MP-1777
Jesusa C. Legaspi, Robert R. Saldana and Norman Rozeff
Assistant Professor and Agricultural Technician
Texas Agricultural Experiment Station
The Texas A&M University System
and
Agriculturist
Rio Grande Valley Sugar Growers, Inc.
The sugarcane industry, constituting approximately 10 percent of all agricultural income in the Lower Rio Grande Valley (LRGV) of Texas, is an essential component of the region's agricultural economy. The LRGV produces all the commercial sugarcane grown in Texas and is the fourth largest source of domestic sugar from cane in the United States. In the 1994-95 growing season, sugarcane acreage totaled about 43,000. Gross cane harvested neared 1.5 million tons, with an economic value of almost $64 million.
Stalkboring moths have been the most serious pests of
sugarcane in the Rio Grande Valley since the early
1970s. At that time, the primary pest was the sugarcane
borer, Diatraea saccharalis F., until the parasite
Cotesia
flavipes (Cameron) brought about biological control. The
primary pest today, the Mexican rice borer, Eoreuma
loftini (Dyar), was first detected in 1980. Surveys taken
in 1996 indicate that the Mexican rice borer currently
constitutes about 98 percent of all stalkborers in the
LRGV. Since 1989, the Mexican rice borer has been
found to attack approximately 20 percent of the
sugarcane inspected. The result is the loss of an
estimated $10 million to $20 million annually. The
Mexican rice borer is moving northeast along the Gulf
Coast, further threatening sugarcane, corn and other
gramineous crops such as rice, sorghum, wheat and
forage grasses throughout the South.
Biology and Ecology of Stalkborers
Sugarcane borer life cycle. The sugarcane borer (SCB) lays its cream-colored eggs, similar in appearance to a patch of fish scales, on green leaf blades. The eggs are flat, oval and laid in flat clusters of about 25 (Fig. 1). In 4 to 9 days, the eggs hatch into larvae, which grow to about 1 inch long. Larvae are yellowish-white with brown spots, although spots are often absent in winter. Spring and summer larvae possess dark brown head capsules, with brown spots called dorsal plates, on a cream-colored body (Fig. 2). The larvae undergo six or seven instars and tunnel vertically within stalks, producing a hollow cavity that may be invaded by red rot fungus (Fig. 3). After 20 to 30 days of feeding, the larvae pupate. The pupae are about 0.7 inches long and have numerous projections or hairs in the abdominal area, resulting in a characteristic "sandpaper" appearance (Fig. 4). The adult emerges after about 9 days. The adult sugarcane borer is a straw-colored moth with wings marked by black dots arranged in a V pattern (Fig. 5). The SCB life cycle was measured at 43 days in the laboratory at 28°C (82.4°F). In the field, the life cycle is completed in 30 to 45 days during the summer. Four to five generations are produced per year.
Mexican rice borer life cycle. The globular eggs are cream colored and laid in masses of five to 100 (Fig. 6). The eggs are usually deposited between layers of dry leaf tissue near the plant base. As with the SCB, larvae are also cream colored but possess four parallel purple- red lines along the length of the body (Fig. 7). The head capsule is orange-brown, in contrast to the cream- colored body. Larvae undergo five or six molts, after which they measure about 0.75 to 1.0 inch long. Young rice borer larvae feed on and in the leaf sheaths, producing a blotched mine with dark red or purple discoloration (Fig. 8). Once they enter the stalks, the larvae tunnel both vertically and horizontally in a girdling fashion, which may result in stalk breakage. Tunnels remain packed with frass (feeding debris and exerement), thereby protecting the larvae from chemical and biological control agents. The mature larva constructs a pupation cell near the stalk surface that is protected by one or two layers of transparent leaf tissue (emergence window). The pupae measure 0.75 inch to less than an inch long and are orange-brown with small projections (tubercles) at the posterior of the abdomen (Fig. 9). Adults, about 0.5 to 0.75 inch long and creamy white (Fig. 10), escape through the emergence window. The adult is distinguished from other stalkborers by a dark spot in the center of each forewing and the absence of other wing markings. Mean developmental times in the laboratory at 27°C (80.6°F) are: egg to larva, 6 to 7 days; larva to pupa, 28.5 days; pupa to adult, 9.6 days; and adult life, about 7 days. The total is about 48.5 days. Mean total fecundity increases from about 260 eggs per female at 20°C (68°F) to a maximum of more than 400 eggs per female at 26°C (78.8°F), and then declines to about 350 at 29°C (84.2°F) and 32°C (89.6 °F). The maximum oviposition rate of about 188 eggs per female per day occurs at 29°C. Four to six generations per year are common in the field. Mexican rice borer larvae are known to undergo diapause in fall and winter and can withstand freezing temperatures. Adequate crop preparation is needed to reduce survival into the next cropping cycle.
Distinguishing between sugarcane borers and
Mexican rice borers. Plant damage caused by the
Mexican rice borer differs from that of the sugarcane
borer and other stalkborers in general. Tunnels may be
vertical like those of other stalkborers but may also be
horizontal or diagonal. Furthermore, Mexican rice borer
tunnels are characteristically packed with frass, unlike
other stalkborer tunnels where the frass is cleared and
deposited between the leaf sheath and stalk. Compared
to that of the Mexican rice borer, the sugarcane borer's
tunnel size is wider because of its larva's large diameter.
Larvae appear similar, both being nearly white and about
26 mm in length. They may be differentiated by the
sugarcane borer's head coloration, which is dark brown,
becoming black near the mouthparts. In contrast, the
Mexican rice borer's head is orange-brown or reddish.
