grew for kudzu bug becoming a soybean pest in the mid-Atlantic as it is in the Southeast. My work has demonstrated that the northern boundary reached in Maryland in 2013 is due to the kudzu bug cold tolerance. Given the limited genetic variation and recent arrival of the insect, it is unlikely that it will rapidly spread further northward in the upcoming years. Kudzu bug will likely spread northward with warming winter temperatures over time.
Producers of soybeans can utilize the cold tolerance information to estimate their risk of kudzu bug annually. For example, producers near the SCP isoline should scout for the bug but it is unlikely for it to build to damaging population levels given the high winter mortality. Additionally, the distribution near the SCP isoline is patchy and heavily dependent on kudzu locations. Producers near or below the LLT50 isoline have a higher risk of damaging population levels given lower winter mortality.
Producers who wish to manage kudzu bug during the winter know to target leaf litter especially in adjacent wooded edges. Furthermore, a reduction of nearby kudzu could help to reduce the kudzu bug pressure on a soybean field. Overall, knowing the cold tolerance and overwintering behavior of the kudzu bug provides producers with annual risk assessment through overwinter survival predictions.
Education & Outreach Activities and Participation Summary
Currently a manuscript is being prepared for publication in a scientific journal. Other publications include:
Grant, J.I., and B. Lamp. (In Prep). Cold Tolerance of Megacopta cribraria (Hemiptera: Plataspidae): an Invasive Pest of Soybeans.
Grant, J.I., A. Leslie, and B. Lamp. 2015. (Fact sheet) Kudzu Bug: An Invasive Pest of Soybean. Available on: mdkudzubug.org Kudzu-Bug-Fact-Sheet
Grant, J.I., A. Leslie, and B. Lamp. 2014. Kudzu Bug, a Potential Soybean Pest, Survives the Harsh Winter. Agronomy News University of Maryland Extension Publication. 5(4): 1-2. Agronomy-News-2014
Results have been presented at professional conferences and extension meetings including:
- Grant J.I., and W. Lamp. Cold tolerance and overwintering behavior of kudzu bugs (Megacopta cribraria) at its northern limit, 25th International Congress of Entomology & Annual Meeting Entomological Society of America. Orlando, FL. (Grant, J.I. Speaker) International-Congress-of-Entomology-with-Entomolgical-Society-of-America-2016-Conference
- Grant J.I., and W. Lamp. Invasive kudzu bug (Megacopta cribraria) in Maryland: the spread and status of a recent invader. The Northeastern Plant, Pest, and Soils Conference: Annual Eastern Branch Meeting of Entomological Society of America. Philadelphia, PA. (Grant, J.I. Speaker) Eastern-Branch-of-the-Entomological-Society-of-America-2016
- Grant, J.I., and W. Lamp. Invasive kudzu bug’s (Megacopta cribraria) status in Maryland. University of Maryland College of Agriculture and Natural Resources Open House. College Park, MD. (Poster) University-of-Maryland-College-of-Agriculture-and-Natural-Resources-Poster
- Grant J.I., and W. Lamp. Overwintering Survival of Kudzu Bugs & its Degree-Day Requirements for Colonization of Soybeans in Maryland. Maryland Commodity Classic. Centreville, MD. (Grant, J.I. Speaker)
Producer adoption focuses on using new information for annual and long-term risk assessment. Knowing the cold tolerance of the kudzu bug allows a producer to predict if the following year will have high or low pest pressure due to temperatures experienced. Furthermore, cold tolerance is currently limiting the northward expansion; therefore, monitoring winter temperatures allows insight for future movement of the kudzu bug into more northern soybean cropland.
Sustainable agriculture is a broad and complex field. My work on this particular project elucidated the intricacy of making pest predictions. Many factors influence and change the dynamics of a pest and its host plants. My understanding of the depth of research for a given commodity has deepened through this project.