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1.12Plant bugs [Hemiptera: Miridae]

Lygus hesperus ^EP and Lygus lineolaris ^EP

Lygus hesperus (western plant bug) and Lygus lineolaris (tarnished plant bug) are not present in Western Australia and are therefore pests of quarantine concern for that state. The biology and taxonomy of these two species are considered sufficiently similar to justify combining them into a single assessment. In this assessment, the term ‘plant bugs’ is used to refer to these two species unless otherwise specified.

The family Miridae includes a large number of species, most of which feed on plants. Mirids are also referred to as plant bugs and are characterised as generalist plant feeding insects that use needle-like mouthparts to extract plant juices from their hosts at all stages of their life, from nymph to adult (University of Missouri 2000; CABI 2011). They may feed upon the fruit of their hosts as well as other reproductive plant parts such as flowers and buds (CABI 2011).

Plant bugs overwinter as adults in dead plants, leaf litter/plant debris and uncultivated areas outside the orchard (CABI 2011; Bentley et al. 2012a). During spring, females will lay eggs in a wide variety of plants that hatch into nymphs that undergo a number of nymphal phases (instars) before becoming adults. Adults are very active and mobile with a short life cycle, which for the L. lineolaris is around 30 days with 2–5 generations per year (CABI 2011). Within California, there have been reports of up to ten overlapping generations in a year for some plant bug species (Bentley et al. 2012a).

Plant bugs lay eggs and feed on both commercial and weedy host plants. The presence of weeds is an important factor that influences the number of plant bugs that may be found in a commercial crop, so control of weeds is usually recommended (CABI 2011).

The risk scenario of concern for L. hesperus and L. lineolaris is the presence of eggs in the imported commodity. As plant bugs are highly mobile and easily disturbed, it is unlikely that nymphal or adult plant bugs would remain associated with imported table grapes. 

Lygus hesperus and L. lineolaris have been assessed in the existing import policy for stone fruit from the United States (Biosecurity Australia 2010). The assessment of L. hesperus and L. lineolaris presented here builds on this previous assessment.

Differences in the host status between table grapes and stone fruit for L. hesperus and L. lineolaris make it necessary to assess the likelihood that L. hesperus and L. lineolaris will be imported into Western Australia with table grapes from California.

The probability of distribution, establishment and spread of L. hesperus and L. lineolaris in Western Australia and the consequences they may cause will be comparable for any commodity from which these species are imported into Western Australia, as these probabilities relate specifically to events that occur in Western Australia and are independent of the importation pathway. Furthermore, the risk scenario of concern is the presence of eggs on fruit and DAFF considers the likelihood of distribution of eggs to be comparable between stone fruit and table grapes. Accordingly, there is no need to reassess these components, and the risk ratings for distribution, establishment, spread and consequences, as set out for L. hesperus and L. lineolaris in the existing import policy for stone fruit from the United States (Biosecurity Australia 2010), will be adopted for this assessment.

1.12.1Probability of entry

The probability of entry is considered in two parts, the probability of importation and the probability of distribution, which consider pre-border and post-border issues, respectively.

Probability of importation

The likelihood that L. hesperus and L. lineolaris will arrive in Western Australia with the importation of table grapes from California is: VERY LOW.

Supporting information for this assessment is provided below:

• 
  Both L. hesperus and L. lineolaris are widely distributed across North America. Lygus hesperus is predominantly reported in the west of North America, ranging from southern British Columbia to northern Mexico (Zhou et al. 2012). In the western United States, it is reported as the dominant plant bug species in a complex that includes L. elisus, L. shulli and L. lineolaris (Zhou et al. 2012). Lygus lineolaris was originally recorded from the eastern United States but has since been reported to be one of the most widely distributed plant bug species in North America (Summers 2001; Mueller et al. 2012).
  
• 
  Both species are highly polyphagous with L. lineolaris reported to feed on more than 385 host plant species (Young 1986) and L. hesperus from over 100 plant species (Godfrey 2000; Zhou et al. 2012).
  
• 
  In California, plant bugs are considered major pests of cotton, fruit, vegetable and seed crops (Godfrey 2000). Lygus hesperus is a key pest of several agricultural crops in California, particularly in the San Joaquin Valley (Godfrey 2000). Its principal hosts include cotton, strawberry, alfalfa and dry beans (Godfrey 2000). Lygus lineolaris is also reported from California (Mueller et al. 2012) and is mainly a pest of apples, carrots, cherries, cotton, lima beans, seed alfalfa, green beans, soybeans, peaches, pears, strawberries, tomatoes and nursery stock (Dixon 2009), although infestation of grapes is also known (Bostanian et al. 2003; Fleury et al. 2006; Fleury et al. 2010). Despite its polyphagy, L. lineolaris mostly feeds on young apples and weeds (Fleury et al. 2010).
  
• 
  Plant bugs are associated with grapevines in the field. There have been reports of L. lineolaris in vineyards of Pennsylvania and in southwestern Quebec, Canada (Bostanian et al. 2003; Fleury et al. 2010). Adults feed on reproductive parts of their host plant (i.e. buds or flowers) or rapidly growing meristematic tissues (Bostanian et al. 2003). Grapevines continuously produce meristematic tissues (as they flower throughout the growing season) which provides new substrates for L. lineolaris generations to feed continuously throughout the season (Fleury et al. 2006; Fleury et al. 2010).
  
