Soybean [Glycine max (L.) Merrill] is the second largest crop grown in the United States in terms of land area and cash value, with 33.9 million hectares planted, yielding US$40.3 billion in 2014 (USDA–NASS, 2015a, 2015b). Forty‐eight percent of soybean planted in the United States was exported in 2014, making it the largest value export crop (ASA, 2014). Yet, soybean suffers from insect pests that cause significant reductions in both seed quality and yield. Insect damage in the US Southeast is greater than that in the rest of the United States due to the warmer climate and longer growing season. Even with integrated pest management (IPM) strategies, growers must apply large amounts of insecticides to control insect pests. In 2002, approximately 771 Mg of insecticide were applied to soybean fields nationwide (Gianessi and Reigner, 2006). Soybean insecticide use quadrupled between 2002 and 2012 due to the introduction of the soybean aphid (Aphis glycines Matsumura) in 2000 (Yang and Suh, 2015).

The kudzu bug (KZB), Megacopta sp. (Hemiptera: Plataspidae), is a recently introduced insect pest from Asia, where it is commonly known as the globular stink bug, lablab bug, or bean plataspid (Eger et al., 2010). The KZB was first found in Georgia in 2009 on kudzu [Pueraria montana (Lour.) Merr. variety lobata (Willd.)] and was thus named the kudzu bug.

Since its introduction in 2009, KZB has quickly spread across the southeastern United States, becoming a major soybean pest in this region. The KZB is now established in 13 states, from the Atlantic Coast to the Mississippi River (Gardner, 2015). A prediction model based on the KZB native range shows its potential to spread north into major soybean production areas of the Midwest (Zhu et al., 2012). However these predictions may not hold, as a decrease in KZB populations across the Southeast was observed during 2014, which is attributed to the abnormally cold winter during 2013–2014 (Thompson, 2014).

While kudzu and soybean are the primary hosts for KZB, they are known to feed on other legumes as well (Medal et al., 2013; Zhang et al., 2012). Chloroplast DNA from loblolly pine (Pinus taeda L.), black walnut (Juglans nigra L.), red oak (Quercus rubra L.), and sweet gum (Liquidambar stryraciflua L.) has been detected in the guts of adult KZBs (Lovejoy and Johnson, 2014), which may indicate that these tree species serve as overwintering and migratory hosts when kudzu and soybean are not available. Adult KZB are known to overwinter near kudzu patches and soybean fields under leaf litter, behind tree bark, and in houses, where they become nuisance pests due to their stench and ability to stain white surfaces (Suiter et al., 2010).

In the spring, adults begin feeding and mating on kudzu and early‐planted soybean (Pozo‐Valdivia and Reisig, 2013). The KZB uses its piercing–sucking mouthparts to feed on the stems, leaves, and pod walls, which results in visible plant damage in the form of black feeding lesions, and ultimately in yield loss. The average yield loss due to KZB infestation in soybean fields in the Southeast is 19%, with losses up to 60% observed (Greene et al., 2012; Seiter et al., 2013). Therefore, growers face the added expense of controlling KZBs with application of a broad‐spectrum insecticide.

Females can oviposit more than 100 eggs on the leaves or stems of a variety of hosts, but nymphs are only known to develop into adults on kudzu, soybean, cowpea [Vigna unguiculata (L.) Walp.], pigeon pea [Cajanus cajan (L) Millsp.], lima bean (Phaseolus lunatus L.), and common bean (Phaseolus vulgaris L.) (Medal et al., 2013; Zhang et al., 2012). First‐instar nymphs ingest their gut endosymbiont bacteria, Candidatus Ishikawaella capsulata Mp., from small brown capsules deposited under the egg mass (Fukatsu and Hosokawa, 2002). KZB develop through five nymphal instars, each lasting approximately 6 to 10 d, before molting into sexually mature winged adults (Pozo‐Valdivia and Reisig, 2013). Adult longevity is 23 to 77 d, so one to three generations can occur each year (Eger et al., 2010).

Using primarily physical characteristics, the KZB present in the United States was originally identified as Megacopta cribraria (Fabricius) (Eger et al., 2010). M. cribraria originates from China, where it is not a major pest of soybean (Hosokawa et al., 2006). However, its Japanese sister species, M. punctatissima (Montandon), is considered a major soybean pest (Hosokawa et al., 2006). Based on comparison of an 8.7‐kb mitochondrial DNA sequence from the native Japanese species with that of the US populations, the pest found in the United States is the Japanese species (Hosokawa et al., 2014). The same conclusion comes from genome sequencing of the gut microbial symbiont, C. I. capsulata (Brown et al., 2014), which determines host preference and pest status on soybean (Hosokawa et al., 2007).

Host plant resistance (HPR) is an integral part of IPM programs, and is considered by some to be the foundation of IPM strategies (Smith, 2005). HPR is recognized as a cost‐effective, environmentally friendly way to control insect pests that has been deployed by breeders for controlling both leaf‐chewing and piercing–sucking insect pests (Hill et al., 2006a, 2006b; Mensah et al., 2005; Mian et al., 2008; Zhu et al., 2006).

Painter (1951) proposed three categories for classifying HPR to insects: preference (antixenosis), antibiosis, and tolerance. Antixenosis refers to the host plant effect on insect behavior that deters oviposition and/or feeding. Natural field infestations, caged field plots, and greenhouse experiments that give insects a choice of plants to feed on are used to identify antixenosis (All et al., 1989; Hill et al., 2004; Rowan et al., 1991). Antibiosis refers to the adverse host plant effect on the physiology and life history of the insect. The adverse effect can be measured as increased mortality, slowed development, or decreased fecundity. No‐choice experiments, which restrict the insect to feed on a single genotype, are used to measure antibiosis. These assays are usually conducted in controlled environments, such as growth chambers or greenhouses (Hill et al., 2004; Zhu et al., 2008).

Simplified visual plant damage ratings and indices have been established by breeders for resistance screening against important insect pests of soybean, such as a defoliation rating for leaf‐chewing insects (All et al., 1989), a seed damage index for stink bug (Gilman et al., 1982), and the aphid index for soybean aphid (Hill et al., 2004; Mensah et al., 2005). These damage ratings facilitate quick and easy screening of lines that does not require tedious count measurements throughout the growing season. However, there is not an established rating for KZB. Xing et al. (2006, 2008) used a percent rating of black mildew on the stem and of purple spots on leaves to screen the Chinese soybean germplasm and map quantitative trait loci (QTLs) for resistance to M. cribraria. The rating was highly correlated (r ≥ 0.87) with nymph counts.

This study tested screening techniques in the field, provides a simplified KZB index for screening lines, identified potential sources of resistance to the KZB in soybean, and characterized the resistance mechanism in growth chamber experiments.