Masters Theses

Date of Award

12-1991

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Gary L. Lentz

Committee Members

Jerome F. Grant, Charles D. Pless

Abstract

Rapeseed is a member of the crucifer genus Brassica (Cruciferae; Brassiceae), a genus with more than 50 plant species utilized for fodder, food, oilseed, or vegetable production (Fribourg et al. 1989). Rapeseed represents both summer and winter forms of rape, Brassica napus ssp. oleifera L., and turnip rape, Brassica campestris ssp. oleifera L., and both species frequently occur in common geographical areas (Appelqvist 1972). Rape, a native to Europe and Northwestern Africa, was first introduced into Canada in 1942, where cultivation is limited to summer forms due to extreme climatic conditions. Until the mid-1980's, rapeseed cultivation in the United States was limited to the Pacific Northwest and the Northern Great Plains (Fribourg et al. 1989).

Usage of rapeseed is dependent upon the level of unsaturated erucic acid and sulfur-containing glucosinolates (Ohlson 1972). High erucic acid rapeseed (HEAR) is a plant cultivar with an oil content of >50% erucic acid and >30 μmol glucosinolates per gram of meal (Fribourg et al. 1989). HEAR oil cultivars are desirable because the oil is used for rubber stabilization, machine lubrication, and textile manufacturing (Ohlson 1972). From 1986 to 1988, field trial experiments were conducted at the Milan Experiment Station in western Tennessee to determine the suitability of local environmental conditions for rapeseed production (Fribourg et al. 1989). This research demonstrated that the rapeseed species, winter rape, Brassica napus ssp. oleifera forma biennis. could be cultivated in western Tennessee to produce HEAR oil. Because rapeseed is a new crop in western Tennessee, the species composition and seasonal incidence of arthropods are unknown; therefore, a two-year study beginning in the spring of 1990 was initiated to address these research areas.

Rapeseed and other crucifers are hosts to more than 150 insect species, with many pest species representing the orders Coleoptera, Diptera, Hemiptera, Homoptera, and Lepidoptera (Hill 1987) . Most homopteran and lepidopteran pests attack the foliage tissues of crucifers; whereas, many coleopteran and hemipteran pests attack the reproductive tissues of crucifers, including rapeseed (Bonnemaison 1965). Dipteran pests are primarily root and foliage feeders (Hill 1987).

During this study, approximately 111 species or groups were collected from rapeseed in western Tennessee. Fortyseven families representing ten insect orders, including Coleoptera, Diptera, Hemiptera, Homoptera, and Lepidoptera, and five families representing one arachnid order were collected and identified. The most extensive diversity of arthropods occurred during the flowering and ripening stages of rapeseed.

The turnip aphid, Lipaphis erysimi (Kaltenbach) (Homoptera: Aphididae), was the predominant aphid species. The turnip aphid was present on the first sampling date (March 9 and 19) in 1990 and 1991, respectively. Turnip aphid populations increased most rapidly during the flowering stage of rapeseed, with population peaks coinciding with rapeseed flowering. Feeding damage in heavily-infested fields caused stunted growth, reduced pod production, and delayed seed maturity. Perhaps due to heavy spring rains, turnip aphid populations in 1991 were 78.8% lower than populations in 1990 as estimated by sweep-net sampling.

In all fields sampled in 1990 and 1991, turnip aphids were frequently attacked by a wasp parasitoid [Diaeretiella rapae (M’Intosh) (Hymenoptera: Aphidiidae)]. Wasp populations in 1991 were 95.8% lower than in 1990, reflecting the reduced level of aphid infestation. Parasitized aphids ceased feeding, attached themselves to plant stems, and became brown mummies.

The coleopteran species, the cabbage seedpod weevil, Ceutorhynchus assimilis (Paykull) (Curculionidae), was collected in sweep-net samples in 1990 and 1991 and was the most common (98.8%) curculionid species collected. Adults were first collected as rapeseed began to flower in early April, when adults were observed to feed and mate on flowers. Damaged seedpods were characterized by the presence of a small, black puncture, with larvae later damaging three to five seeds per pod. During both years of this study, overwintering adult populations peaked during or shortly after the peak flowering of rapeseed. The summer generation (F1) adults emerged three to four weeks later during the ripening (5.1 to 5.5) stage of rapeseed, and were observed to feed on any remaining green tissue.

Several thysanopterans attracted to rapeseed flowers included flower thrips, Frankliniella tritici (Fitch), tobacco thrips, F. fusca (Hinds), and soybean thrips, Neohydatothrips variabilis (Beach) (all Thripidae). Flower thrips was the most abundant species (87.8%) in 1990 and (97.9%) in 1991. Flower thrips were most abundant during the flowering (4.1 to 4.4) and early ripening (5.1) stages, and numbers of flower thrips decreased rapidly after flowering terminated. No visual damage to infested plants was observed, and plants appeared to develop normally. The implication of thrips infestation in rapeseed is that the crop may serve as an early-season host for later infestation of other crops (e.g., cotton).

In western Tennessee, the tarnished plant bug, Lvous lineolaris (Palisot de Beauvois) (Miridae), was the most frequently (>95%) collected pod-feeding hemipteran. During both years of this study, tarnished plant bug populations peaked during the ripening (5.1 to 5.3) stage of rapeseed. The tarnished plant bug is believed to utilize rapeseed as an early-season host in western Tennessee; therefore, the tarnished plant bug may build up populations in rapeseed, and later infest other crops (e.g., cotton).

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