Cucurbitacins of Cotyledons of Cucurbitaceae Cultivars as Related to Diabroticite Beetle Attack

Cucurbit Genetics Cooperative Report 5:42-43 (article 21) 1982

J. E. Ferguson, Esther R. Metcalf, Robert L. Metcalf and A. M. Rhodes
Departments of Entomology and Horticulture, University of Illinois, Urbana, Champaign

The worldwide culture of many species and varieties of Cucurbitaceae is complicated by the feeding of many species of rootworms, cucumber beetles, pumpkin beetles, of the order Coleoptera, family Chrysomelidae, tribe Luperini. A common feature of the preference of this group of beetles for cucurbits is the presence of the oxygenated tetracyclic triterpenoid cucurbitacins that act as kairomones promoting arrest and compulsive feeding. In the United States cucurbit seedlings are especially severely attacked and often defoliated by the spotted cucumber beetles Diabrotica undecimpunctata undecimpunctataMannerheim and D. u. howardi Barber, by the banded cucumber beetle D. balteata LeConte and by the striped cucumber beetles Acalymma vittata Fabricius, and A. trivittata (Mannerheim).

The cucurbitacins seem clearly to have arisen by coevolutionary selection as intensely bitter substances effective as feeding deterrents to herbivores. Their presence renders cucurbitaceous fruits completely unpalatable to man and our present cultivars selected over thousands of years for palatability, are essentially devoid of Cucs (1, 2). However, the extent of Diabroticite feeding on the cotyledons of many varieties of Cucurbita, Cucumis, and Citrullus suggests that substantial amounts of Cucs are present. Cuc synthesis in these genera is initiated by a single dominant gene B1 (3) but non-bitter fruit may develop from bitter seedlings in the presence of a modifier suppressing Cuc synthesis in the fruit.

We have investigated the Cuc content of the cotyledons of 19 species and 47 cultivars of Cucurbita, Cucumis, and Citrullus by extracting the Cucs, separating them by thin-layer chromatography, and localizing them by the feeding spots produced after exposure to adult D. undecimpunctata and D. balteata. Some cultivars have also been characterized by high pressure liquid chromatography. Substantial amounts of Cucs were found in 30 of 47 cultivars (Table 1) and 10 of 11 wild species examined (Citrullus colocynthis, Cucumis hardwickii, Cucurbita andreanna, C. ficifolia, C. Foetidissima, G. lundelliana, C. martinezii, C. okeechobeensis, C. palmeri, and C. texana). Cotyledonary Cuc content was found to be directly related to seedling beetle damage on the field but unrelated to fruit or leaf beetle damage.

Although other factors may be involved in Diabroticite feeding on Cucurbit cultivars, they are negligible in the presence of the powerful kairomones, the cucurbitacins. The screening of Cucurbits in the seedling stage is paramount in developing non-Cuc containing cultivars for incorporation into PIM programs to lessen Diabroticite attack upon the Cucurbitaceae.

Table 1. Cucurbitacin content of cotyledons of Cucurbitaceae cultivars as estimated by Diabroticite feeding on TLC plates developed from standard chloroform extracts. Species abbreviations and characteristic major Cucs: Cuc E and Cuc-glycosides – LAN = Citrullus lanatus; Cuc C – SAT = Cucumis sativus; Cuc B-ANG = Cucumis anguria, MELO – Cucumis melo, MAX = Cucurbita maxima, MIX = C. mixta, MOS = C. moschata, PEPO = C. pepo.

Cucurbitacin Content* : Species – Cultivar
Large	Moderate	None detected
LAN	LAN	MELO
Charleston Gray	New Hampshire	Early Dawn
Iopride	MELO	Gold Star
Sugar Baby	Golden Rind	SAT
Yellow Doll	Honey Mist	Saticoy
ANG	SAT	MIX
West Indian Gherkin	Marketmore	Gold Striped Cushaw
SAT	Pot Luck	MOS
Liberty Hybrid	Wis. SMR-18	Dickinson Field
Palomar	MAX	Early Butternut
MAX	Boston Marrow	PEPO
Mammoth Gold	Golden Hubbard	Bush Table King
MOS	Pink Banana Jumbo	Crookneck
Tahiti	PEPO	Early White Squash
PEPO	Blackjack	Goldbar
Ambassador	Caserta	Goldneck
Black	Greyzini	Patty Green Tint
Diplomat	Seneca Butterbar	Scottsdale
Gourmet Globe		Seneca Prolific
Greenbay		St. Pat Scallop
Cocozelle		Straightneck
Scallopini
Storr’s Green
Striato Striped

*Cuc content verified by HPLC Iopride (large) = 39.3 μg Cuc E/g fwt cotyledons; Black (large) = 23.0 μg Cuc B/g fwt; Blackjack (moderate) = 10.8 μg Cuc B/g fwt; and Goldbar (none detected), 0.25 μg fwt.

Literature Cited

1. Metcalf, R.L., R.A. Metcalf and A.M. Rhodes, 1980. Cucurbitacins as kairomones for diabroticite beetles. Proc. Nat. Aca. Sci. 77: 3769-3772.
2. Metcalf, R.L., A.M. Rhodes, R.A. Metcalf, Jane Ferguson, Esther R. Metcalf and Po-Young Lu, 1982. Cucurbitacin contents and Diabroticite feeding upon Cucurbita spp. In Press. Env. Entom.
3. Robinson, R.W., H.M. Munger, T.W. Whitaker and G.W. Bohn, 1976. Genes of the Cucurbitaceae, Hort. Scoi. 11(6):554-568.