common name: Myakka bug (unofficial common name)
scientific name: Ischnodemus variegatus (Signoret) (Insecta: Hemiptera: Blissidae)

Introduction - Synonymy - Distribution - Description - Life Cycle and Biology - Hosts - Damage - Economic Importance - Natural Enemies - Selected References

Introduction

West Indian marsh grass, Hymenachne amplexicaulis (Rudge) Nees (Poaceae), is a robust, stoloniferous, semiaquatic, perennial grass native to the Neotropics (tropical Central and South America). This perennial grass is considered valuable forage in its native range (Tejos 1978, Enriquez-Quiroz et al. 2006). It reproduces from stolons or seeds in areas with fluctuating water levels and can survive long periods of flooding, but only persists along the edges of permanent deep water (Tejos 1980). West Indian marsh grass is especially adapted to low lying fresh water wetlands and flood plains containing high nutrient and sediment influx (Csurches et al. 1999).

In the 1970s and 1980s, H. amplexicaulis began invading wetlands in Florida (Langeland and Craddock-Burks 1998). Although the introduction pathway of this grass into Florida is uncertain, intentional introduction into Florida is possible due to its high forage value (Antel et al. 1998, Diaz et al. 2009, Kibbler and Bahnisch 1999). Hymenachne amplexicaulis, a Florida Exotic Pest Plant Council Category I invasive plant, competitively displaces native vegetation in wetland areas due to its aggressive growth patterns during the rainy season (Diaz et al. 2009, FLEPPC 2009).

In 2000, the "Myakka bug," Ischnodemus variegatus (Signoret) (Hemiptera: Blissidae), was first reported causing severe damage to H. amplexicaulis at Myakka River State Park, Sarasota County, Florida (Brambila and Santana 2004). Ischnodemus variegatus was identified as a new record for the continental United States by the Florida Department of Agriculture and Consumer Services (FDACS) (Halbert 2000). Research was conducted by University of Florida scientists on the biology, host specificity, and potential impact for this newly introduced, exotic insect species.

Synonymy

The taxonomic status of this species was reviewed by Slater (1987) who raised I. variegatus (Signoret) from synonymy with Ischnodemus oblongus Fabricius.

Distribution

The native distribution of I. variegatus includes Central and South America. Collection records indicate H. amplexicaulis may be the only host (Baranowski 1979, Slater 1987).

As mentioned above, both I. variegatus and H. amplexicaulis now occur in Florida.

In 1988, H. amplexicaulis was released in Queensland for use as "ponded pasture" (Csurhes 1999).

Description

Members of the genus Ischnodemus are characterized by elongate, parallel sided bodies, closed fore coxal cavities, terete (cylindrical) antennae, a straight apical (tip) margin, and a forewing membrane with a distinctive morphological texture (i.e. the clause and corium are well differentiated) (Slater and Wilcox 1969). Slater (1976) classified this genus as a 'Type I' body shape which includes species with elongate, slender body shape that is usually slightly flattened. The 'Type I' body shape is advantageous for insects living on the stems of grasses.

Laboratory and field observations indicate the 1st through 4th instars are typically found in aggregations while 5th instars and adults are often observed exploring as individuals. If nymphs or adults are disturbed, they secrete a strong odor from the scent glands located in the thorax and abdomen (Diaz et al. 2008).

scent glands

Adults: Females (7.23 mm in length, ± 0.56, n=28) are larger than males (6.05 mm in length, ± 0.22, n=49) and both genders have a distinctive "M" pattern at the base of the hemelytra. Female sclerites (hardened plates) at the ventral, or top side, tip of the abdomen are triangular in shape. The last sclerites of males are more rounded.

adult female

adult sclerites

Adult flying is restricted to short hops of a few meters or less. Gravid females mostly walk, possibly due the large size of their abdomens (Diaz et al. 2008).

Eggs: The egg length is approximately 3 mm (0.1 inches). Eggs are laid in masses (averaging 12 eggs per mass, with a range of 1 to 38) between the leaf sheath and the culm (or stem of the plant), preferentially near the node. Newly deposited eggs are white and older eggs turn bright red (Diaz et al. 2008).

egg mass

Nymphs: Ischnodemus variegatus has five nymphal instars. Instars initially remain aggregated near the site of oviposition, or egg laying. Later nymphal instars migrate to tightly appressed spaces between leaves and stems. Fourth and 5th instars are darker in color than early instars.

