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Dislodgment of conidia of Nomuraea rileyi from cadavers of cabbage looper, Trichoplusia ni
JOURNAL
OF INVERTEBRATE
~islodg~ent
PATHOLOGY
30,114-
116 (1977)
of Conidia of ~o~ur~e~ ri~eyi from Cadavers of Cabbage Looper, ~ric~o~/usi~ ni
Wind is probably one of the most important means of dislodging and dispersing spores of many phytopathogenic and entomopathogenic fungi (P. H. Gregory, Trans. Brit. Mycol. Sot. 28, 26-72, 1974). Thus the annual epizootics of caterpillars in soybean ecosystems that are caused by No~uraea rileyi probably result from dispersal by wind of conidia from larvae that died earlier in the season (C. M. Ignoffo, C. Garcia, D. L. Hostetter, and R, E. Pinnell, J. Invertebr. Pathol. 29, 147-152, 1977). Consequently, we were interested in determining what velocity of air will dislodge conidia of N. rileyi. The results of our laboratory studies of this subject are reported herein. The device depicted in Figure IA was constructed to determine the minimum velocity of air necessary to dislodge conidia of N. rileyi from a spore-laden cadaver. It consists of a longitudinal section of plastic pipe (a) (inside diameter, 12.5 cm; wall thickness, 0.62 cm; length, 45 cm) that supports rings (b,c,d) which, in turn, hold a slide screen and a wooden dowel. The air current, generated by compressed air at 90 psi (g), is channeled through a flow meter (f) and a funnel (e) and then directed at a conidia-laden larva (d) mounted on a wooden dowel with plastic cement. Dislodged conidia are collected on a glass slide coated with a thin layer of 70% Sorbol.’ The glass slide is mounted on a 0.85 x 1.35cm wire screen (0.25cm mesh) with rubber bands (b). The screen with slide is then placed in the airstream. The lengths of the ring holding the slide (b), the spacer ring (c), and the dowel supporting the larva (d) are 5, 1.25, and 1.25 cm, respectively. The distance from the edge of the funnel to the larvae is 15 cm; from the edge of the funnel to the slide it is 20 cm. Air velocities were determined for various flow-meter settings
(Mark III, Model 448324; Fisher Scientific Co., St. Louis, Missouri) by using an anemometer (Taylor Model 3132; Sybron Corp., Arden, North Carolina). l Only dead, fifth-instar cabbage looper larvae, Trichoplusia ni, heavily laden with eonidia of N~~uraea rileyi were used in our experiments. The technique for obtaining these was previously described (C. M. Ignoffo, B. Puttler, D. L. Hostetter, and W. A. Dickerson, J. Znvertebr. Pathol., 28, 259-262, 1976). In short, second-instar larvae were exposed to soybean leaflets containing an LC5,, dose of conidia (approximately three conidia/mm2). Then after 48 hr, the larvae were transferred to diet and reared at 22-24°C. Heavy mycelial growth was evident on dead, mature larvae 7 to 10 days after initial exposure. Sporulation began within 2-4 days of the appearance of mycelium, and within another 2-6 days the cadavers were heavily laden with conidia. Counts of conidia were made by using phase microscopy (Zeiss Universal model). Approximately 15 fields/slide were counted at random. A magnification of 640x was used; the field area was 0.0745 mm2. Although some conidia were released at less than 1.6 kmlhr, the minimum velocity to cause consistent disl~gement was 2.7 km/hr (Table 1). A steady increase in the percentage of microscopic fields with conidia, and the average number of conidia per field were recorded for velocities between 2.7 and 5.9 km/hr. Almost all of the fields examined had conidia after exposure to a velocity of 5.9 km/hr. No spores were obtained from slides exposed to still air before or after each series of tests. * Mention of a commercial or proprietary product does not constitute endorsement or recommendation of this product by the U.S. Department of A~cul~re. 114
Copyright 6 1917 by Academic Press. Inc. Ml rights of reproduction in any form reserved
ISSN 0022-201 I
NOTES
FIG. 1. (A), Device used to measure dislodgment of conidia of cabbage loopers, Trichoplusia ni. The separate items of slide holder, (c) spacer, (d) larva holder, (e) funnel, (f) flow meter, (b) used to support screen with slide and dowel with conidia-laden support (a) and larval mount (d).
