Nantucket pine tip moth (Rhyncionia frustrata) is one of the most abundant insect pests in southeastern pine forests, attacking primarily seedlings and saplings. Changes in timber management in this area over the past two decades, particularly herbicide applications to reduce competition and increase tree growth that limits vegetationial diversity, may have adverse effects on natural enemies. Several species of the egg parasitoid Trichogramma occur in this area, causing up to 74% parasitism. A locally adapted strain of T. exiguum, one of the naturally-occurring species, was chosen for augmentative releases because it is a vigorous species that retained high quality under long-term mass rearing.
Inundative releases of T. exiguum were made in first-year loblolly pine, Pinus taeda, plantations. Wasps were released three times at 7 day intervals during second-generation pine tip moth egg deposition. Nantucket pine tip moth population levels were high, presenting a challenging environment for testing augmentative releases for pest suppression. Parasitism of pine tip moth eggs was 29% higher in the treated plots than in the control plots, although natural parasitism was relatively high at 42%. Moth egg hatching was reduced by 46%. Although larval populations were significantly lower (60%) in the treated plots, there was no significant difference in the percentage of terminals damaged. The length of terminal damage was reduced, however, and fewer top whorl shoots were damaged.
Wasp survival and emergence is reduced the longer temperatures remain above 35ºC. Microhabitat significantly influences the temperature and the number of consecutive hours per day that are at or above that level. Areas of bare soil have the hottest conditions, followed by soil surface with herbaceous cover, and canopies of small trees. When larger trees (0.9-1.8 m tall) are present moderate temperature conditions tend to prevail. Ground cover and its effect on microclimate significantly influenced emergence of released wasps. Taking into consideration the effect of management practices on ground cover and its impact on Trichogramma releases may eventually lead to better control of Nantucket pine tip moth.
Although these studies demonstrated that mass releases of Trichogramma wasps reduced Nantucket pine tip moth populations, additional studies are needed to determine if augmentative releases can be operationally feasible.
Source:
Orr, D. B. and C. P. C. Suh. 2000. Evaluation of inundative releases of Trichogramma exiguum (Hymenoptera : Trichogrammatidae) for suppression of nantucket pine tip moth (Lepidoptera : Tortricidae) in pine (Pinaceae) plantations. Can. Entomol. 132(3):373-386.
Gypsy Moth Fungus in the Air
The fungal pathogen of gypsy moth, Entomophaga maimaiga, produces two types of spores. Resting spores, which provide overwintering survival, are produced mainly in late-instar cadavers. Conidia are produced externally on mainly early-instar cadavers and are critical for disease transmission during a season. After an infected gypsy moth larva dies, it usually remains attached to twigs, branches or tree trunks. Conidia are actively ejected from the dead larvae under humid conditions. However, little is known about the survival, activity or abundance of these airborne conidia.
Conidia were sampled during 1992 and 1993 in a red oak woodlot in a state forest in central New York where an E. maimaiga epizootic had occurred in 1991. Conidia were abundant in the air during sporadic intervals when gypsy moth populations were relatively high (>20,000 egg masses/ha in 1992), but were almost undetectable when gypsy moth larvae were scarce (<100 egg masses/ha at the beginning of the 1993 field season). Conidia were only present in the air after periods of leaf wetness, with a lag of 5 to 16 hours from the start of leaf wetness to conidial presence, and large releases of conidia only occurred during extended stormy periods.
Infection in the natural populations of gypsy moth larvae increased only after the first peak of abundance of airborne conidia. Infection among larvae caged at 1 1/2 feet above the ground was associated with leaf wetness, suggesting that moisture is critical for conidial survival and infection. Larvae caged on the ground (therefore, exposed to both resting spores and conidia of E. maimaiga) became infected throughout the field season, while, larvae caged at 1 1/2 feet (exposed only to airborne conidia) were infected sporadically when conidial levels were high (1992) and never when conidia could not be detected (1993).
Not all the conidia produced from dead larvae becomes airborne; larvae walking or resting next to sporulating cadavers could become infected by those spores directly rather than through the air. And although airborne conidia do cause infection in gypsy moth popuations, the relative contributions of short-range and long-range dispersal by conidia to epizootic development is not known.
Source:
Hajek, A. E., C. H. Olsen, and J. S. Elkinton. 1999. Dynamics of airborne conidia of the gypsy moth (Lepidoptera : Lymantriidae) fungal pathogen Entomophaga maimaiga (Zygomycetes : Entomophthorales). Biol. Control 16(1):111-117.
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