Reduced Emergence of Cylas formicarius elegantulus (Coleoptera: Curculionidae) from Sweet Potato Roots by Heterorhabditis indica

Okinawan sweet potato, Ipomoea batatas, is an important food staple and export crop for the Island of Hawaii. Cylas formicarius elegantulus, sweet potato weevil, is a major quarantine pest that causes severe destruction to the crop. Root malformation and a bitter taste occur when larvae feed and tunnel within the storage root. Off-grade roots are often left in the field after harvest and serve as a reservoir for the weevils. Current management involves the unsustainable practice of moving to virgin land for the next cropping cycle. Strains of Heterorhabditis indica isolated from the Hawaiian Islands were tested for their efficacy at causing mortality of C. formicarius and reducing the emergence of adults from infested roots. In well plate assays, H. indica caused mortality of 88% larvae, 96% pupae, and 4% adults after 48 h. When applied to infested roots, the nematodes caused an average mortality of 78% larvae, 66% pupae, and 32% adults. Greater mortality was observed at the highest inoculum levels (10,000 infective juveniles per storage tuber) but a reduction of 90% inoculum density was still effective at weevil management. In simulated field trials, infestation of storage roots was reduced by 42–99.6% when planted among infested roots that had been inoculated with H. indica. Rates of 2.5 billion IJs/hectare were just as effective as 5 billion IJs/hectare. Application of local H. indica strains in sweet potato production has the potential to manage C. formicarius populations and allow for consecutive cropping seasons.

A New Hyptadienic Acid Derivative from Hyptis Verticillata (Jacq.) with Insecticidal Activity

A new triterpene, A(1)-1-benzoate-19α-hydroxy-urs-2(3),12(13)-dien-28-oic acid (1), was isolated from Hyptis verticillata, in addition to the known triterpenes tormentic acid, isolated as the diacetate (2), betulinic acid (3), α- and β-amyrin, oleanolic acid and ursolic acid. The structure of 1 was elucidated by spectroscopic analysis. Compounds 1–3 exhibited insecticidal activity against the sweet potato weevil Cylas formicarius elegantulus (Summers).

Insect Resistance in Sweetpotato Plant Introductions

One hundred U.S. sweetpotato [Ipomoea batatus (L.) Lam.] plant introductions (PIs) and four control cultivars were screened for insect injury in 1993. Of the least injured by insects, 56 and 31 were tested again in 1994 and 1995, respectively. Among control cultivars, the most highly resistant was `Regal' (moderately resistant), followed by `Beauregard' (susceptible), `Centennial' (susceptible), and `Jewel' (susceptible). Stem and root injury by the sweetpotato weevil (SPW) [Cylas formicarius elegantulus (Summers)] and root injury by the wireworm (Conoderus sp.)–Diabrotica sp. (cucumber beetle)– Systena sp. (flea beetle) (WDS) complex were measured. SPW stem injury was less severe (P ≤ 0.05) in 1994 and 1995 in PIs 508523, 531116, and 564107 than in control cultivars. PIs 508523 and 531116 also suffered less SPW root injury than did `Regal'. In the six PIs with least SPW root injury, PIs 538354, 564149, 508523, 538286, 531116, and 564103, 70% to 85% of the roots were not injured compared with 36% in `Regal' and 6% in `Jewel'. SPW root injury scores (0 = no injury; 5 = severe injury) in those PIs averaged 0.5 vs. 2.3 for `Regal'. Only in PI 538286 was WDS injury to roots less than in `Regal' over 2 years. However, eight additional accessions suffered less WDS injury than `Regal' in 1995 and four of those were among the six with least SPW injury. The lower levels of combined insect injury found in these four PIs (compared to `Regal') show that PIs have potential use for increasing insect resistance in sweetpotato improvement programs.