• 
  Laboratory studies have shown that adults of L. lineolaris are capable of feeding on all phenological stages of grapevines (Fleury et al. 2006). Most feeding in the berry development stages of grapevine growth occurs in the grape bunch on the upper part of the pedicel (Fleury et al. 2006).
  
• 
  Although eggs may be laid into fruit from around mid May until late in the season, females preferentially deposit eggs in stems, leaf parts and flowers of orchard weeds such as Amaranthus spp. (pigweed), Brassica spp. (wild mustard), Capsella bursa-pastoris (shepherd's-purse), Centaurea solstitialis (yellow starthistle), Chenopodium album (lambsquarters), Hemizonia spp. (tarweed), Melilotus officinalis (sweet clover), Raphanus raphanistrum (wild radish), Salsola tragus (Russian thistle), and Vicia spp. (vetch) (Anthon 1993; Fleury et al. 2010; CABI 2011; Caprile et al. 2011). For L. hesperus, winter and early spring weeds serve as a reservoir and provide a link to summer crops in the Central Valley of California (Godfrey 2000).
  
• 
  It is noted that nymphs are not commonly seen in orchards, suggesting that eggs are preferentially laid into other hosts. The availability and sequence of flowering in weedy hosts is thought to be a critical factor in their population dynamics (CABI 2011). The presence of adult plant bugs in orchards is linked to the drying up of primary host material around the orchard, at which time the adults migrate to the irrigated areas (Bentley et al. 2012a).
  
• 
  Adult and nymphal plant bugs are highly mobile and easily disturbed. Adults of L. lineolaris can easily and rapidly move between crops, and have been shown to fly over 12km in 12h, and 5km without interruption (Fleury et al. 2010). The process of harvesting table grapes is likely to disturb or dislodge any plant bugs associated with the fruit, but if any eggs are present, they may not be affected.
  
• 
  Unless fruit damage or other symptoms of infestation are obvious, fruit infested with eggs would not be expected to be removed by harvest and post-harvest quality assurance operations, particularly given the enclosed nature of table grape bunches.
  
• 
  Plant bugs overwinter as adults beneath weeds, on the orchard floor or in bordering uncultivated areas (Anthon 1993). Lygus lineolaris overwinters as diapausing adults beneath plant litter (Fleury et al. 2006) and resumes activity in spring when temperatures are greater than 8^oC (Anthon 1993; Bostanian et al. 2003).
  
• 
  Eggs are the life stage expected to be associated with the imported commodity. It has been shown that eggs can survive temperatures of 10^oC for 15 days without any notable level of mortality (Snodgrass and McWilliams 1992). However, there is no evidence that eggs exposed to the colder temperatures – such as those experienced during in-transit cold storage under commercial conditions – would result in significant mortality.
  

The high mobility of adults, limited reports citing infestation of table grapes and the predominant association of eggs and nymphs with weedy hosts support a likelihood estimate for importation of ‘very low’.

Probability of distribution

The probability of distribution for L. hesperus and L. lineolaris is being based on the assessment for stone fruit from the United States (Biosecurity Australia 2010). That assessment used the same methodology as described in Chapter of this report. The rating from the previous assessment was: MODERATE.

Overall probability of entry (importation  distribution)

The overall probability of entry is determined by combining the probabilities of importation and of distribution using the matrix of rules shown in Table 2.2.

The likelihood that L. hesperus and L. lineolaris will enter Western Australia as a result of trade in table grapes from California and be distributed in a viable state to a susceptible host is: VERY LOW.

1.12.2Probability of establishment and spread

The probability of establishment and of spread for L. hesperus and L. lineolaris is being based on the assessment for stone fruit from the United States (Biosecurity Australia 2010). That assessment used the same methodology as described in Chapter of this report. The ratings from that assessment were:

Probability of establishment: HIGH

Probability of spread: MODERATE

1.12.3Overall probability of entry, establishment and spread

The overall probability of entry, establishment and spread is determined by combining the probability of entry, of establishment and of spread using the matrix of rules shown in Table 2.2.

The likelihood that L. hesperus and L. lineolaris will enter Western Australia as a result of trade in table grapes from the United States, be distributed in a viable state to a susceptible host, establish in Western Australia and subsequently spread within Western Australia is: VERY LOW.

1.12.4Consequences

The consequences of the establishment L. hesperus and L. lineolaris in Western Australia have been estimated previously for stone fruit from the United State (Biosecurity Australia 2010). That assessment used the same methodology as described in Chapter of this report. The ratings from that assessment can be used in this review for Western Australia because the geographic level in the consequence impact scores did not exceed Regional. The estimate of impact scores from that analysis is provided below:

Plant life or health E Significant at the regional level

Other aspects of the environment B Minor significance at the local level

Eradication, control etc. D Significant at the district level

Domestic trade C Significant at the local level

International trade C Significant at the local level

Environment B Minor significance at the local level

Based on the decision rules described in Table 2.4, that is, where the consequences of a pest with respect to one or more criteria are ‘E’, the overall consequences are estimated to be MODERATE.

1.12.5Unrestricted risk estimate

Unrestricted risk is the result of combining the probability of entry, establishment and spread with the estimate of consequences. Probabilities and consequences are combined using the risk estimation matrix shown in Table 2.5.

Unrestricted risk estimate for L. hesperus and L. lineolaris
Overall probability of entry, establishment and spread	Very low
Consequences	Moderate
Unrestricted risk	Very low

As indicated, the unrestricted risk estimate for L. hesperus and L. lineolaris has been assessed as ‘very low’, which achieves Australia’s ALOP. Therefore, no specific risk management measures are required for this pest.