Further, the sugarcane borer possesses light brown setae
(hairs) and pinacula (plates) on its body, except during
winter; the Mexican rice borer has no conspicuous setae
or pinacula. Perhaps the most obvious difference is the
four purple-red lines traversing the length of the Mexican
rice borer larva. After pupation, the exit hole through
which the sugarcane borer adult emerges is covered with
a silky webbing (Fig. 11), whereas that of the Mexican rice
borer is usually clear (Fig. 12). Microscopic examination
of the genitalia is useful for determining the sex and
species identity of individual moths.
|
|
| Fig 1. Sugarcane borer, Diatraea saccharalis egg mass | Fig 2. Sugarcane borer, D. saccaralis larva |
|
|
| Fig 3. Tunnel made by sugarcane borer, D. saccharalis, and fungal red rot | Fig 4. Sugarcane borer, D. saccharalis pupa |
|
|
| Fig 5. Sugarcane borer, D. saccharalis adult | Fig 6. Mexican rice borer, Eoreuma loftini egg mass |
|
|
| Fig 7. Mexican rice borer, E. loftini larva | Fig 8. Tunnel with frass made by Mexican rice borer, E. loftini |
|
|
| Fig 9. Mexican rice borer, E. loftini pupa | Fig 10. Mexican rice borer, E. loftini adult |
|
|
| Fig 11. Sugarcane borer, D. saccharalis adult exit hole | Fig 12. Mexican rice borer, E. loftini adult exit hole |
Managing Stalkborers
Cultural control. Cultural control incorporates a variety of techniques to alter the crop habitat and make it less favorable for pest reproduction and survival. In sugarcane, cultural control methods may be used to create a clean crop to reduce the possibility of stalkborer infestation. Weeds such as Johnsongrass should be controlled because they serve as an alternate host for the stalkborers. Raking and burning plant trash and stalk pieces after harvest can reduce Mexican rice borer survival in the field because dead shoots and dried leaves provide rice borers with sites for laying eggs and borers may overwinter in stalk pieces. The use of clean seed is important in promoting stalk quality and reducing damage by insects and diseases. Improved plant vigor and resistance to borer damage may be achieved by reducing other plant stresses caused by poor soil fertility, disease, improper irrigation, salinity and poor weed management.
Chemical control. Chemical control of the Mexican rice borer has met with limited success because of the borer's tunneling behavior, which restricts contact with pesticides. Plant density is also high and restrictive. The rice borer is active throughout the production season; therefore, reliance on chemical control requires repeated applications, resulting in only temporary suppression of pest populations and entailing significant cost. For these reasons, some farmers of the LRGV have abandoned insecticides altogether as a method of controlling the Mexican rice borer. When insecticides are being considered, scouting for Mexican rice borers should begin before the first aboveground internode is formed and continue regularly on an individual field basis. Based on scouting, the economic threshold of 7 to 10 percent infestation of leaf sheaths and blades by young larvae should be reached before spraying. Sprays must be applied when young larvae have not penetrated the stalks. For current insecticide application and dosage recommendations, consult product labels, your county Extension agent, or the co- op agriculturist.
Host plant resistance. In this method, natural or acquired plant tolerance to insect pests is incorporated into commercial varieties, thereby producing insect-resistant clones. The identification of these resistant clones and the subsequent development and marketing of commercial clones may reduce loss to insect pests in sugarcane. Much work has been done in Louisiana to develop sugarcane resistance to the sugarcane borer. However, only recently has differential resistance to Mexican rice borer been found in the clones evaluated. Fifty-four precommercial clones were developed within the sugarcane breeding program of the Texas A&M University Research and Extension Center at Weslaco, Texas. These clones, together with five commercial ones, were evaluated in the field for resistance to the Mexican rice borer. One commercial clone was found to be highly resistant to the rice borers; however, seven precommercial ones exhibited similar levels of resistance, and two had even less rice borer damage than the commercial clone.
Biological control. This approach involves
the use
of predators, pathogens and parasites. The most
promising biological control method has been the use
of parasites. Since the early 1980s, about 50 species
of parasites have been imported to the LRGV from
different countries and released to control Mexican
rice borers. Some have been found to establish in the
LRGV. Most parasites recovered from stalkborers
collected in 1996 were the exotic species Allorhogas
pyralophagus Marsh and Alabagrus stigma (Brulle),
and the native species Chelonus sonorensis Cameron
(Fig. 13) and Digonogastra solitaria Wharton and
Quick. Despite their successful establishment, none
has produced control comparable to that of the earlier
introduction of C. flavipes (Fig. 14) against sugarcane
borers. However, new exotic parasites are continually
being evaluated and released in the LRGV.
|
|
| Fig 13. Chelonus sonorensis, a native parasite of the Mexican rice borer | Fig 14. Cotesia flavipes, an exotic parasite of the sugarcane borer |
References
Acknowledgments
We thank Rick Hernandez (Texas Agricultural Experiment Station) for help with the photographs. Photographs courtesy of Texas Agricultural Experiment Station. Funding for this publication was provided by the Rio Grande Valley Sugar Growers, Inc.
The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the Cooperative Extension Service is implied.
Educational programs conducted by the Texas Agricultural Extension Service are open to all people without regard to race, color, sex, disability, religion, age or national origin.
Copies printed: 1,000
1997
Return
to top of page
Return
to Departmental Home Page
Last modified: November 20, 1997