1 I5
of Nomuraea rileyi from the cadavers the device are: (a) ring support, (b) (g) pressure meters and valve. (B), Ring cadaver (d). (C), Funnel-end view of ring
116
NOTES TABLE
1
PERCENTAGES OF MICROSCOPIC FIELDS CONTAINING CONIDIA OF NOMURAEA RILEYI AND AVERAGE NUMBERS OF CONIDIA PER FIELD AFTER ExPOSURE OF LARVAE BEARING CONIDIA TO VARIOUS AIR CURRENTS”
Air velocity (km/hr)
Number of fields examined
Fields with conidia (5%)
Number conidiai field (X)
O.ob 0.3 Et 0.2 0.8 + 0.2 1.9 k 0.2 2.7 2 0.3 4.2 k 0.3 5.9 + 0.3 0.0’
90 86 86 86 82 82 98 90
0.0 2.3 1.2 0.0 11.0 50.0 96.9 0.0
0.00 0.02 0.20 0.00 9.18 33.13 85.85 0.00
(1All values are results of six replicates; 2 indicates the standard error of mean. * Before tests. r After tests.
After 30 min of continuous exposure at 5.9 kmlhr, a total of 9393 conidia was counted in 66 microscopic fields (one larva/ replicate, four replicates). The percentages of the total conidia collected on slides after 1, 2, 3, 5, 15, and 30 min were 66.2, 16.3, 6.0, 4.5, 4.6, and 2.4%, respectively. Thus, approximately 90% of the conidia were collected within the first 3 min; thereafter the percentage collected leveled off. This does not mean that the cadavers were denuded of conidia. More conidia could be dislodged if the air velocity was increased; however, higher velocities of >16 km/hr eventually disintegrated some cadavers. C. GARCIA C. M. IGNOFFO Biological Control of Insects Research Agricultural Research Service U.S. Department of Agriculture Columbia, Missouri 65201 Received September 17, 1976
OF INVERTEBRATE
~islodg~ent
PATHOLOGY
30,114-
116 (1977)
of Conidia of ~o~ur~e~ ri~eyi from Cadavers of Cabbage Looper, ~ric~o~/usi~ ni
Wind is probably one of the most important means of dislodging and dispersing spores of many phytopathogenic and entomopathogenic fungi (P. H. Gregory, Trans. Brit. Mycol. Sot. 28, 26-72, 1974). Thus the annual epizootics of caterpillars in soybean ecosystems that are caused by No~uraea rileyi probably result from dispersal by wind of conidia from larvae that died earlier in the season (C. M. Ignoffo, C. Garcia, D. L. Hostetter, and R, E. Pinnell, J. Invertebr. Pathol. 29, 147-152, 1977). Consequently, we were interested in determining what velocity of air will dislodge conidia of N. rileyi. The results of our laboratory studies of this subject are reported herein. The device depicted in Figure IA was constructed to determine the minimum velocity of air necessary to dislodge conidia of N. rileyi from a spore-laden cadaver. It consists of a longitudinal section of plastic pipe (a) (inside diameter, 12.5 cm; wall thickness, 0.62 cm; length, 45 cm) that supports rings (b,c,d) which, in turn, hold a slide screen and a wooden dowel. The air current, generated by compressed air at 90 psi (g), is channeled through a flow meter (f) and a funnel (e) and then directed at a conidia-laden larva (d) mounted on a wooden dowel with plastic cement. Dislodged conidia are collected on a glass slide coated with a thin layer of 70% Sorbol.’ The glass slide is mounted on a 0.85 x 1.35cm wire screen (0.25cm mesh) with rubber bands (b). The screen with slide is then placed in the airstream. The lengths of the ring holding the slide (b), the spacer ring (c), and the dowel supporting the larva (d) are 5, 1.25, and 1.25 cm, respectively. The distance from the edge of the funnel to the larvae is 15 cm; from the edge of the funnel to the slide it is 20 cm. Air velocities were determined for various flow-meter settings
