Occurrence and causes of mortality among active and resting larvae of Pectinophora gossypiella (Lepidoptera, Gelechiidae) in Giza, Egypt

Rates of natural sickness and mortality among active larvae of Pectinophora gossypiella Saunders, found in cotton squares flowers and bolls throughout two active seasons (July-September, 1974 and 1975) were very low: 0–0.6% sick and 3.5–9.4 dead. During the resting season, however, rate of natural sickness and mortality increased gradually from November till May. Throughout the two resting seasons 1973/74 and 1974/75, the respective average rates of healthy larvae were 54.4 and 57.8%, sick larvae carrying dermal brown lesions associated with pathogenic bacteria were 4.1 and 1.7%, larvae killed by the ectoparasitic mite Pyemotes herfsi 24.5 and 33.9%, larvae parasitized by Exeristes roborator 1.4 and 1.7%, larvae parasitized by Parasierola sp. 0.9 and 0.7%, predated larvae 1.3 and 2.0%, dead, soft-bodied and v-shaped larvae (mostly due to virus infection) 5.4 and 1.7%, dead larvae carrying dermal brown lesions 2 and 0.4%, and dead larvae showing no specific symptoms 5.6 and 0.1%. However, by the end of the resting season (in May), rate of mortality among resting larvae reached 80.4–96.8%, out of which 44.9–73.9% were killed by Pyemotes herfsi. Two pathogenic bacterial strains, Bacillus thuringiensis var. finitimus and B. cereus, were isolated from sick or dead resting larvae of P. gossypiella carrying dermal brown lesions. No significant correlation was found between the mortality rates of resting larvae, and the monthly averages of daily mean temperature or relative humidity. Zusammenfassung Untersuchungen uber die Mortalitat der Aktiv- und Ruhelarven von Pectinophora gossypiella Saund. (Lepidoptera, Gelechiidae) in Giza, Agypten Der Anteil an Kranken und Gestorbenen unter den aktiven Larven von P. gossypiella innerhalb der Baumwollbluten und -kapseln war wahrend der zwei Untersuchungsperioden (Juli-September 1974 und 1975) mit 0–0,6% Kranken und 3,5–9,4% Toten sehr gering. Wahrend der Ruheperiode (November-Mai) stiegen jedoch die Raten fortwahrend an auf folgende Werte: gesunde Larven 54,4 und 57,8%; kranke Larven mit braunen Hautlesionen (Bakteriosen) 4,1 und 1,7%; von der ektoparasitischen Milbe Pyemotes herfsi vernichtete Larven 24,5 und 33,9%; von der Schlupfwespe Exeristes roborator Parasitierte 1,4 und 1,7%; von der Schlupfwespe Parasierola sp. Parasitierte 0,9 und 0,7%; von Predatoren Gefressene 1,3 und 2,0%; tote, schlaffe und v-gestaltige Larven (Virosen) 5,4 und 1,7%; tote Larven mit braunen Hautlesionen 2 und 0,4%; tote Larven ohne spezifische Symptome 5,6 und 0,1%. Am Ende der Ruheperiode (im Mai) erreichte die Sterblichkeit der Ruhelarven 80,4 und 96,8%, woran Pyemotes herfsi 44,9–73,9% Anteil hatte. Zwei pathogene Bakterienstamme wurden aus den kranken oder toten P. gossypiella-Larven isoliert: Bacillus thuringiensis var. finitimus sowie Bacillus cereus. Zwischen der Sterblichkeit der Ruhelarven und den Witterungsbedingungen bestanden keine gesicherten Beziehungen.