Macadamia felted coccid impact on macadamia nut yield in the absence of a specialized natural enemy, and economic injury levels

Highlights

• Macadamia nut yield was measured over a gradient of infestation levels of macadamia felted coccid.

• Substantial yield reductions occurred, and where most substantial in drier areas.

• Economic injury levels are estimated for selected macadamia varieties.

• A relatively easily implemented monitoring process is described.

Abstract

Macadamia nut is among the most important edible crops produced in Hawaii. Macadamia also accounts a significant amount of agricultural land in the state at about 6920 ha of active production. Currently, macadamia has one major insect pest that requires considerably more severe management than others; the macadamia felted coccid, Acanthococcus (previously Eriococcus) ironsidei. Horticultural oils and insect growth regulator insecticides are frequently used to control this pest, without any knowledge of economic injury level currently. This study quantified yield loss and impact on nut quality related to A. ironsidei in Hawaii, on two varieties in two different environments, one mesic, and the other dry. Based on these data, EIL estimates were developed for A. ironsidei on macadamia. There was significant variation in yield loss to A. ironsidei in the two varieties and locations. Some varieties of macadamia nut had severe yield losses under dry-habitat conditions, while others tolerated higher A. ironsidei infestations better, with a lower rate of yield loss. Kernel quality was largely not impacted by A. ironsidei infestation, and reduction in yield was the only impact detected. There were correspondingly variable EILs estimates from the different varieties and environments. The implications of high yield loss in a crop in the absence of a natural enemy of the primary pest are discussed. Results of this study will help to contribute to developing an effective integrated pest management program for A. ironsidei management in Hawaii.

Introduction

With the translocation of plants to different parts of the world for agricultural purposes, many pests of those plants are left behind – the crop is able to achieve high productivity in the absence of the impacts from specific pests. In other cases, insects from the native provenance of the plants become invasive in the new range of the crop and cause substantial yield losses in the absence of natural enemies that successfully suppress the pest. This scenario creates the opportunity to assess the potential impact that insects have on plants, but is not evident in their place of origin owing to the actions of natural enemies there. The beneficial impact of highly evolved natural enemies on plant fitness can thus be extrapolated. Macadamia nut (Macadamia integrifolia, Proteaceae) is originally from Australia and is cultivated in a number of other countries. Hawaii was the first place to develop macadamia nuts as a commercial crop. Today, macadamia nuts are grown in Australia, South Africa, Kenya, Guatemala and Brazil. The invasion of Hawaii (and more recently, South Africa, Schoeman and Millar, 2018) by a specialist macadamia nut insect, Acanthococcus (formerly Eriococcus) ironsidei (Hemiptera; Eriococcidae, macadamia felted coccid, MFC) has resulted in substantial yield losses in the industry there, while in Australia, A. ironsidei is a minor and sometimes sporadic pest. It is evidently suppressed through mortality inflicted by a specialized parasitoid wasp in Australia, Metaphychus macadamiae (Polaszek et al., 2020), which has not yet been released in Hawaii as a biological control agent.

Macadamia nut (Macadamia integrifolia, Proteaceae) is one of the most important edible crops produced in Hawaii, with a farm value of $42 million (based on wet-in-shell weight) for the 2018–2019 crop (USDA-NASS, 2019). Approximately 6920 ha of macadamia nut trees are in active production in Hawaii (USDA-NASS, 2019). This represents a significant portion of agricultural land in Hawaii, and therefore the agricultural practices of macadamia nut growers including pesticide applications may have significant implications for the region in terms of environmental impacts.

The macadamia felted coccid is currently the most significant insect pest of macadamia nut in Hawaii, and has also recently become invasive in South Africa. Macadamia felted coccid was first recorded on the Island of Hawaii in 2005 in a macadamia orchard located in South Kona, and by 2009 its distribution had extended to the east and north areas of the Island (Wright and Conant, 2009). A. ironsidei females develop from egg hatch to adult stage in 32 days and their lifetime can exceed 50 days; males become adults in 16 days and die soon after mating. An individual female can lay up to 97 eggs in its lifetime (Zarders and Wright, 2016). A. ironsidei produces multiple generations per year and population densities typically increase in the drier season in Hawaii (Gutierrez-Coarite et al., 2017).