(Mark III, Model 448324; Fisher Scientific Co., St. Louis, Missouri) by using an anemometer (Taylor Model 3132; Sybron Corp., Arden, North Carolina). l Only dead, fifth-instar cabbage looper larvae, Trichoplusia ni, heavily laden with eonidia of N~~uraea rileyi were used in our experiments. The technique for obtaining these was previously described (C. M. Ignoffo, B. Puttler, D. L. Hostetter, and W. A. Dickerson, J. Znvertebr. Pathol., 28, 259-262, 1976). In short, second-instar larvae were exposed to soybean leaflets containing an LC5,, dose of conidia (approximately three conidia/mm2). Then after 48 hr, the larvae were transferred to diet and reared at 22-24°C. Heavy mycelial growth was evident on dead, mature larvae 7 to 10 days after initial exposure. Sporulation began within 2-4 days of the appearance of mycelium, and within another 2-6 days the cadavers were heavily laden with conidia. Counts of conidia were made by using phase microscopy (Zeiss Universal model). Approximately 15 fields/slide were counted at random. A magnification of 640x was used; the field area was 0.0745 mm2. Although some conidia were released at less than 1.6 kmlhr, the minimum velocity to cause consistent disl~gement was 2.7 km/hr (Table 1). A steady increase in the percentage of microscopic fields with conidia, and the average number of conidia per field were recorded for velocities between 2.7 and 5.9 km/hr. Almost all of the fields examined had conidia after exposure to a velocity of 5.9 km/hr. No spores were obtained from slides exposed to still air before or after each series of tests. * Mention of a commercial or proprietary product does not constitute endorsement or recommendation of this product by the U.S. Department of A~cul~re. 114
Copyright 6 1917 by Academic Press. Inc. Ml rights of reproduction in any form reserved
ISSN 0022-201 I
NOTES
FIG. 1. (A), Device used to measure dislodgment of conidia of cabbage loopers, Trichoplusia ni. The separate items of slide holder, (c) spacer, (d) larva holder, (e) funnel, (f) flow meter, (b) used to support screen with slide and dowel with conidia-laden support (a) and larval mount (d).
1 I5
of Nomuraea rileyi from the cadavers the device are: (a) ring support, (b) (g) pressure meters and valve. (B), Ring cadaver (d). (C), Funnel-end view of ring
116
NOTES TABLE
1
PERCENTAGES OF MICROSCOPIC FIELDS CONTAINING CONIDIA OF NOMURAEA RILEYI AND AVERAGE NUMBERS OF CONIDIA PER FIELD AFTER ExPOSURE OF LARVAE BEARING CONIDIA TO VARIOUS AIR CURRENTS”
Air velocity (km/hr)
Number of fields examined
Fields with conidia (5%)
Number conidiai field (X)
O.ob 0.3 Et 0.2 0.8 + 0.2 1.9 k 0.2 2.7 2 0.3 4.2 k 0.3 5.9 + 0.3 0.0’
90 86 86 86 82 82 98 90
0.0 2.3 1.2 0.0 11.0 50.0 96.9 0.0
0.00 0.02 0.20 0.00 9.18 33.13 85.85 0.00
(1All values are results of six replicates; 2 indicates the standard error of mean. * Before tests. r After tests.
After 30 min of continuous exposure at 5.9 kmlhr, a total of 9393 conidia was counted in 66 microscopic fields (one larva/ replicate, four replicates). The percentages of the total conidia collected on slides after 1, 2, 3, 5, 15, and 30 min were 66.2, 16.3, 6.0, 4.5, 4.6, and 2.4%, respectively. Thus, approximately 90% of the conidia were collected within the first 3 min; thereafter the percentage collected leveled off. This does not mean that the cadavers were denuded of conidia. More conidia could be dislodged if the air velocity was increased; however, higher velocities of >16 km/hr eventually disintegrated some cadavers. C. GARCIA C. M. IGNOFFO Biological Control of Insects Research Agricultural Research Service U.S. Department of Agriculture Columbia, Missouri 65201 Received September 17, 1976