Avocado Researchers Examine Possibility that Nocturnal Insects Pollinate Avocado Trees at Night

A new research paper explores nocturnal pollination of ‘Hass’ avocado trees and the presence of nocturnal insects that may play a role in pollination. The paper, entitled “Low Overnight Temperatures Associated with a Delay in ‘Hass’ Avocado (Persea americana) Female Flower Opening, Leading to Nocturnal Flowering,” is based on observations by researchers in New Zealand.

As avocado growers are aware, avocado flowers open first as female flowers, then close and open the following day as male flowers. ‘Hass’ avocado flowers, which are categorized as type A cultivars, typically open as female in the morning and as male in the afternoon of the next day. However, low temperatures have been shown to affect the timing of the opening of the flowers. Because New Zealand is subject to cold spring weather, the researchers set out to examine how cool overnight temperatures affected the timing of when ‘Hass’ avocado female flowers opened. Because avocados have sticky pollen and must rely primarily on insects, not wind, for pollination, the researchers also sought to determine if nocturnal insects played a role in pollination.

Researchers collected data from 2011 – 2013 in three avocado orchards using time-lapse images of the ‘Hass’ avocado flowers during the flowering stages. Observations were drawn only from “complete sequences of flower opening and closing in either male or female phase.” Temperature also was recorded.

In addition, the researchers collected nocturnal insects on evenings when both female- and male-phase flowers remained open after dark. The insects were examined under a stereomicroscope to identify and count adhered avocado pollen.

According to the researchers, “low overnight minimum temperatures are associated with the delay in the female flower phase in ‘Hass’ avocados such that the flowers remain open overnight.” Further, a greater proportion of female-phase flowers remained open overnight as the temperatures decreased in prior nights. They also observed that male-phase flowers remained open “almost every night,” which means that on cooler evenings there is a greater occurrence of male-female phase overlap at night. They noted that while cold overnight temperatures triggered nocturnal flowering, the following evening when the female flower remained open was generally more mild in temperature.

Of the active insects collected, the largest diversity was seen in Coleoptera, Diptera and Lepidoptera orders, with Coleoptera, Diptera and Neuroptera insects carrying more pollen grains at greater frequencies. The possibility of a new set of nocturnal pollinators — moths, flies and beetles — could be of interest to avocado growers.

Based on this data, the researchers determined that nocturnal pollination could be important in regions with strong fluctuations in temperature during the flowering period. However, they note that it has yet to be determined whether the nocturnal female-phase flowers can be pollinated, fertilized and produce viable seeds or fruit because data from other food crops has shown that these processes can be vulnerable to temperature stress.

The researchers also suggested that further studies be conducted to better understand the activity patterns of nocturnal insects and whether nocturnal conditions could inhibit fertilization. Because avocado growers primarily rely on honey bees for pollination, the researchers also note it’s worth examining whether nocturnal flowers can set fruit without honey bee visitation.

Interested California avocado growers can review the entire paper, which has been published in the Journal of Pollination Ecology.