1st instar

2nd instar

3rd instar

4th instar

5th instar

Life Cycle and Biology

Average total development time from egg to adult is 40 days. Eggs take an average of 12 days to hatch at 30.5° C (86.9°F). The nymphal stage reaches adulthood in an average of 29 days at 30.5° C (86.9°F). The preoviposition period is about seven days at 28°C (82.4°F) (Diaz et al. 2008). Females lay their eggs in tight spaces between the leaf sheath and the stem. When the eggs hatch, the 1st instar nymph remains together near the site of emergence (Diaz et al. 2008).

Scientists developed a temperature-dependent development model to predict the number of generations that I. variegates could complete per year at different locations in Florida. In North Florida, the model predicts that I. variegatus can complete two to three generations per year. In South Florida, the predicted number of generations increases to four to five per year (Diaz et al. 2008).

generational model

Developmental time and survival of eggs as well as immature stages are affected by temperature. When I. variegatus is exposed to low temperatures from 8°C (46.4°F) to 18°C (64.4°F) and to a high temperature of 38°C (100.4°°F), low survivorship occurs. Nymphs died within a few days at higher temperatures of 38°C (100.4°F) and after weeks at lower extreme temperatures, suggesting that I. variegatus has a broader lower temperature threshold compared to the upper threshold.

The optimal temperature range for development and survival is between 28°C (82.4°F) and 33°C (91.4°F). These ideal conditions for I. variegatus development match with the weather conditions in central Florida from April to October (Diaz et al. 2008).

Hosts

Due to economic and ecological importance of grasses, scientists at the University of Florida studied the host range of I. variegatus. They found that H. amplexicaulis is the preferred host of I. variegatus in laboratory and field conditions. In laboratory conditions, developmental host range of I. variegatus was examined on 57 plant species across seven plant families. Complete development was obtained from H. amplexicaulis (23.4%), compared to water paspalum, Paspalum repens (0.4%); beaked panicgrass, Panicum anceps (2.2%); and fire flag, Thalia geniculata (0.3%). In field experiments, H. amplexicaulis had higher densities of I. variegatus than other species (Diaz et al. 2009).

H. amplexicaulis

The seasonal cycle of H. amplexicaulis in Florida begins in spring during seed germination and new shoot growth. Increases in the water level as well as favorable day-length and temperature in the summer allow the grass to grow aggressively. Maximum biomass for H. amplexicaulis is reached by late summer. Later in the fall, short days trigger flower production (Tropical Weeds Research Centre 2006). During winter, some parts of the grass die, but the stolons and seeds remain dormant underwater until spring. Based on herbarium specimens collected in the native range, a predictive model of the potential distribution of H. amplexicaulis in Florida was created suggests that its northern limit in Florida will be Alachua County.

potential distribution of H. amplexicaulis

Damage

Damaged leaves turn dark red, due to the accumulation of anthocyanins (a type of pigment in the host plant). Persistent infestations eventually result in leaves turning brown and dying. Feeding effects of I. variegatus diminish carbon dioxide assimilation, growth rate, photosynthetic capacity and biomass of H. amplexicaulis (Overholt et al. 2004). Greenhouse experiments demonstrated that I. variegatus feeding damage negatively affected growth of H. amplexicaulis seedlings (Diaz 2008).

feeding damage to H. amplexicaulis

Economic Importance

Population outbreaks of I. variegatus during the summer produce a major stress on West Indian marsh grass plants growing in poor conditions (shallow canals). However, plants growing in resource rich environments (deep floodplains, high nutrients runoff) can sustain some damage by I. variegatus without impact on the plant's reproductive output (Diaz 2008).

Natural Enemies

Ischnodemus variegatus has two natural enemies in Florida: the egg parasitoid Eumicrosoma sp. (Hymenoptera: Scelionidae) and the entomopathogen Beauveria bassiana (Balsamo) Vuillemin (Deuteromycotina: Hyphomycetes). The egg parasitoid was identified as a potentially accidentally introduced, non-native species for North America (T. Nuhn 2005, personal communication). It attacks young and old eggs, and parasitized eggs turn black. Field sampling in Florida demonstrated that the impact of these natural enemies is minimal to I. variegatus populations

Selected References

Authors: Cecil O. Montemayor, Rodrigo Diaz and William A. Overholt, University of Florida; Amanda Hodges, SPDN, University of Florida
Photographs and Illustrations: Rodrigo Diaz, University of Florida
Project Coordinator: Thomas R. Fasulo, University of Florida
Publication Number: EENY-471
Publication Date: February 2010
Copyright 2010 University of Florida

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