Ann E. Koehler

Effects of trapping on rat populations and subsequent damage and yields of macadamia nuts

Abstract During the 1990–1991 and 1991–1992 crop cycles, the effects of snap trapping on rat populations in a macadamia orchard and subsequent damage and yields of nuts were evaluated. During 1990–1991, 1681 roof rats ( Rattus rattus ), 22 Polynesian rats ( R. exulans ), and one Norway rat ( R. norvegicus ) were captured; 360 rats of undetermined species were captured during 1991–1992. Cumulative rat damage for the entire season varied from 0.36 to 1.34% of total annual production in the trapped sections, and from 1.71 to 3.62% of total annual production in the reference sections. However, trapping had no effect on yields: the number of nuts, mass per nut and the total mass of undamaged nuts did not differ between the trapped and reference sections. The results suggest the need to examine crop yield more closely in assessing methods for managing rodent infestations in macadamia orchards. The commonly used indices based on rodent activity and proportion of nuts damaged may overestimate the impact of rodent depredations and exaggerate the effectiveness of control measures in macadamia orchards. A large incidental take of birds points to the need for more selective techniques before trapping is utilized as a damage control measure in Hawaiian macadamia orchards.

Effects of simulated rat damage on yields of macadamia trees

Abstract Rattus rattus damages 5–10% of the developing macadamia ( Macadamia integrifolia ) nut crop each year, but the impact on yields of mature nuts has not been well documented. We evaluated the effects of simulated damage on yields of mature nuts at two locations on the island of Hawaii during the 1995 crop season. We removed 10 or 30% of the developing nut clusters from 5-year-old trees at 90, 120, or 150 days post-anthesis (dpa) and evaluated yields of mature nuts at 210–215 dpa. Removal of 10% of the crop load had no measurable effect on yields of mature nuts regardless of when damage was inflicted. Yields of trees with 30% of nut clusters removed differed from the control (no nut clusters removed) only when damage was inflicted at 150 dpa. These results raise questions about the cost-effectiveness of current rodent control programs, especially during early nut development. Growers may be able to tolerate damage to 10% of their developing nuts without suffering economic losses, and may be able to sustain losses as high as 30% provided that damage is incurred before 120 dpa. Damage control efforts should focus on reducing damage after 150 dpa. However, high rat populations and damage prior to 150 dpa might indicate the need to apply measures to reduce damage later in the crop cycle.

Bait placement and acceptance by rats in macadamia orchards

Black rats (Rattus ruttus) cause extensive damage in Hawaiian macadamia (A4ucndamia integrifolia) orchards. Many growers apply rodenticides to reduce rat populations in orchards, but improper placement of bait may reduce the effectiveness of many baiting programs. We evaluated the optimum placement of bait in macadamia orchards among the three locations specified on current rodenticide labels. We placed a non-toxic oat bait treated with 0.75% tetracycline hydrochloride, an antibiotic that chelates with calcium in growing bones and teeth and fluoresces under UV light, in burrows, on the ground and in trees in separate orchard sections. We consistently captured the greatest percentage of marked rats (53-91%) in sections where we placed the bait in trees and the lowest percentage of marked rats (O-36%) where we broadcast bait on the ground. Placement of bait in burrows produced intermediate results (40-70%). These results suggest that broadcasting bait on the orchard floor reduces the effectiveness of rat control programs. Placing baits in trees targets rats that not only are most likely to eat the poison bait, but also are most likely to damage developing nuts. Published by Elsevier Science Ltd

Weatherability and acceptance of selected commercial zinc phosphide rodent baits

Abstract We evaluated the chemical and physical characteristics of five commercial zinc phosphide (Zn3P2) rodenticide baits at selected intervals during 16 days of exposure to prevailing weather under simulated field conditions, and conducted laboratory feeding trials to assess the effects of weathering on the acceptance and toxicity of one of the baits to Norway rats (Rattus norvegicus), roof rats (R. rattus), and Polynesian rats (R. exulans). In bait weatherability trials, the concentration of Zn3P2 declined most rapidly in an oat bait that was overcoated with this toxicant. The Zn3P2 incorporated into pelleted formulations was less susceptible to physical weathering. All baits softened as they absorbed moisture but, with the exception of Ridall-Zinc® Field & Agricultural Bait, hardened again as they dried. Mold colonization was not apparent until after at least 4 days of exposure and occurred earlier and was more prevalent under wetter conditions. Hopkins® Zinc Phosphide Pellets were most durable in terms of retaining both their physical form and Zn3P2 concentration. Applying 2·5 cm of simulated rain (with or without drying) did not affect consumption of 1·0 cm Hopkins Zinc Phosphide Pellets by roof rats and Polynesian rats, but may have enhanced consumption by Norway rats. Mortality was similar among treatments for Norway rats and Polynesian rats, but was lower for roof rats offered pellets that received added moisture. Mortality for all species and treatment groups was inadequate for operational control purposes.

Comparison of bait markers for blacks rats

Rodenticides are a necessary part of successful large-scale rodent control programs, but poor bait acceptance by rats compromises the effectiveness of many baiting programs. Non-toxic bait markers are one approach for assessing consumption and identifying the cause of failure. In single-feeding laboratory bioassays, we evaluated metallic flakes, Solvent Blue 36 oil-soluble dye, and tetracycline hydrochloride (THC) as bait markers for monitoring consumption of oat groats by captive black rats (Rattus rattus). We detected metallic flakes (0.3% [g/g] dietary concentration) in the gastrointestinal (GI) tracts of all 12 rats examined immediately following a single 24-hour feeding trial but in only 1 of 12 rats examined 48 hours later. We easily detected Solvent Blue 36 oil-soluble dye (0.1% to 0.5% [g/g]) in subcutaneous, abdominal, and genital fat of 7 of 8 rats 2 days after feeding, but found it in only 6 of 12 rats examined 5 days after feeding. Dye applied at 1.0% and 1.5% (g/g) persisted in all 8 rats inspected 5 days after feeding, but reduced consumption (P < 0.05) at these levels suggests that concentrations ≥1% are unpalatable to rats. Dietary concentrations of THC from 0.5% to 1.0% (g/g) produced a golden-yellow fluorescence under ultraviolet (UV) illumination on the incisors and growing points of the mandibles of all rats examined 3 and 14 days after feeding. Because THC had no apparent effect on consumption (P = 0.28), it offers a reliable means of detecting consumption by black rats and thus, of determining whether poor results with rodenticides are due to poor bait acceptance.

Exploration of lauric acid as a potentiator for enhancing warfarin toxicity to rats

Abstract We conducted laboratory feeding trials to evaluate the effectiveness of lauric acid, a naturally occurring short-chain fatty acid, for enhancing the toxicity of warfarin to three species of Hawaiian rats ( Rattus norvegicus, R. rattus, R. exulans ). We offered oats treated with one of three concentrations trations of (0.01, 0.1 or 1.0%) lauric acid and either of two concentrations (0.0125 or 0.005%) of warfarin to individually caged rats during a series of 24-h feeding trials. None of the warfarin-lauric acid combinations enhanced mortality in any of the three species. Nor did pre-exposing rats to lauric acid improve its effectiveness. R. rattus and R. exulans exposed to the highest concentration of lauric acid (1.0%) reduced their consumption of toxic baits. At the concentrations evaluated, lauric acid appears to have little potential as a bait additive for enhancing the toxicity of warfarin to rats.