A. ironsidei primarily infest the trunks and branches of the macadamia tree, and at high population densities, will populate the leaves, racemes, and husks of the nuts. Up to 1700 MFC crawlers per 6.5 cm² (1 square inch) have been recorded on macadamia trees in Hawaii (Gutierrez-Coarite et al., 2019). MFC feed by inserting their mouthparts into the plant and extracting plant fluids. At high infestation levels, their feeding can lead to distorted and stunted tree growth, causing yellow spotting on mature leaves, premature drop of developing fruit, and branch dieback (Jones, 2002) affecting nut production. Young foliage with a high rate of photosynthesis on macadamia nut trees is essential for the development of nuts, and the accumulation of oils in them (Stephenson and Gallagher, 1990). MFC can reduce young healthy foliage when densities are high. The exact extent of yield reduction in relation to MFC density has not been quantified though, and it is essential for developing an effective integrated pest management for the pest.

Control of A. ironsidei using pesticides is difficult considering the size that macadamia trees and canopies can reach (up to 15–20 m in Hawaii, with dense canopies), making full coverage spray applications that are nearly impossible and expensive. Despite these difficulties, current management programs for MFC in Hawaii rely primarily on insecticide applications, while biological control and cultural management options are being developed. Growers rely on insect growth regulators (pyriproxyfen, Esteem® 35 WP, Valent U.S.A. Corporation, Walnut Greek, CA and buprofezin, Centaur®, Nichino America Inc., Wilmington, DE), in combination with paraffinic horticultural oils (Saf-T-Side®, Lawn and Garden Products, Inc. Fresno, CA and TriTek®, Brandt Consolidated, Inc. Springfield, IL) (Gutierrez-Coarite et al., 2017). There are constraints on the use of the insect growth regulators insecticides, including a 60 day pre-harvest interval of buprofezin, and impacts on non-target species. In addition to limited insecticide options, the dense canopies, access to water, moving heavy equipment throughout the orchards and the costs associated with these factors contribute to reducing the economic sustainability of insecticidal control of MFC. There are resident natural enemy populations that may contribute to reductions in A. ironsidei populations with predation and parasitism of up to 60% and 4.3% respectively (Gutierrez-Coarite et al., 2018). However, their contribution to MFC mortality is inadequate to control this pest when population peaks occur in some seasons of the year. A specialist parasitoid from Australia, Metaphychus macadamiae, is a potential biological control agent that is under consideration for release in Hawaii.

Insecticides have been used to control A. ironsidei in Hawaii with no specific action thresholds in place, and without a thorough understanding of yield loss attributable to MFC. The economic injury level (EIL) is a significant element of determining cost-benefit in integrated pest management (IPM) programs and is an important decision-making instrument in the utilization of pesticides (Shipp et al., 2000). Pedigo et al. (1986) defined EIL as the point where the costs of applying pest control measures are equal to the benefits of the pest management actions. Pest infestations below the EIL do not justify pest control actions; however, economic damage may accrue when the pest populations surpass the EIL. The parameters included in EIL calculations comprise the cost of controlling the pest, actual market price of the product, yield loss rate attributable to the pest, and efficiency of controlling the pest using the management options applied (Higley and Pedigo, 1996).

A. ironsidei is currently the only insect pest in Hawaii that requires consistent and aggressive management in macadamia, and it adds a significant cost for management in terms of equipment, fuel, labor, water and insecticide purchases that growers were not incurring prior to the invasion and spread of MFC. There are of course also concerns with the use of insecticides in terms of environmental impacts. Development of IPM decision-making tools that will assist in reducing dependence on insecticides for A. ironsidei will be a valuable contribution to sustainable management of MFC.

This study examines the impact of A. ironsidei at a range of population densities on macadamia nut yield in the absence of an effective biological control agent, in two cultivated varieties, and under two different sets of environmental conditions. Further, to provide growers with a decision-making tool for MFC IPM, this study provides estimates for the economic injury level of A. ironsidei in macadamia orchards in Hawaii.